CIS3601 CIS 360: Introduction to Computer Systems Course Notes Mary Beth Zak Lohse Rick.

CIS360 1

CIS 360: Introduction to Computer SystemsCourse Notes

Mary Beth Zak Lohse ([email protected]) http://www.cis.ohio-state.edu/~lohse

Rick Parent ([email protected]) http://www.cis.ohio-state.edu/~parent

Copyright © 1998-2003 by Rick Parent, Todd Whittaker, Bettina Bair, Pete Ware, Wayne Heym

CIS360 2

Information Representation 1 Positional Number Systems: position of character

in string indicates a power of the base (radix). Common bases: 2, 8, 10, 16. (What base are we using to express the names of these bases?)– Base ten (decimal): digits 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 form

the alphabet of the decimal system. E.g., 31610 =

– Base eight (octal): digits 0, 1, 2, 3, 4, 5, 6, 7 form the alphabet.

E.g., 4748 =

CIS360 3

Information Representation 2– Base 16 (hexadecimal): digits 0-9 and A-F.

E.g., 13C16 =

– Base 2 (binary): digits (called “bits”) 0, 1 form the alphabet.

E.g., 100110 =

– In general, radix r representations use the first r chars in {0…9, A...Z} and have the form dn-1dn-2…d1d0. Summing dn-1rn-1 + dn-2rn-2 + … + d0r0 will convert to base 10. Why to base 10?

CIS360 4

Information Representation 3 Base Conversions

– Convert to base 10 by multiplication of powers E.g., 100125 = ( )10

– Convert from base 10 by repeated division E.g., 63210 = ( )8

– Converting base x to base y: convert base x to base 10 then convert base 10 to base y

CIS360 5

Information Representation 4– Special case: converting among binary, octal, and

hexadecimal is easier Go through the binary representation, grouping in sets of 3 or

4. E.g., 110110012 = 11 011 001 = 3318

110110012 = 1101 1001 = D916

E.g., C3B16 = ( )8

CIS360 6

Information Representation 5 What is special about binary?

– The basic component of a computer system is a transistor (transfer resistor): a two state device which switches between logical “1” and “0” (actually represented as voltages on the range 5V to 0V).

– Octal and hexadecimal are bases in powers of 2, and are used as a shorthand way of writing binary. A hexadecimal digit represents 4 bits, half of a byte.1 byte = 8 bits. A bit is a binary digit.

– Get comfortable converting among decimal, binary, octal, hexadecimal. Converting from decimal to hexadecimal (or binary) is easier going through octal.

CIS360 7

Information Representation 6Binary Hex Decimal Binary Hex Decimal

0000 0 0 1000 8 8

0001 1 1 1001 9 9

0010 2 2 1010 A 10

0011 3 3 1011 B 11

0100 4 4 1100 C 12

0101 5 5 1101 D 13

0110 6 6 1110 E 14

0111 7 7 1111 F 15

CIS360 8

Information Representation 7 Ranges of values

– Q: Given k positions in base n, how many values can you represent?

– A: nk values over the range (0…nk-1)10

n=10, k=3: 103=1000 range is (0…999)10

n=2, k=8: 28=256 range is (0…255)10

n=16, k=4: 164=65536 range is (0…65535)10

– Q: How are negative numbers represented?

CIS360 9

Information Representation 8 Integer representation:

– Value and representation are distinct. E.g., 12 may be represented as XII, C16, 1210, and 11002. Note: -12 may be represented as -C16, -1210, and -11002.

– Simple and efficient use of hardware implies using a specific number of bits, e.g., a 32-bit string, in a binary encoding. Such an encoding is “fixed width.”

– Four methods: (fixed-width) simple binary, signed magnitude, binary coded decimal, and 2’s complement.

– Simple binary: as seen before, all numbers are assumed to be positive, e.g., 8-bit representation of6610 = 0100 00102 and 19410 = 1100 00102

CIS360 10

Information Representation 9– Signed magnitude: simple binary with leading sign bit.0 = positive, 1 = negative. E.g., 8-bit signed mag.:

6610 = 0100 00102

-6610 = 1100 00102

What ranges of numbers may be expressed in 8 bits?Largest:Smallest:

Extend 1100 0010 to 12 bits:

CIS360 11

Information Representation 10Problems: (1) Compare the signed magnitude numbers1000 0000 and 0000 0000. (2) Must have “subtraction” hardware in addition to “addition” hardware.

– Binary Coded Decimal (BCD): use a 4 bit pattern to express each digit of a base 10 number

0000 = 0 0001 = 1 0010 = 2 0011 = 3 0100 = 4 0101 = 5 0110 = 6 0111 = 7 1000 = 8 1001 = 9 1010 = + 1011 = -

E.g., 123 : 0000 0001 0010 0011+123 : 1010 0001 0010 0011-123 : 1011 0001 0010 0011

CIS360 12

Information Representation 11BCD Disadvantages:

– Takes more memory. 32 bit simple binary can represent more than 4 billion discrete values. 32 bit BCD can hold a sign and7 digits (or 8 digits for unsigned values) for a maximum of110 million values, a 97% reduction.

– More difficult to do arithmetic. Essentially, we must force the Base 2 computer to do Base 10 arithmetic.

BCD Advantages:– Used in business machines and languages, i.e., in COBOL for

precise decimal math.– Can have arrays of BCD numbers for essentially arbitrary

precision arithmetic.

CIS360 13

Information Representation 12– Two’s Complement

Used by most machines and languages to represent integers. Fixes the -0 in the signed magnitude, and simplifies machine hardware arithmetic.

Divides bit patterns into a positive half and a negative half (with zero considered positive); n bits creates a range of [-2n-1… 2n-1 -1].

CODE0000000100100011010001010110011110001001101010111100110111101111

Simple0123456789101112131415

Signed+01234567-0-1-2-3-4-5-6-7

2’s comp01234567-8-7-6-5-4-3-2-1

CIS360 14

Information Representation 13– Representation in 2’s complement; i.e., represent i in

n-bit 2’s complement, where -2 n-1 i +2 n-1-1 Nonnegative numbers: same as simple binary Negative numbers:

– Obtain the n-bit simple binary equivalent of | i | – Obtain its negation as follows:

• Invert the bits of that representation• Add 1 to the result

Ex.: convert -32010 to 16-bit 2’s complement

Ex.: extend the 12-bit 2’s complement number1101 0111 1000 to 16 bits.

CIS360 15

Information Representation 14 Binary Arithmetic

– Addition and subtraction only for now– Rules: similar to standard addition and subtraction, but

only working with 0 and 1. 0 + 0 = 0 0 - 0 = 0 1 + 0 = 1 1 - 0 = 1 0 + 1 = 1 1 - 1 = 0 1 + 1 = 10 10 - 1 = 1

– Must be aware of possible overflow. Ex.: 8-bit signed magnitude 0101 0110 + 0110 0011 =

Ex.: 8-bit signed magnitude 0101 0110 - 0110 0011 =

CIS360 16

Information Representation 15 2’s Complement binary arithmetic

– Addition and subtraction are the same operation – Still must be aware of overflow.

Ex.: 8 bit 2’s complement: 2310 + 4510 =

Ex.: 8 bit 2’s complement: 10010 + 4510 =

Ex.: 8 bit 2’s complement: 2310 - 4510 =

CIS360 17

Information Representation 16– 2’s Complement overflow

Opposite signs on operands can’t overflow If operand signs are same, but result’s sign is different, must

have overflow Can two positives sum to positive and still have overflow?

Can two negatives?

CIS360 18

Information Representation 17 Characters and Strings

– EBCDIC, Extended Binary Coded Decimal Interchange Code Used by IBM in mainframes (360 architecture and descendants). Earliest system

– ASCII, American Standard Code for Information Interchange. Most common system

– Unicode, http://www.unicode.org New international standard Variable length encoding scheme with either 8- or 16-bit minimum “a unique number for every character, no matter what the platform,

no matter what the program, no matter what the language.”

CIS360 19

Information Representation 18 ASCII

– see table 1.7 on pg. 18. In Unix, run “man ascii”.

– 7 bit code Printable characters for human interactions Control characters for non-human communication (computer-

computer, computer-peripheral, etc.)

– 8-bit code: most significant bit may be set Extended ASCII (IBM), includes graphical symbols and lines ISO 8859, several international standards Unicode’s UTF-8, variable length code with 8-bit minimum

CIS360 20

ASCII Easy to decode

– But takes up a predictable amount of space Upper and lower case characters are 0x20 (3210)

apart ASCII representation of ‘3’ is not the same as the

binary representation of 3. – To convert ASCII to binary (an integer), ‘3’-‘0’ = 3

Line feed (LF) character– 000 10102 = 0x0a = 1010

– ‘\n’ = 0xa

Character ASCII Binary ASCII Hex‘ ’ 010 0000 0x20‘A’ 100 0001 0x41‘a’ 110 0001 0x61‘R’ 101 0010 0x52‘r’ 111 0010 0x72‘0’ 011 0000 0x30‘3’ 011 0011 0x33

CIS360 21

Information Representation 19 String: definition is programming language dependent.

– C, C++: strings are arrays of characters terminated by a null byte. Decode:

1000001, 1010011, 1000011, 1001001, 1001001, 0100000, 1101001, 1110011, 0100000, 1100101, 1100001, 1110011, 1111001, 0000000

– Or (in hex):41 53 43 49 49 20 69 73 20 65 61 73 79 00

How many bytes is this? What’s the use of the ’00’ byte at the end?

CIS360 22

Information Representation 20 Simple data compression

– ASCII codes are fixed length. – Huffman codes are variable length and based on

statistics of the data to be transmitted. Assign the shortest encoding to the most common character.

– In English, the letter ‘e’ is the most common.– Either establish a Huffman code for an entire class of messages,– Or create a new Huffman code for each message, sending/storing

both the coding scheme and the message. “a widely used and very effective technique for compressing

data; savings of 20% to 90% are typical, depending on the characteristics of the file being compressed.” (Cormen, p. 337)

CIS360 23

ECL - Expected Code LengthChar Fixed len

encodingFreq Var len

encoding# bits Expected

# bits

00 .5 1 1 .5

01 .25 01 2 .5

10 .15 001 3 .45

11 .10 000 3 .3

Avg len 2 1.75

CIS360 24

Information Representation 21 Huffman Tree for “a man a plan a canal panama”

– Examine data set and determine frequencies of letters (example ignores spaces, normally significant)

– Create a forest of single node trees. Choose the two trees having the smallest total frequencies (the two “smallest” trees), and merge them together (lesser frequency as the left subtree, for definiteness, to make grading easier). Continue merging until only one tree remains.

Count Frequency‘ a’ 10 0.476190‘ c’ 1 0.047619‘ l ’ 2 0.095238‘ m’ 2 0.095238‘ n’ 4 0.190476‘ p’ 2 0.095238

CIS360 25

Information Representation 22

Huffman Tree for "a man a plan a canal panama"

'a'.4762

'n'.1905

'c'.0476

'l'.0952

.1428

'm'.0952

'p'.0952

.1905

.3333

.5238

1.0

Reading a ‘1’ calls for following the left branch.

Reading a ‘0’ calls for following the right branch.

Decoding using the tree:To decode ‘0001’, start at root and follow r_child, r_child, r_child, l_child, revealing encoded ‘m’.

CIS360 26

Information Representation 23 Comparison of Huffman and 3-bit code example

– 3-bit: 000 011000100 000 101010000100 000 001000100000010 101000100000011000 = 63 bits

– Huffman: 1 0001101 1 00000010101 1 001110110010 0000101100011 = 46 bits

– Savings of 17 bits, or 27% of original message

3-bit code Huffman Code Count H length 3 length‘a’ 000 1 10 10 30‘c’ 001 0011 1 4 3‘l’ 010 0010 2 8 6‘m’ 011 0001 2 8 6‘n’ 100 01 4 8 12‘p’ 101 0000 2 8 6Totals 46 63

CIS360 27

Parity: Simple error detection Data transmission, aging media, static

interference, dust on media, etc. demand the ability to detect errors.

Single bit errors detected by using parity checking.

Parity, here, is the “the state of being odd or even.”

CIS360 28

Information Representation 24– How to detect a 1-bit error:

Ex.: send ASCII ‘S’: send 1010011, but receive 1010010?

Add a 1-bit parity to make an odd or even number of bits per byte.

Parity bit is stripped by hardware after checking. Sender/receiver both agree to odd or even parity.

2 flipped bits in the same encoding are not detected.

‘ S’ ‘ E’ASCII 101 0011 100 0101Even parity 0101 0011 1100 0101Odd Parity 1101 0011 0100 0101

CIS360 29

Information Representation 25 Two meanings for Hamming distance. 2nd is generalization

of 1st. 1st is: distance between two encodings of the same length.1. A count of the number of bits different in encoding 1 vs. encoding 2.

E.g., dist(1100, 1001) =dist(0101, 1101) =

2. Generalize to an entire code by taking the minimum over all distinct pairs (2nd meaning).

– The ASCII encoding scheme has a Hamming distance of 1.– A simple parity encoding scheme has a Hamming distance of 2.

Hamming distance serves as a measure of the robustness of error checking (as a measure of the redundancy of the encoding).

CIS360 30

ISEM FAQ 1 Editing, Assembling, Linking, and Loading

– There are three components to the Instructional SPARC Emulator (ISEM) package that we use for this class:

the assembler, the linker, and the emulator/debugger.

CIS360 31

ISEM FAQ 2 Editing

– There are a number of programs that you can use to create your source files.

Emacs is probably the most popular; vi is also available, but its command syntax is difficult to learn

and use; using pine program, you can use the pico editor, which

combines many features of Emacs into a simple menu-driven facility.

– Start Emacs by “xemacs sourcefile.s &”, which creates the file called sourcefile.s.

– Use the tutorial, accessed by typing "Ctrl-H Ctrl-H t". – For other editors, you are on your own.

CIS360 32

Example Sparc Assembly Language Instructions

% type xmp0.s .data ! Assembler directive: data starts here. A_m, B_m, andA_m: .word ’?’ ! C_m are symbolic constants. Furthermore, eachB_m : .word 0x30 ! is an address of a certain-sized chunk of memory. Here,C_m : .word 0 ! each chunk is four bytes (one word) long. When the

! program gets loaded, each of these chunks stores a ! number in 2’s complement encoding, as follows: At ! address C_m, zero; at B_m, 48; at A_m, 0x3F = 077 = 63.

.text ! Assembler directive, instructions start herestart: ! Label (symbolic constant) for this address set A_m, %r2 ! Put address A_m into register 2 ld [%r2], %r2 ! Use r2 as an indirect address for a load (read) set B_m, %r3 ! Put address B_m into register 3 ld [%r3], %r3 ! Read from B_m and replace r3 w/ value at addr B_m sub %r2, %r3, %r2 ! Subtract r3 from r2, save in r2 set C_m, %r4 ! Put address C_m into register 4 st %r2, [%r4] ! Store (write) r2 to memory at address C_mterminate: ! Label for address where ’ta 0’ instruction stored ta 0 ! Stop the programbeyond_end: ! Label for address beyond the end of this program

CIS360 33

ISEM FAQ 3 Assembling

– The assembler is called "isem-as", and is the GNU Assembler (GAS), configured to cross-assemble to a SPARC object format.

– It is used to take your source code, and produce object code that may be linked and run on the ISEM emulator.

– The syntax for invoking the assembler is:

isem-as [-a[ls]] sourcefile.s -o objectfile.o

– The input is read from sourcefile.s, and the output is written to objectfile.o.

– The option "-a" tells the assembler to produce a listing file. The sub-options "l" and "s" tell the assembler to include the assembly source in the listing file and produce a symbol table, respectively.

CIS360 34

ISEM FAQ 4 The listing file

– Will identify all the syntactic errors in your program, and it will warn you if it identifies "suspicious" behavior in your source file.

– Column 1 identifies a line number in your source file. – Column 2 is an offset for where this instruction or data resides in

memory.– Column 3 is the image of what is put in memory, either the

machine instructions or the representation of the data. – The final column is the source code that produced the line. – At the bottom of the file you will find the symbol table. – Again, the symbols are represented as offsets that are relocated

when the program is loaded into memory.

CIS360 35

isem-as -als labn.s -o labn.o >! labn.lst

1 .data 2 0000 0000003F A_m: .word ’?’ 3 0004 00000030 B_m: .word 0x30 4 0008 00000000 C_m: .word 0 5 000c 00000000 .text 6 start: 7 0000 05000000 set A_m, %r2 7 8410A000 8 0008 C4008000 ld [%r2], %r2 9 000c 07000000 set B_m, %r3 9 8610E000 10 0014 C600C000 ld [%r3], %r3 11 0018 84208003 sub %r2, %r3, %r2 12 001c 09000000 set C_m, %r4 12 88112000 13 0024 C4210000 st %r2, [%r4] 14 terminate: 15 0028 91D02000 ta 0 16 002c 01000000 beyond_end: DEFINED SYMBOLS xmp0.s:2 .data:00000000 A_m xmp0.s:3 .data:00000004 B_m xmp0.s:4 .data:00000008 C_m xmp0.s:6 .text:00000000 start xmp0.s:14 .text:00000028 terminate xmp0.s:16 .text:0000002c beyond_end NO UNDEFINED SYMBOLS

Line in source file (.s)

Offset to address

in memory

Contents at

address in

memoryLabels are

symbolic offsets

CIS360 36

ISEM FAQ 5 Linking

– Linking turns a set of raw object file(s) into an executable program. – From the manual page, "ld combines a number of object and archive files,

relocates their data and ties up symbol references. Often the last step in building a new compiled program to run is a call to ld."

– Several object files are combined into one executable using ld; the separate files could reference symbols from one another.

– The output of the linker is an executable program.– The syntax for the linker is as follows:

isem-ld objectfile.o [-o execfile]

Examples

% isem-ld foo.o -o foo Links foo.o into the executable foo. % isem-ld foo.o Links foo.o into the executable a.out.

CIS360 37

ISEM FAQ 6 Loading/Running

– Execute the program and test it in the emulation environment. – The program "isem" is used to do this, and the majority of its features are

covered in your lab manual. – Invoke isem as follows

isem [execfile]

Examples

% isem foo Invokes the emulator, loads the program foo % isem Invokes the emulator, no program is loaded

– Once you are in the emulator, you can run your program by typing "run" at the prompt.

CIS360 38

ISEM Debugging Tools 1% isem xmp0 Instructional SPARC EmulatorCopyright 1993 - Computer Science Department University of New Mexico ISEM comes with ABSOLUTELY NO WARRANTY ISEM Ver 1.00d : Mon Jul 27 16:29:45 EDT 1998 Loading File: xmp02000 bytes loaded into Text region at address 8:20002000 bytes loaded into Data region at address a:4000 PC: 08:00002020 nPC: 00002024 PSR: 0000003e N:0 Z:0 V:0 C:0 start : sethi 0x10, %g2 ISEM> runProgram exited normally.

Assembly language programs are not notoriously chatty.

CIS360 39

ISEM Debugging Tools 2 reg

– Gives values of all 32 general registers

– Also PC symb

– Shows the resolved values of all symbolic constants

dump [addr]– Either symbol or hex

address– Gives the values stored in

memory

ISEM> reg ----0--- ----1--- ----2--- ----3--- ----4--- ----5--- ----6--- ----7---G 00000000 00000000 0000000f 00000030 00004008 00000000 00000000 00000000O 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000L 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000I 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 PC: 08:0000204c nPC: 00002050 PSR: 0000003e N:0 Z:0 V:0 C:0 beyond_end : sethi 0x0, %g0 ISEM> symbSymbol List A_m : 00004000 B_m : 00004004 . . . terminate : 00002048 ISEM> dump A_m0a:00004000 00 00 00 3f 00 00 00 30 00 00 00 0f 00 00 00 00 ...?...0........0a:00004010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................0a:00004020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................

CIS360 40

ISEM Debugging Tools 3 break [addr]

– Set breakpoints in execution– Once execution is stopped, you can look at the contents of

registers and memory. trace

– Causes one (or more) instruction(s) to be executed– Registers are displayed– Handy for sneaking up on an error when you’re not sure where it

is. For the all-time “most wanted” list of errors (and their

fixes)– http://www.cis.ohio-state.edu/~heym/360/common/faq.html

CIS360 41

D F lip F lopD ataI n

C lock

D ataO ut

one cycle

Basic Components 1 Terminology from Ch. 2:

– Flip flop: basic storage device that holds 1 bit– D flip flop: special flip flop that outputs the last value

that was input to it (a data signal).– Clock: two different meanings: (1) a control signal that

oscillates (low to high voltage) every x nanoseconds; (2) the “write select” line for a flip flop.

CIS360 42

Basic Components 2– Register: collection of flip flops with parallel load.

Clock (or “write select”) signal controlled. Stores instructions, addresses, operands, etc.

– Bus: Collection of related data lines (wires).

d7 d6 d5 d4 d3 d2 d1 d0I nput B us

O utput B us

C lock 8 B it R egister

8

8

C lock

CIS360 43

Basic Components 3– Combinational circuits: implement Boolean functions.

No feedback in the circuit, output is strictly a function of input.

Gates: and, or, not, xor

E.g., xy + z

AN D O R N O T X O R

x

y

z f

CIS360 44

Basic Components 4– Gates can be used in combination to implement a

simple (half) adder. Addition creates a value, plus a carry-out.Z = X YCO = X Y

X Y Z CO

0 0 0 0

0 1 1 0

1 0 1 0

1 1 0 1

X

Y

Z

CO

CIS360 45

Basic Components 5– Sequential Circuits: introduce feedback into the circuit.

Outputs are functions of input and current state.

– Multiplexers: combinational circuits that use n bits to select an output from 2n input lines.

4 to 1 M UX

s 0 s 1

f

i0i1i2i3

D

C

Q

CIS360 46

Basic Components 6 Von Neumann

Architecture– Can access either

instructions or data from memory in each cycle.

– One path to memory(von Neumann bottleneck)

– Stored program system. No distinction between programs and data

M ain M em ory S ys tem

O perational Regis ters

P rogram Coun ter

Arithm etic and Logic Unit

Contro l Unit

Input/O u tpu t S ys tem

Addres sP athw ay

D ata andIns truc tionP athw ay

CIS360 47

Basic Components 7Examples of Von Neumann architecture to be

explored in this course: SAM: tiny, good for learning architecture MIPS: text’s example assembly language SPARC: labs

Roughly, the order of presentation in this course is as follows:

A couple of days on the Main Memory System Weeks on the Central Processing Unit (CPU) Finish the course with the I/O System

CIS360 48

Basic Components 8 Memory: Can be viewed as an array of storage

elements. – The index of each element is called the address.– Each element holds the same number of bits. How

many bits per element? 8, 16, 32, 64?

0

1

2

...

n-1

0

1

2

...

n-1

0

1

2

...

n-1

0

1

2

...

n-1

8 bits = 1 byte 16 bits 32 bits 64 bits

CIS360 49

Memory Element & Address Sizes

•If a machine’s memory is 5-bit addressable, then, at each distinct address, 5 bits are stored. The contents at each address are represented by 5 bits.•If 3 bits are used to represent memory addresses, then the memory can have at most 23 = 8 distinct addresses.•Such a memory can store at most 8 5 = 40 bits of data.•If the data bus is 10 bits wide, then up to 10 bits at a time can be transferred between memory and processor; this is a 10-bit word.

AddressContentsDecima

lBinary

0 000 00011

1 001 01111

2 010 01110

3 011 10100

4 100 00101

5 101 01110

6 110 10100

7 111 10011

CIS360 50

Basic Components 9 Let’s look deeper.

– Suppose each memory element is stored in a bank and given a relative address.

– You could have several such banks in your memory.

– The GLOBAL address of each element would be:[relative address] & [bank address].

– To get two elements at a time, start reading from bank 0 (don’t start from bank 1; this would be a “memory address not aligned” error).

000001010011100101

Bank 0

000001010011100101

Bank 0000001010011100101

Bank 1000 0001 0010 0011 0100 0101 0

000 1001 1010 1011 1100 1101 1

Global addresses,not contents.

Think of the contents as beingunderneath the global addresses.

CIS360 51

Basic Components 10– Memory alignment: Assume a byte addressable

machine with 4-byte words. Where are operands of various sizes positioned?

bytes: on a byte boundary (any address) half words: on half word boundary (even addresses) words: on word boundary (addresses divisible by 4) double words: on double word boundary (addresses divisible

by 8)

CIS360 52

Contrast with bit ordering

Basic Components 11 Byte ordering: how numeric data is stored in memory

– Ex.: 24789651110 = 0EC699BF16

– Stored at address 0

0 OE

1 C6

2 99

3 BF

0 BF

1 99

2 C6

3 0E

Big Endian

High order (big end) is at byte 0

7 6 5 4 3 2 1 0

1 0 1 1 1 1 1 1

Little Endian

Low order (little end) is at byte 0

CIS360 53

Basic Components 12 Read/Write operations: must know the address to

read or write. (read = fetch = load, write = store) CPU puts address on address bus

CPU sends read signal– (R/W=1, CS=1)– (Read/don’t Write, Chip Select)

Wait

Memory puts data ondata bus

– reset (CS=0)

D0D1

D(n-1)

A0A1

A(m-1)

CS

R/ W

CIS360 54

Basic Components 13– Types of memory:

ROM: Read Only Memory: non-volatile (doesn’t get erased when powered down; it’s a combinational circuit!)

PROM: Programmable ROM: use a ROM burner to write data to it initially. Can’t be re-written.

EPROM: Erasable PROM. Uses UV light to erase. EEPROM: Electrically Erasable PROM. RAM: Random access memory. Can efficiently read/write any

location (unlike sequential access memory). Used for main memory.

– Many variations (types) of RAM, all volatile• SDRAM, DDR SDRAM• RDRAM• www.tomshardware.com

CIS360 55

Basic Components 14 CPU: executes instructions -- primitive operations

that the computer can perform.– E.g., arithmetic A+B

data movement A := Bcontrol if expr goto labellogical AND, OR,

XOR… Instructions specify both the operation and the operands.

Operands are usually locations in memory where the actual operands may be found (addresses of actual operands).

CIS360 56

Basic Components 15– Instruction set: all instructions for a machine.

Instruction format specifies number and type of operands.

Ex.: Could have an instruction likeADD A, B, RWhere A, B, and R are the addresses of operands in memory. The result is R := A+B.

8

9

1 7

0

4

8

C

A

B

R

M em oryAddr Label

CIS360 57

Basic Components 16– Actually, the “instruction” might be represented in a

source file as:0x41444420412C20422C20520A. … A D D A , B , RAs such, it is an assembly language instruction.

– An assembler might translate it to, say, 0x504C, the machine’s representation of the instruction.As such, it is a machine language instruction.

CIS360 58

A Simple Instruction Set 1 Simple instruction set: the Accumulator machine.

– Simplify instruction set by only allowing one operand. Accumulator implied to be the second operand.

– Accumulator is a special register. Similar to a simple calculator.

ADD addr ACC ACC + M[addr] SUB addr ACC ACC - M[addr] MPY addr ACC ACC * M[addr] DIV addr ACC ACC / M[addr] LOAD addr ACC M[addr] STORE addr M[addr] ACC

CIS360 59

A Simple Instruction Set 2 Ex.: C = AB + CD

LOAD 20 ! 1)Acc<-M[20]MPY 21 ! 2)Acc<-Acc*M[21]STORE 30 ! M[30]<-AccLOAD 22 ! 3)Acc<-M[22]MPY 23 ! 4)Acc<-Acc*M[23]ADD 30 ! 5)Acc<-Acc+M[30]STORE 22 ! M[22]<-Acc

– Machine language: Converting from assembly language to machine language is called assembling.

20

21

22

23

. . .

A

B

C

D

tem p30

Accumulator1)2)3)4)5)

CIS360 60

A Simple Instruction Set 3 Assume 8-bit architecture. Each instruction may be 8 bits. 3

bits hold the op-code and 5 bits hold the operand.

How much memory can we address? How many op-codes can we have? Convert the mnemonic op-codes into binary codes.

7 5 4 0

o p - c o d e o p e r a n d

Operation CodeADD 000SUB 001MPY 010DIV 011LOAD 100STORE 101

CIS360 61

A Simple Instruction Set 4 Hand assemble our program:

LOAD 20 100 10100MPY 21 010 10101STORE 30 101 11110... ...

Instructions are stored in consecutive memory:Addr Memory Mnemonic0 100 10100 LOAD A1 010 10101 MPY B2 101 11110 STORE temp3 100 10110 LOAD C4 010 10111 MPY D5 000 11110 ADD temp6 101 10110 STORE C… …20 4 A21 5 B22 6 C23 7 D… …30 20 temp

CIS360 62

A Simple Instruction Set 5

M A R M D R

I R

D ecode

2 to 1M

UXA C C

2 to 1

MUX P C

I N C

T im ing andC ontr ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

CIS360 63

A Simple Instruction Set 6– Control signals: control functional units to determine

order of operations, access to bus, loading of registers, etc.

Number Operation Number Operation0 ACC bus 8 ALU ACC1 load ACC 9 INCPC2 PC bus 10 ALU operation3 load PC 11 ALU operation4 load IR 12 Addr bus5 load MAR 13 CS6 MDR bus 14 R/W7 load MDR

CIS360 64

A Simple Instruction Set 7P C to busload M A RI N C to P C

load P C

C S , R /W

M D R to busload I R

A ddr to busload M A R

O P =stor e

A C C to busload M D R

C S

C S , R /W

M D R to busload A C C

O P =load

M D R to busA L U to A C C

A L U opload A C C

F etch

E xecute

0

123

State

Y N

45

Y N

78

6

CIS360 65

State 0: Control Signals 2, 5, 9, 3

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C

T im ing andC ont r ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

P C to busload M A RI N C to P C

load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Put the address of the next instruction in the Addr Register and Inc. PC.

CIS360 66

State 1: Control Signals 13, 14

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Fetch the word of memory at Address, and load into Data Register.

CIS360 67


M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Send the word from the Data Register to the Instruction Register.

CIS360 68


M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C

T iming andC ontr ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Put the address from the instruction in the Address Register.

CIS360 69

After State 3, what values are now stored in each register?

PC MAR MDR IR ACC

CIS360 70


M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W


A ddr t o busload M A R

O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Take the value from the ACCumulator and store it in the Data Register.

CIS360 71

State 5: Control Signal 13

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Write the data from the Data Register to the address stored in the MAR.

CIS360 72


M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Load the word at the Address from the Addr Reg into the Data Register.

CIS360 73

After State 6, what values are now stored in each register?

PC MAR MDR IR ACC

CIS360 74


M A R M D R

I R

D ecode

2 to 1M

UXA C C

2 to 1

MUX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Load the word from Data Register into the ACCumulator.

CIS360 75

State 8: Control Signals 6, 8, 10/11, 1

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

Use word from the Data Register for Arith Op and put result in ACC.

CIS360 76

New Instruction•What is necessary to implement a new instruction?

•New states?•New control signals?•New fetch/execute cycle?

•An Example: •SWAP

Exchange value in Accumulator with value at Address

•SWAP addr ! Acc <- #M[addr], M[addr] <- #Acc

CIS360 77

New Instruction What changes to fetch/execute cycle?

– The fetch part of the cycle usually remains the same. – Recall the values stored in registers after each state

E.g., After State 6, what values are in each register?

– PC– MAR– MDR– IR– ACC

Handy to have #M[addr] in MDR

– Start after state 6 then… .


load P C

C S , R /W



O P =stor e


C S

C S , R /W


O P =load


A L U opload A C C

F etch

E xecute

CIS360 78

New State 9: Control Signals 6, 5

M A R M D R

I R

D ecode

2 to 1M

UXA C C

2 to 1

MUX P C

I N C

T iming andC ont r ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

Save the Data value from the MDR in the Address Register.

MDR -> busLoad MAR

CIS360 79

New State 10: Control Signals 0, 7

M A R M D R

I R

D ecode

2 to 1M

UXA C C

2 to 1

MUX P C

I N C

T iming andC ontr ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

Send the ACCumulator value to the Data Register.

ACC -> busload MDR

CIS360 80

New State 11: Control Signals ?, 1

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C


M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

Put the saved value from the MAR into the ACCumulator.

MAR->busload ACC

Note: there is no control signal in the current architecture opposite of5 (Load MAR), so we would have to create a new control signal (MAR to bus) in addition to creating these new states.

CIS360 81

New State 12 (Old 3): Control Signals 12, 5

M A R M D R

I R

D ecode

2 to 1M

UXA C C

2 to 1

MUX P C

I N C

T iming andC ont r ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

Put (reload) the address from the instruction in the Address Register.

Addr -> busload MAR

CIS360 82

New State 13 (Old 5): Control Signals 13

M A R M D R

I R

D ecode

2 to 1M

UXA C C2 t

o 1M

UX P C

I N C

T im ing andC ont r ol

M emor y

B us

A ddr O p

0

1

2

3

4

5 6 7

8

9

A L U

10 11

12

13 14

Write the data from the Data Register to the address stored in the MAR.

CS

CIS360 83

New Instruction Example Summary

Changes to States, added 9 thru 13 Changes to Signals, added 15: MAR -> bus Changes to Fetch/Execute, new register transfer language (RTL)

PC -> bus, load MAR, INC -> PC, Load PCCS, R/wMDR -> bus, load IRAddr -> bus, load MARCS, R/w MDR -> bus, load MARACC -> bus, load MDRMAR -> bus, load ACCAddr -> bus, load MARCS

CIS360 84

Instruction Set Architectures 1 RISC vs. CISC

– Complex Instruction Set Computer (CISC): many, powerful instructions. Grew out of the need for high code density. Instructions have varying lengths, number of operands, formats, and clock cycles in execution.

– Reduced Instruction Set Computer (RISC): fewer, less powerful, optimized instructions. Grew out of opportunity for simpler, faster hardware. Instructions have fixed length, number of operands, formats, and similar number of clock cycles in execution.

CIS360 85

Instruction Set Architectures 2 Motivation: memory is comparatively slow.

– 10x to 20x slower than processor.– Need to minimize number of trips to memory.

Provide faster storage in the processor -- registers. Registers (16, 32, 64 bits wide) are used for intermediate

storage for calculations, or repeated operands. Accumulator machine

– One data register -- ACC.– 2 memory accesses per instruction -- one for the instruction and

one for the operand. Add more registers (R0, R1, R2, …, Rn)

CIS360 86

Instruction Set Architectures 3 How many addresses to specify?

– With binary operations, need to know two source operands, a destination, and the operation.

E.g., op (dest_operand) (src_op1) (src_op2)– Based on number of operands, could have:

3 addr. machine: both sources and dest are named. 2 addr. machine: both sources named, dest is a source. 1 addr. machine: one source named, other source and dest. is

the accumulator. 0 addr. machine: all operands implicit and available on the

stack.

CIS360 87

Instruction Set Architectures 4 1-address architecture: a:=ab+cde

– Memory only Using registers

1½-address architecture: one operand must always be a register. (½ address is register, 1 address is the memory operand: LOAD 100, R1).

– Like an accumulator machine, but with many accumulators.

Code # mem refsLOAD 100 2MPY 104 2STORE 100 2LOAD 108 2MPY 112 2MPY 116 2ADD 100 2STORE 100 2

Code # mem refsLOAD 100 2MPY 104 2STORE R2 1LOAD 108 2MPY 112 2MPY 116 2ADD R2 1STORE 100 2

CIS360 88


– Using memory only:

– Using registers:

– What about instruction size?

Code # mem refsMPY 100, 100, 104 ;a:=abMPY 200, 108, 112 ;t:=cdMPY 200, 116, 200 ;t:=etADD 100, 200, 100 ;a:=t+a

Code # mem refsMPY R2, 100, 104 ;t1:=abMPY R3, 108, 112 ;t2:=cdMPY R3, 116, R3 ;t2:=et2ADD 100, R3, R2 ;a:=t1+t2

Memory100 (a)104 (b)108 (c)112 (d)116 (e)...200 (t)

CIS360 89


– Using memory only:

– Using registers:

– Most CISC arch. this way, making 1 operand implicit

Code # mem refsMPY 100, 104 ;a:=ab 4MOVE 200, 108 ;t:=c 3MPY 200, 112 ;t:=td 4MPY 200, 116 ;t:=te 4ADD 100, 200 ;a:=t+a 4

Memory100 (a)104 (b)108 (c)112 (d)116 (e)...200 (t)

Code # mem refsMPY 100, 104 ; a: =ab 4MOVE R2, 108 ; R2: =c 2MPY R2, 112 ; R2: =R2d 2MPY R2, 116 ; R2: =R2e 2ADD 100, R2 ; a: =t +a 3

CIS360 90


– Stack machine: All operands are implicit. Only push and pop touch memory. All other operands are pulled from the top of stack, and result is pushed on top.E.g., HP calculators.

Code # mem refsPUSH A 2PUSH B 2MPY 1PUSH C 2PUSH D 2PUSH E 2MPY 1MPY 1ADD 1POP A 2

Stack43210

CIS360 91

Instruction Set Architectures 8 Load/Store Architectures -- RISC

– Use of registers is simple and efficient. Therefore, the only instructions that can access memory are load and store. All others reference registers.

Code # mem refsLOAD R2, 100 ;R2a 2LOAD R3, 104 ;R3b 2LOAD R4, 108 ;R4c 2LOAD R5, 112 ;R5d 2LOAD R6, 116 ;R6e 2MPY R2, R2, R3 ;R2ab 1MPY R3, R4, R5 ;R3cd 1MPY R3, R3, R6 ;R3(cd)e 1ADD R2, R2, R3 ;R2ab+(cd)e 1STORE 100, R2 ;aab+(cd)e 2

CIS360 92

Instruction Set Architectures 9 Why load/store architectures?

– Number of instructions (hence, memory references to fetch them) is high, but can work without waiting on memory.

– Claim: overall execution time is lower. Why? Clock cycle time is lower (no micro code interpretation). More room in CPU for registers and memory cache. Easier to overlap instruction execution through pipelining.

– Side effects: Register interlock: delaying execution until memory read completes. Instruction scheduling: rearranging instructions to prevent register

interlock (loads on SPARC) and to avoid wasting the results of pipelined execution (branches on SPARC).

CIS360 93

SPARC Assembly Language 1 SPARC (Scalable Processor ARChitecture)

– Used in Sun workstations, descended from RISC-II developed at UC Berkeley

– General Characteristics: 32-bit word size (integer, address, register size, etc.) Byte-addressable memory RISC load/store architecture, 32-bit instruction, few addressing

modes Many registers (32 general purpose, 32 floating point, various

special purpose registers)– ISEM: Instructional SPARC Emulator - nicer than a real

machine for learning to write assembly language programs.

CIS360 94

SPARC Assembly Language 2 Structure

– Line oriented: 4 types of lines Blank - Ignored Labeled -

– Any line may be labeled. Creates a symbol in listing. Labels must begin with a letter (other than ‘L’), then any alphanumeric characters. Label must end with a colon “:”. Label just assigns a name to an address.

Assembler Directives - E.g., .data .word .text, etc.

Instructions– Comments start after “!” character and

go to the end of the line.

.data

x_m: .word 0x42y_m: .word 0x20z_m: .word 0

.text

start:

set x_m, %r2 ld [%r2], %r2 set y_m, %r3 ld [%r3], %r3

! Load x into reg 2! Load y into reg 3

CIS360 95

SPARC Assembly Language 3 Directives: Instructions to the assembler

– Not executed by the machine .data -- following section contains declarations

– Each declaration reserves and initializes a certain number of bits of storage for each of zero or more operands in the declaration.• .word -- 32 bits• .half -- 16 bits• .byte -- 8 bits

E.g., .dataw: .half 27000x: .byte 8y: .byte ’m’, 0x6e, 0x0, 0, 0z: .word 0x3C5F

.text -- following section contains executable instructions

CIS360 96

SPARC Assembly Language 4 Registers -- 32 bits wide

– 32 general purpose integer registers, known by several names to the assembler

%r0-%r7 also known as %g0-%g7 global registers -- Note, %r0 always contains value 0.

%r8-%r15 also known as %o0-%o7 output registers %r16-%r23 also known as %l0-%l7 local registers %r24-%r31 also known as %i0-%i7 input registers Use the %r0-%r31 names for now. Other names are used in

procedure calls.

– 32 floating point registers %f0-%f31. Each reg. is single precision. Double prec. uses reg. pairs.

CIS360 97

SPARC Assembly Language 5 Assembly language

– 3-address operations - format different from bookop src1, src2, dest !opposite of textE.g., add %r1, %r2, %r3 !%r3 %r1 + %r2

or %r2, 0x0004, %r2 !%r2 %r2 b-w-or 0x0004

– Contrast SPARC with MiPs (used in the book) indirect address notation: @addr vs [addr] operand order, especially the destination register register notation: R2 vs. %r2 branches

CIS360 98

SPARC Assembly Language 6– 2-address operations: load and store

ld [addr], %r2 ! %r2 M[addr]st %r2, [addr] ! M[addr] %r2

Often use set to put an address (a label, a symbolic constant) into a register, followed by ld to load the data itself.set x, %r1 !put addr x into %r1ld [%r1],%r2 !use addr in %r1 to load %r2

– Immediate values: instruction itself contains some data to be used in execution.

CIS360 99

SPARC Assembly Language 7– Immediate values (continued)

E.g., add %rs, siconst13, %rd !%rd%rs+const Constant is coded into instruction itself, therefore available

after fetching the instruction (no extra trip to memory for an operand).

On SPARC, no special notation for differentiating constants from addresses because no ambiguity in a load/store architecture.

Immediate value coded in 13 bit sign-extended value. Range is, then, -212…212-1 or -4096 to 4095.

Immediate values can be specified in decimal, hexadecimal, or octal.E.g., add %r2, 0x1A, %r2 ! %r2 %r2 + 26

CIS360 100

SPARC Assembly Language 8– Synthetic Instructions: assembler translates one

“instruction” into several machine instructions. set : used to load a 32-bit signed integer constant into a

register. Has 2 operands - 32 bit value and register number. How does that fit into a 32 bit instruction?E.g., set iconst32, %rd

set -10, %r3set x, %r4set ’=’, %r8

clr %rd : used to set all bits in a register to 0. How? mov %rs, %rd : copies a register. neg %rs, %rd : copies the negation of a register.

CIS360 101

SPARC Assembly Language 9– Operand sizes

double word = 8 bytes, word = 4 bytes, half word = 2 bytes, byte = 8 bits. Recall memory alignment issues.set x, %r2 !Put addr x in %r2ld [%r2], %r1 !load wordldsb [%r2], %r1 !load byte, sign extendedldub [%r2], %r1 !load byte, extend with 0’s

st %r1, [%r2] !store word, addr is mult of 4stb %r1, [%r2] !store byte, any addresssth %r1, [%r2] !store half word, address is even

– Characters use 8 bits ldub to load a character stb to store a character

CIS360 102

SPARC Assembly Language 10– Traps : provides initial help with I/O, also used in

operating systems programming. ta 0 : terminate program ta 1 : output ASCII character from %r8 ta 2 input ASCII character into %r8 ta 4 : output integer from %r8 in unsigned hexadecimal ta 5 : input integer into %r8, can be decimal, octal, or hex

E.g.,set ’=’, %r8 !put ’=’ in %r8ta 1 !output the ’=’ta 5 !read in value into %r8mov %r8, %r1 !copy %r8 into %r1set 0x0a, %r8 !load a newline into %r8ta 1 !output the newline

CIS360 103

SPARC Assembly Language 11– More assembler directives (.asciz and .ascii):

Each of the following two directives is equivalent:– msg01: .asciz "a phrase"– msg01: .byte 'a', ' ', 'p', 'h', 'r' .byte 'a', 's', 'e', 0

Note that .asciz generates one byte for each character between the quote (") marks in the operand, plus a null byte at the end.

The .ascii directive does not generate that extra byte. Each of the following three directives is equivalent:– digits: .ascii "0123456789"– digits: .byte '0', '1', '2', '3', '4', '5' .byte '6', '7', '8', '9'

– digits: .byte 0x30, 0x31, 0x32, 0x33, 0x34 .byte 0x35, 0x36, 0x37, 0x38, 0x39

CIS360 104

SPARC Assembly Language 12– Quick review of instructions so far:

ld [addr], %rd ! %rd M[addr] st %rd, [addr] ! M[addr] %r2 op %rs1, %rs2, %rd! op is ALU op op %rs, siconst13, %rd ! %rd%rs op const set siconst32, %rd ! %rdconst ta # ! trap signal

– Have actually seen many more variants, e.g., ldub, ldsb, sth, clr, mov, neg, add, sub, smul, sdiv, umul, udiv, etc. Can evaluate just about any simple arithmetic expression.

CIS360 105

Review: Sparc Loads, Stores .datax_m: .word 0xa1b2c3d4

.skip 12 .text set x_m, %r2 ld [%r2], %r3 ldsb [%r2], %r4 ldub [%r2], %r5 st %r3, [%r2+4] sth %r3, [%r2+8] stb %r3, [%r2+12] ta 0

After this runs, what values are in %r2-5, and memory locations starting at byte address x_m?

CIS360 106

Flow of Control 1 In addition to sequential execution, need ability to

repeatedly and conditionally execute program fragments.– High level language has: while, for, do, repeat, case, if-then-else,

etc. – Assembler has if, goto. – Compare: high level vs. pseudo-assembler, implementation of f=n!

f = 1 i = 2loop: if (i > n) goto done f = f * i i = i + 1 goto loopdone: ...

f = 1;i = 2;while (i <= n){ f = f * i; i = i + 1;}

CIS360 107

Flow of Control 2– Branch -- put a new address in the program counter.

Next instruction comes from the new address, effectively, a “goto”.

– Unconditional branch (book) BRANCH addr ! PC addr

(SPARC) ba addr ! PC addr– Conditional branch

(book) BRcc R1, R2, target“if R1 cc R2 then PC target” and cc is comparison operation (e.g., LT is , GE is , etc.)

CIS360 108

Flow of Control 3– Evaluating conditional

branches Evaluate condition If condition is true, then

PC target, else PC PC+1

– Consider changes to the fetch-execute cycle given earlier for accumulator machine. What needs to change?

OP =B R cc

P C to bu s, etc.

OP =B R A N C H

A ddr to bu s, loadP C

C ond=T

Y es

N oY es

N o N o

Y es

F etch

E xecute

CIS360 109

Flow of Control 4 Other conditions (from text, very similar to MIPS)

Can implement high level control structures now. Back to the factorial example using the book’s assembly language:

LOAD R1, #1 ; R1 = f = 1LOAD R2, #2 ; R2 = i = 2LOAD R3, n ; R3 = n

loop: BRGT R2, R3, done ; branch if i > n

MPY R1, R1, R2 ; f = f * iADD R2, R2, #1 ; i = i + 1BRANCH loop ; goto loop

done: STORE f, R1 ; f = n!

BRLT Rn, Rm, targetBRLE Rn, Rm, targetBREQ Rn, Rm, targetBRNE Rn, Rm, targetBRGE Rn, Rm, targetBRGT Rn, Rm, target

; if Rn Rm then PCtarget; if Rn Rm then PCtarget; if Rn Rm then PCtarget; if Rn Rm then PCtarget; if Rn Rm then PCtarget; if Rn Rm then PCtarget

CIS360 110

Flow of Control 5– Condition Codes

Book’s assembly language has 3-address branches. SPARC uses 1-address branches. Must use condition codes.

Non-MIPS machines use condition codes to evaluate branches. Condition Code Register (CCR) holds these bits. SPARC has 4-bit CCR.

N: Negative, Z: Zero, V: Overflow, C: Carry. All are shown in a trace, or in the reg command under ISEM.

Condition codes are not changed by normal ALU instructions. Must use special instructions ending with cc, e.g., addcc.

N Z V C

CIS360 111

Flow of Control 6 .textstart: set 1, %r2 set 0xFFFFFFFE, %r1 ! –2 in 32-bit 2’s compcc_set: subcc %r1, %r2, %r3 ! r3<= -2-1end: ta 0

ISEM> reg ----0--- ----1--- ----2--- ----3--- ----4--- ----5--- ----6--- ----7---G 00000000 fffffffe 00000001 00000000 00000000 00000000 00000000 00000000O 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000L 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000I 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 PC: 08:00000028 nPC: 0000002c PSR: 0000003e N:0 Z:0 V:0 C:0 cc_set : subcc %g1, %g2, %g3 ISEM> trace ----0--- ----1--- ----2--- ----3--- ----4--- ----5--- ----6--- ----7---G 00000000 fffffffe 00000001 fffffffd 00000000 00000000 00000000 00000000O 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000L 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000I 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 PC: 08:0000002c nPC: 00000030 PSR: 00b0003e N:1 Z:0 V:0 C:0

CIS360 112

Flow of Control 7– Setting the condition codes

Regular ALU operations don’t set condition codes. Use addcc, subcc, smulcc, sdivcc, etc., to set condition

codes. E.g., Suppose %r1 contains -4 and %r2 contains 5.

addcc %r1, %r2, %r3subcc %r1, %r2, %r3subcc %r2, %r1, %r3subcc %r1, %r1, %r3

N Z V C

CIS360 113

ALU Hardware 1 How does a computer add?

– Design a circuit that adds three single digit binary numbers. Results in a sum, and a carry out.

Cin X Y Sum Cout0 0 0 0 01 0 0 1 00 0 1 1 01 0 1 0 10 1 0 1 01 1 0 0 10 1 1 0 11 1 1 1 1

1

x y

cout

cin

Sum

FA

x y

cout cin

Sum

CIS360 114

ALU Hardware 2 Now cascade the full adder hardware

How are CCR bits set? (Above is a ripple-carry adder.)– C-bit = Cout – V-bit = Cout Cn-1– Z-bit = (rzn-1 rzn-2 rzn-3 ... rz0)– N-bit = rzn-1

FA 0

register x register y

register z

FAcout FAFA FA

CIS360 115

Flow of Control 8– Branches use logic to evaluate CCR (SPARC)

Operation Assembler Syntax Branch Condition

Branch always ba target 1 (always)

Branch never bn target 0 (never)

Branch not equal bne target Z

Branch equal be target Z

Branch greater bg target (Z (N V))

Branch less or equal ble target (Z (N V))

Branch greater or equal bge target (N V)

Branch less bl target N V

Branch greater, unsigned bgu target (C Z)

Branch less or equal, unsigned bleu target C Z

Branch carry clear bcc target C

Branch carry set bcs target C

Branch positive bpos target N

Branch negative bneg target N

Branch overflow clear bvc target V

Branch overflow set bvs target V

CIS360 116

Flow of Control 9– Setting Condition Codes (continued)

Synthetic instruction cmp %rs1, %rs2– Sets CCR, but doesn't modify any registers.– Implemented as subcc %rs1, %rs2, %g0

Back to the factorial example (SPARC)set 1, %r1 ! %r1 = f = 1set 2, %r2 ! %r2 = i = 2set n, %r3 ! Get loc of nld [%r3], %r3 ! Put n in %r3

loop: cmp %r2, %r3 ! Set CCR (i?n)bg done ! i > n donenop ! Branch delay

umul %r1, %r2, %r1 ! f = f * iadd %r2, 1, %r2 ! i = i + 1

ba loop ! Goto loopnop ! Branch delay

done: set f, %r3 ! Get loc of fst %r1, [%r3] ! f = n!

CIS360 117

Flow of Control 10– Branch delay slots: unique to RISC architecture

Non-technical explanation: processor is running so fast, it can’t make a quick turn.

– Instruction following branch is always executed. Technical explanation: the efficiency advantage of pipelining

is greater if the following instruction, which has almost completed execution, is allowed to complete.

Compilers take advantage of branch delay slots by putting a useful instruction there if possible.

For our purposes, use the nop (no operation) instruction to fill branch delay slots. Beware! Forgetting the nop will be a large source of errors in your programs!

CIS360 118

High Level Control Structures 1 Converting high level control structures

– You get to be the “compiler”. Some compilers convert the source language (C, Pascal,

Modula 2, etc.) into assembly language and then assemble the result to an object file. GNU C, C++ do this to GAS (Gnu Assembler).

– if-then-else, while-do, repeat-until are all possible to create in a structured way in assembly language.

CIS360 119

High Level Control Structures 2 General guidelines

– Break down into independent (or nested) logical units– Convert to if/goto pseudo-code.

– Mechanical, step-by-step, non-creative process

f=1 i=2loop: if (i>n) goto done f = f*i i = i+1 goto loopdone: ...

f = 1;

for (i=2; i<=n; i++)

f = f * i;

CIS360 120

High Level Control Structures 3 if-then-elseif (a<b) c = d + 1;else c = 7;

if/goto if (a >= b) goto elsec = d + 1goto end

else: c = 7 end:

init: set a, %r2 ! get &a into r2 ld [%r2], %r2 ! get a into r2 set b, %r3 ! get &b into r3 ld [%r3], %r3 ! get b into r3if: cmp %r2, %r3 ! a ?? b (want >=) bge else ! a >= b, do then nop set d, %r5 ! get &d into r5 ld [%r5], %r5 ! get d into r5 add %r5, 1, %r4 ! r4 <- d+1 ba end nopelse: set 7, %r4 ! get 7 into r4end: set c, %r5 ! get &c into r5 st %r4, [%r5] ! c <- r4

CIS360 121

High Level Control Structures 4 while loops:

while (a<b) a = a+1;c = d;

if/goto:whle: if (a>=b) goto done

body: a = a+1goto whle

done: c = d

init: set a, %r4 ! get &a into r4 ld [%r4], %r2 ! get a into r2 set b, %r3 ! get &b into r3 ld [%r3], %r3 ! get b into r3whle: cmp %r2, %r3 ! a ?? b (want >=) bge done ! a >= b skip body nopbody: add %r2, 1, %r2 ! r2 = a + 1 st %r2, [%r4] ! a = a + 1 ba whle ! repeat loop body nopdone: set c, %r5 ! get &c into r5 ...

CIS360 122

High Level Control Structures 5 repeat-until loops:repeat …until (a>b)

if/goto:repeat: …

if (a<=b) goto repeat

rpt: ... ... set a, %r2 ; get &a into r2 ld [%r2], %r2 ; get a into r2 set b, %r3 ; get &b into r3 ld [%r3], %r3 ; get b into r3 cmp %r2, %r3 ; a <= b? ble rpt ; do body again

nop

CIS360 123

High Level Control Structures 6 Complex condition

if((a<b)and(b>=c)) …

if((a<b)or(b>=c)) …

These can be combined and used in if/else or while loops.

Primitive Language

if (a>=b) then goto skip if (b<c) then goto skipbody: ... ...skip: ...

Primitive Language

if (a<b) then goto body if (b<c) then goto skipbody: ... ...skip: ...

CIS360 124

Flow of Control 11– Optimizing code: change order of instructions, combine

instructions, take advantage of branch delay slots. Factorial example again. (for i:=n downto 1 do…)

Reduced 7 instructions in loop to just 4. (You gain no advantage if you optimize code in your labs.)

set 1, %r1 ! %r1=f=1set n, %r2 ! Get loc of nld [%r2], %r2 ! Put n in %r2

loop: umul %r1, %r2, %r1 ! f=f*nsubcc %r2, 1, %r2 ! Decrement nbg loop ! Repeatnop ! Branch delay set f, %r3 ! Get loc of fst %r1, [%r3] ! f=n!

CIS360 125

Synthetic Instructions Remember lab0? .datax_m: .word 0x42y_m: .word 0x20z_m: .word 0 .textstart: set x_m, %r2 ld [%r2], %r2 set y_m,%r3

ld [%r3], %r3 and so on…

Suppose you gave this command to ISEM (after loading):ISEM> dump startstart 05 00 00 10 84 10 a0 00 c4 00 80 00 07 00 00 10

Could you find the set instruction?

CIS360 126

Instruction Encodings 1 First, Instruction Encoding is how instructions are

assembled– All instructions must fit into 32 bits.

Register-register: op=10, i=0

Register-immediate: op=10, i=1

Floating point: op=10, i=0

op rd op3 rs1 asii rs2

3130 29 25 24 19 18 14 1312 5 4

op rd op3 rs1 simm13i

opf rs2op rd op3 rs1 i

CIS360 127

Instruction Encodings 2 Call instructions: op=01

Branch instructions: op=00, op2=010

SETHI instructions: op=00, op2=100

Ex.: add %r2, %r3, %r4

in hexadecimal: 88008003

op disp30

3130 29

op rd op2 imm22

10 00100 000000 00010 000000000 00011

3130 29 25 24 19 18 14 1312 5 4

condop i

3130 29 28 25

op2

24 22

disp22

21

a

CIS360 128

Understanding SET SyntheticUsually used to put the value of an address in memory into a register.

For example, set 0x4004, %r3 Can do neither ‘add %r0, 0x4004, %r3’ nor ‘or %r0, 0x4004, %r3’. Why not?

SET is a synthetic instruction which may be implemented in two steps.

bit positions 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

sethi 0x10, %r3 ! Puts 0x10 in the Most Significant 22 bits hex value%r3 0 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0 0 0 1 0 0 1 0 0 1 0 0 1 0 0 0 0x124812480x10 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 x x x x x x x x x x 0x10sethi%r3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0x4000

or %r3, 0x0004, %r3 ! Puts 0x0004 in the least significant bits%r3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0x40000x0004 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0x00000004OR%r3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0x4004

#2

#1

sethi 0x10, %r3 0 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0x 07 00 00 10or %r3, 4, %r3 1 0 0 0 0 1 1 0 0 0 0 1 0 0 0 0 1 1 1 0 0 0 0 0 0 0 0 0 0 1 0 0 0x 86 10 E0 04

Machine language encoding for 'set 0x4004, %r3'

CIS360 129

Decoding an Instruction05 00 00 1016 0000 0101 0000 0000 0000 0000 0001 00002

Instruction Group (bits 30:31) = 00

Destination Register (bits 25:29) = 00010

Op Code (bits 22:24) = 100

Constant (bits 0:21) = 0000000000000000010000

Meaning: sethi 0x10, %r2%r2 <-- 00000000000000000100000000000000 (0x4000)

CIS360 130

More Decoding

Binary Group OP

Rd Rs1 Rs2 SICONST

84 10 A0 00 1000 0100 0001 0000 1010 0000 0000 0000

C4 00 80 00

07 00 00 10

86 10 E0 04

CIS360 131

SET Synthetic Instruction set iconst, rd

sethi %hi(iconst), rdor rd, %lo(iconst), rd

--or--sethi %hi(iconst), rd

--or--or %g0, iconst, rd

CIS360 132

Bitwise Operations 1 Bit Manipulation Instructions

– Bitwise logical operations and %rs1, %rs2, %rd

10010011… (32 bits)01111001…

or %rs1, %rs2, %rd10010011… (32 bits)01111001…

xor %rs1, %rs2, %rd10010011… (32 bits)01111001…

x y xy0 0 00 1 01 0 01 1 1

x y x+y0 0 00 1 11 0 11 1 1

x y xy0 0 00 1 11 0 11 1 0

CIS360 133

Bitwise Operations 2 andn %rs1, %rs2, %rd

10010011… (32 bits)01111001…

orn %rs1, %rs2, %rd10010011… (32 bits)01111001…

not %rs, %rd10010011… (32 bits)

Recall the cc operations, so andcc, orcc, etc. are available. (However, there is no notcc; use xnorcc.)

x y x y0 0 00 1 01 0 11 1 0

x y x y0 0 10 1 01 0 11 1 1

x x0 11 0

CIS360 134

Bitwise Operations 3 For what kinds of things are these bit level operations used?

Recall the synthetic operation clr, and mov.clr %r2 or %r0, %r0, %r2mov %r2 %r3 or %r0, %r2, %r3

Masking operations: Want to select a bit or group of bits from a set of 32. E.g., convert lower (or upper) to upper case:

‘a’ in binary is 01100001‘A’ in binary is 01000001

All we need to do is “turn off” the bit in position 5.0xDF in binary is 11011111

and %r1, 0xDF, %r1 will turn off that bit! What if we subtract 32 from %r1? What about converting upper to lower case?

CIS360 135

Bitwise Operations 4– Bitwise shifting operations

Shift logical left: sll %rs1, %rs2, %rd%rs1: data to be shifted%rs2: shift count%rd: destination register

E.g., set 0xABCD1234, %r2sll %r2, 3, %r3

%r2: 1010 1011 1100 1101 0001 0010 0011 0100%r3: 0101 1110 0110 1000 1001 0001 1010 0000

sll is equivalent to multiplying by a power of 2 (barring overflow). (In the decimal system, what’s a shortcut for multiplying by a power of ten?)

CIS360 136

Bitwise Operations 5 Shift Logical Right: srl %rs1, %rs2, %rd

– Shifts right instead of left, inserting zeros. Arithmetic shifts: propagate the sign bit when shifting right,

e.g., sra. (Left shift doesn't change.)– Equivalent to dividing by a power of 2.

Rotating shifts: Bits that would have gone into the bit bucket are shifted in instead. (E.g., rr, rl)

– Rotate not implemented in SPARC

Rotate R ight Rotate Lef t

CIS360 137

More SPARC Assembly Language Assembler directives

Are not encoded as machine instructions Memory alignment: .align 4

– Used when mixing allocations of bytes, words, halfwords, etc. and need word boundary alignment

Reserve bytes of space: .skip 20– Useful for allocating large amounts of space (e.g., arrays)

Create a symbolic constant: .set mask, 0x0f– Can now use the word “mask” anywhere we could use the

constant 0x0f previously

All this is leading to additional addressing modes, which help us work with pointers, arrays, and records in assembly language.

CIS360 138

Addressing Modes 1 Addressing Modes

– How do we specify operand values? In a register, location is encoded in the instruction. As a constant, immediate value is in the instruction. In memory, operand is somewhere in memory, location may

only be known at runtime.

– Memory operands: Effective address: actual location of operand in memory. This

may be calculated implicitly (e.g., by a displacement in the instruction) or may be calculated by the programmer in code.

CIS360 139

Addressing Modes 2– Summary of addressing modes:

Mode Example Loc. Of Operand Suitable for SPARC? Immediate add %r1, 100, %r1 instruction Constants Yes

Register Direct add %r1, %r2, %r1 %r2 Integers, constants Yes

Memory Direct add %r1, [2000], %r2 mem[2000] Integers, constants No

Memory Indirect add %r1, [[2000]], %r2 mem[mem[2000]] Pointers No

Register Indirect ld [%r1], %r2 mem[%r1] Pointers Yes

Register Indexed st %r1, [%r2+%r3] mem[%r2+%r3] Arrays Yes

Register Displaced

st %r1, [%r2+x] mem[%r2+x] Records Yes

Post Increment ld [%r1]+, %r2 mem[%r1] increment %r1

Arrays, strings, stacks

No

Pre Decrement ld -[%r1], %r2 decrement %r1, mem[%r1]

Arrays, strings, stacks

No

CIS360 140

Addressing Modes 3– Memory Direct addressing

Entire address is in the instruction (not in SPARC).E.g., accumulator machine: each instruction had an opcode and a hard address in memory.

– Can’t be done on SPARC because an address is 32 bits, which is the length of an instruction. No room for opcodes, etc. Can be done in CISC because multi-word instructions are permitted.

– Memory Indirect addressing Pointer to operand is in memory. Instruction specifies location

of pointer. Requires three memory fetches (one each for instruction, pointer, and data). Not in RISC machines because instruction is too slow; such an instruction would cause its own register interlock!

CIS360 141

Addressing Modes 4– Register Indirect addressing

Register has address of operand (a pointer). Instruction specifies register number, effective address is contents of register.Ex.:

.datan_m: .word 5 ; initialize n to 5

.textset n_m, %r1 ; %r1 has n_m,

pointer to nld [%r1], %r3 ; fetch n into

%r3

CIS360 142

Addressing Modes 5 Ex.: sum up array of integers:

.datan_m: .word 5 ! Size of arraya_m: .word 4,2,5,8,3 ! 5 word arraysum_m: .word 0 ! Sum of elementsb_m: .skip 5*4 ! another 5 word array

.textclr %r2 ! r2 will hold sumset n_m, %r3 ! r3 points to nld [%r3], %r3 ! r3 gets array sizeset a_m, %r4 ! r4 points to array a

loop: ld [%r4], %r5 ! Load element of a into r5add %r5, %r2, %r2! sum = sum + elementadd %r4, 4, %r4 ! Incr ptr by word sizesubcc %r3, 1, %r3! Decrement counterbg loop ! Loop until count = 0nop ! Branch delay slotset sum_m, %r1 ! r1 points to sumst %r2, [%r1] ! Store sumta 0 ! done

0 54321

a_ma_m+4a_m+8a_m+12a_m+16

r2 r3 r4 r5Pre-looplooploop+1loop+2loop+3

5 n_ma_ma_m+4a_m+8a_m+12a_m+16sum_m

42583

CIS360 143

Addressing Modes 6 C-style example of pointer data type

char x; // object of type characterchar * ptr; // pointer to character typeptr = &x; // ptr has address of x (points to x)*ptr = ‘a’; // store ‘a’ at address in ptr

Assembly language equivalent.data

x_m: .byte 0 ! reserve character space; x_m = &x; [x_m] = x

.align 4 ! align to word boundaryptr_m: .word 0 ! pointer variable; [ptr_m] = ptr

.textset x_m, %r1 ! get address x_m into %r1set ptr_m, %r2 ! get address ptr_m into %r2st %r1, [%r2] ! make [ptr_m] point to [x_m]set ’a’, %r3 ! put character ‘a’ into r3set ptr_m, %r2 ! get address ptr_m into %r2ld [%r2], %r1 ! get address [ptr_m], i.e. x_m,

into %r1stb %r3, [%r1] ! store ‘a’ at address [ptr_m],

i.e., ptr

x_mptr_m

‘a’

‘a’x_m, i.e., addr of x

x_m:ptr_m:

r1r2r3

CIS360 144

Addressing Modes 7– Register Indexed addressing

Suitable for accessing successive elements of the same type in a data structure.

Ex.: Swap elements A[i] and A[k] in array

Effective address calculations!

.data A: .skip 24*4 ! reserve array[0..23] of int

! assume i is in %r2 and k is in %r3 .text set A, %r4 ! beginning of array ptr. sll %r2, 2, %r2 ! “multiply” i by 4 sll %r3, 2, %r3 ! “multiply” k by 4 ld [%r2+%r4], %r7 ! r7 <- a[i] ld [%r3+%r4], %r8 ! r8 <- a[k] st %r8, [%r2+%r4] ! a[i] <- r8 st %r7, [%r3+%r4] ! a[k] <= r7

AA+4A+8A+12

001 0010 A100 1000

r2 r3 r4 r7 r8

after sll<-

CIS360 145

Addressing Modes 8 Simulating Register Indirect addressing on SPARC

– SPARC doesn't truly have register indirect addressing. We can write st %r2, [%r1] but assembler converts this automatically into st %r2, [%r1+%r0]

Array mapping functions: used by compilers to determine addresses of array elements. Must know upper bound, lower bound, and size of elements of array.

– Total storage = (upper - lower + 1)*element_size– Address offset for element at index k = (k - lower)*element_size– Address (byte) offset for A[3] = (3-0)*4 = 12– This is for 1 dimensional arrays only!

CIS360 146

Addressing Modes 9 1D array mapping functions: Want an array of n elements,

each element is 4 bytes in size, array starts at address arr.– Total storage is 4n bytes– First element is at arr+0– Last element is at arr+4(n-1) – kth (k can range from 0…n-1) element is at arr+4k. Array uses

zero-based indexing.

k=0 k=1 k=2 k=3 k=4 k=5

ar r +0 ar r +4 ar r +8 ar r +12 ar r +16 ar r +20

ar r ay of 6 elements, 4 bytes each

CIS360 147

Addressing Modes 10 2D array mapping functions: must linearize the 2D concept;

e.g., map the 2D structure into 1D memory.

– Convert into 1D array in memory

0,0 0,1 0,2 0,3 0,4

1,0 1,1 1,2 1,3 1,4

2,0 2,1 2,2 2,3 2,4

0 1 2 3 4

0

1

2

3 R ows(0...2)

5 C olumns (0...4)

0,0 0,1 0,2 0,3 0,4 1,0 1,1 2,3 2,4.....

CIS360 148

Addressing Modes 11 2 ways to convert to 1D

– Row major order (Pascal, C, Modula-2) stores first by rows, then by columns. E.g.,

– Column major order (FORTRAN) stores first by columns then by rows. E.g.,

– Row major 2D array mapping function: Given an array starting at address arr that is x rows by y columns, each element is m bytes in size, and indices start at zero, then element (i, j) may be found at location: arr + (y i + j) m

0,0 0,1 0,2 0,3 0,4 1,0 1,1 2,3 2,4.....

0,0 1,0 2,0 0,1 1,1 2,1 0,2 1,4 2,4.....

CIS360 149

Addressing Modes 12 3D array mapping function: natural extension of 2D function.

Store by row, then column, then depth.

– Array starting at arr with x rows, y columns, depth z, m element size. Element (i, j, k) is found at location:arr + (zyi + j) + k)m

0,0,1 0,1,1 0,2,1 0,3,1 0,4,1

1,0,1 1,1,1 1,2,1 1,3,1 1,4,1

2,0,1 2,1,1 2,2,1 2,3,1 2,4,1

0,0,0 0,1,0 0,2,0 0,3,0 0,4,0

0,1,0 1,1,0 1,2,0 1,3,0 1,4,0

2,0,0 2,1,0 2,2,0 2,3,0 2,4,0

3 R ows, 5 C olumns, 2 D epth

1,0,0

+0

+1

+2 +4 +6 +8

+3 +5 +7 +9

+10

+12

+14

+16

+18

CIS360 150

Addressing Modes 13CALCULATE:total storageoffset for A(i,j,k)address for A(i,j,k)

1D 2D 3Delement size (#bytes) 4 2 1# rows (x) 7 3 3# cols (y) 1 5 5# depth (z) 1 1 2starting addr (0) 4 8 12i= 1 1 0j= 0 1 1k= 0 0 1

CIS360 151

Addressing Modes 14! Example that adds 1 to every element of columns 1 and 2, not 0, of a 5 by 3 array

.data

.set rows, 5 ! define symbolic constants

.set cols, 3arr_m: .skip rows * cols * 4 ! allocate space (.skip 60 same)

.text...

set arr_m, %r3 ! get address of arrayclr %r1 ! %r1 is i (row)

loop1: cmp %r1, rows ! done if i >= rowsbge donenopset 1, %r2 ! %r2 is j (col); start at one (skip col zero)

loop2: cmp %r2, cols ! if at last column, done with rowbge inc1nopumul %r1, cols, %r4 ! # elements to skip for current rowadd %r4, %r2, %r4 ! then which column being accessedumul %r4, 4, %r4 ! change from element to byte offsetld [%r3+%r4], %r5 ! get arr[i][j]add %r5, 1, %r5 ! add 1 to the element valuest %r5, [%r3+%r4] ! store it back to arr[i][j]

inc2: add %r2, 1, %r2 ! next columnba loop2 ! continue inner loop over columnsnop

inc1: inc %r1 ! next rowba loop1 ! continue outer loop over rowsnop

done: ...

CIS360 152

Addressing Modes 15– Displacement Addressing

Suitable for accessing the individual fields of record data structures. Each field can be of a different type.

Use .set directive to establish offsets to fields within records. Then use displacement addressing to access those fields.

20 C har acter s

I nteger

I nteger

N ame

A ge

D O B

L ogicalview of a

r ecor d

20 bytes 4 bytes 4 bytes

A ctual layout of r ecor d in memor y

per son+0 per son+20 per son+24

CIS360 153

Addressing Modes 16 Ex.: Add 1 to the age field in a person record

Problem: alignment in memory. May have to waste some space in the person record in order to have the integer fields align on a word boundary.

.data .set name, 0 ! offset to name field .set age, 20 ! offset to age field .set dob, 24 ! offset to date of birthperson: .skip 28 ! size of a person record

.text.... set person, %r1 ! get addr of person record ld [%r1+age], %r2 ! get the age of the person add %r2, 1, %r2 ! increment age by 1 st %r2, [%r1+age] ! store back to record

CIS360 154

Addressing Modes 17– Auto-increment and Auto-decrement addressing

SPARC does not support these modes. They may be simulated using register indirect addressing followed by an add or subtract of the size of the element on that register.

Useful for traversing arrays forward (auto-increment) and backward (auto-decrement). Also useful for stacks and queues of data elements.

CIS360 155

Subroutines 1– Subroutines and subroutine linkage

Subroutines: programming mechanism to facilitate repeated computations and modularization.

– Use of subroutines Basis for structured and disciplined programming Compact code (no need to write monolithic loops) Relatively easy to debug (no cut-and-paste errors) Requires little hardware support, mostly protocols and

conventions to handle parameters.

CIS360 156

Subroutines 2– Terminology

Caller: the code (which could be a subroutine itself) which invokes the subroutine of interest

Callee: the subroutine being invoked by the caller Function: subroutine that returns one or more values back to

the caller and exactly one of these values is distinguished as the return value

Return value: the distinguished value returned by a function

CIS360 157

Subroutines 3– Terminology (continued)

Procedure: a subroutine that may return values to the caller (through the subroutine’s parameter(s)), but none of these values is distinguished as the return value

Return address: address of the subroutine call instruction Parameters: information passed to/from a subroutine (a.k.a.

arguments) Subroutine linkage: a protocol for passing parameters between

the caller and the callee

CIS360 158

Subroutines 4– Subroutine linkage

Calling a subroutine– Assembly language syntax for calling a subroutine

call labelnop

– Must change the program counter (as in a branch instruction) however, we must also keep track of where to resume execution after the subroutine finishes. Call instruction handles this atomically (i.e., without interruption) by:

%r15 #PC(PC #nPC)nPC label

Returning from a subroutine– Assembly language syntax for returning from a subroutine

retlnop

CIS360 159

Subroutines 5 Returning from a subroutine (continued)

– Again, must change the program counter to return to an instruction after the one that called the subroutine. The address of the instruction that called it was saved in %r15, and we must skip over the branch delay slot as well. So, this is accomplished by:nPC %r15+8

Parameter passing: 2 approaches– Register based linkage: pass parameters solely through registers.

Has the advantage of speed, but can only pass a few parameters, and it won’t support nested subroutine calls. Such a subroutine is called a leaf subroutine.

– Stack based linkage: pass parameters through the run-time stack. Not as fast, but can pass more parameters and have nested subroutine calls (including recursion).

CIS360 160

Register-based Linkage 1– Subroutine linkage:

Startup Sequence: load parameters and return address into registers, branch to subroutine.

Prologue: if non-leaf procedure then save return address to memory, save registers used by callee.

Epilogue: place return parameters into registers, restore registers saved in prologue, restore saved return address, return.

Cleanup Sequence: work with returned values

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CIS360 161

Register-based Linkage 2– Example: Print subroutine.

.textmain: set 1, %r1 ! Initialize r1 and r2

set 3, %r2mov %r1, %r8 ! Print %r1call printnopmov %r2, %r8 ! Print %r2call printnopadd %r1, %r2, %r8 ! Do our calculationcall print ! Print the result (expect ‘4’)nopta 0

print: set ‘0’, %r1 ! Ascii value of zeroor %r8, %r1, %r2 ! Treat r8 as parametermov %r2, %r8 ! Move into output registerta 1 ! Output charactermov ‘\n’, %r8ta 1 ! Output end of line (newline)retl ! Returnnop

What’s wrong with the above code?

CIS360 162

Register-based Linkage 3– Which registers can leaf subroutines change?

Convention for optimized leaf procedures:

Any other registers the subroutine touches must be saved to memory somewhere, and restored before returning to the caller.

Problem: how can a subroutine call another subroutine? How can a subroutine call itself?

Register(s) Use Changeable? %r0 Zero No %r1 Temporary Yes %r2-%r7 Caller’s variables No %r8 Return value Yes %r8-%r13 Parameters Yes %r14 Stack pointer No %r15 Return address No %r30 Frame pointer No %r16-%r29, %r31 Caller’s variables No

CIS360 163

Register-based Linkage 4– Example: procedure to print linked list of ints.

. dat a . set dt a, 0 ! off set i n r ecor d t o dat a . set pt r , 4 ! off set i n r ecor d t o next poi nt erhead: . wor d 0 . t extmai n: . . . . ! does al l i ni t and al l ocat i on of l i st set head, %r 8 ! pr epar e par amet er t o t r aver se pr oc l d [ %r 8] , %r 8 ! f ol l ow head poi nt er t o fi r st node cal l t r av ! cal l subr out i ne nop ! br anch del ay . . . .t r av: mov %r 8, %r 1 ! copy poi nt er t o %r 1l oop: cmp %r 1, 0 ! check f or nul l poi nt er be done ! nul l poi nt er means we ar e done nop ! br anch del ay l d [ %r 1+dt a] , %r 8 ! f ol l ow poi nt er and get dat a fi el d t a 4 ! pr i nt dat a fi el d l d [ %r 1+pt r ] , %r 1 ! get poi nt er t o next r ecor d ba l oop nop ! br anch del aydone: r et l

5 7 4 1 nilhead

nop

CIS360 164

Parameter Passing 1– Parameter passing review:

Pass by value copy: parameters to subroutine are copies upon which the subroutine acts.

Pass by result copy: parameters are copies of results produced by the subroutine.

Pass by reference copy: parameters to subroutine are (copies of) addresses of values upon which the subroutine acts. Callee is responsible for saving each result to memory at the location referred to by the appropriate parameter.

Hybrid: some parameters passed by value copy, some by result copy, and/or some by reference copy. Callee is responsible for saving results for reference parameters.

CIS360 165

Parameter Passing 2– Parameter passing notes:

Array or record parameters typically are passed by reference copy (efficiency reasons). Primitive data types may be passed either way.

Conventions among languages allows any language to call functions in any other language:

– Pascal: VAR parameters are passed by reference copy; all others are passed by value copy.

– C: all parameters are passed by value copy. Must explicitly pass a pointer if you want a reference parameter.

– C++: like Pascal, can pass by value or reference copy.– FORTRAN: all things passed by reference copy (even

constants).– ADA: pass by value/result copy.

CIS360 166

Parameter Passing 3 .text ! Example 10.1 of Lab Manual! pr_str – print a null terminated string! Parameters: %r8 – pointer to string (initially)!! Temporaries: %r8 – the character to be printed! %r9 – pointer to string!pr_str: mov %r8, %r9 ! we need %r8 for the “ta 1” belowpr_lp: ldub [%r9], %r8 ! load character cmp %r8, 0 ! check for null be pr_dn nop ta 1 ! print character ba pr_lp inc %r9 ! increment the pointer (in ! branch delay slot)pr_dn: retl nop

CIS360 167

Parameter Passing 4 Summary from text (p. 220)

– Pass by value copy: For small “in” parameters. Subroutines cannot alter the originals whose copies are passed as parameters.

– Pass by value/result copy: For small “in/out” parameters. Caller’s cleanup sequence stores values of any “in/out” parameters.

– Pass by reference copy: for “in/out” parameters of all sizes, and large “in” parameters. “Out” values are provided by changing memory at those addresses. (Note: pass by reference copy is passing an address by value copy.)

CIS360 168

Parameter Passing 5– Write Sparc code for the caller and callee for the

following subroutine using register based parameter passing

! global_function Integer subchr (A, B, C)! Substitutes character C for all B in string [A],! and returns count of changes.! ! // In comments, "[A+index]" is denoted by "ch".! index = 0! count = 0! LOOP: if [A+index]=0 go to END // while (ch != 0) { ! if [A+index]B go to INC // if (ch == B) {! [A+index]=C // ch = C;! count=count+1 // count++; }! INC: index=index+1 // index++;! go to LOOP // }! END:

.data ! data sectionC_m: .byte ’I’ ! parameter CB_m: .byte ’i’ ! parameter BA_m: .asciz "i will tip" ! parameter A .align 4R_m: .word 0 ! for storing result count

Assume

CIS360 169

Stack-based Linkage 1 Stack based linkage

– Advantages Permits subroutines to call others. Allows a larger number of parameters to be passed. Permits records and arrays to be passed by value copy. Saving of registers by callee is “built-in”. A way for callee to reserve memory for other uses is “built-in”, too.

– Disadvantages Slower than register based More complex protocol

– Why a stack? Subroutine calls and returns happen in a last-in first-out order (LIFO).

Also known as a runtime stack, parameter stack, or subroutine stack.

CIS360 170

Stack-based Linkage 2 Items “saved” on the stack

in one activation record– Parameters to the

subroutine– Old values of registers

used in the subroutine– Local memory variables

used in subroutine– Return value and return

address Say A() calls B(), B()

calls C(), and C() calls A()

1st stackfr ame for A

1st stackfr ame for B

1st stackfr ame for C

2nd stackfr ame for A

L ocal var iables

S aved gener al pur poser egister s

R etur n addr esses

R etur n values

P ar ameter s

R unt ime S tack E xpanded V iew

CIS360 171

Stack-based Linkage 3– Stack based linkage parameter passing

convention Startup sequence:

– Push parameters– Push space for return value

Prologue– Push registers that are changed

(including return address)– Allocate space for local variables

Epilogue– Restore general purpose registers– Free local variable space– Use return address to return

Cleanup Sequence– Pop and save returned values– Pop parameters

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CIS360 172

Stack-based Linkage 4– Stack based parameter passing example:

Register %r14 %sp stack pointer– Invariant: Always indicates the top of the stack (it has the

address in memory of the last item on stack, usually a word).– Moved when items are “pushed” onto the stack.– Due to interruptions (system interrupts (I/O) and exceptions),

values stored above %sp (at addresses less than %sp) can change at any time! Hence, any access above %sp is unsafe!

Register %r30 %fp frame pointer– Indicates the previous stack pointer. Activation record is from

(some subroutine-specific number of words before) the %fp to the %sp.

– Invariant: %fp is constant within a subroutine (after prologue).

CIS360 173

Stack-based Linkage 5– Stack based parameter passing example:

Want to implement the following subroutine (also a caller):

! global_function Integer subchr (A, B, C)! Substitutes character C for all B in string A,! and returns count of changes.! ! // In comments, "*(A+index)" is denoted by "ch".! index = 0! count = 0! LOOP: if *(A+index)=0 go to END // while (ch != 0) { ! if *(A+index)B go to INC // if (ch == B) {! *(A+index)=C // ch = C;! count=count+1 // count++; }! INC: index=index+1 // index++;! go to LOOP // }! END:

.data ! data sectionC_m: .byte ’I’ ! parameter CB_m: .byte ’i’ ! parameter BA_m: .asciz "i will tip" ! parameter A .align 4R_m: .word 0 ! for storing result count

CIS360 174

Stack-based Linkage 6 .data ! data sectionC_m: .word ’I’ ! parameter CB_m: .word ’i’ ! parameter BA_m: .asciz "i will tip" ! parameter A .align 4 ! align to word addressstack: .skip 250*4 ! allocate 250 word stackbstak: ! point to bottom of stackR_m: .word 0 ! reserve for count .text! Program’s one-time initializationstart: set bstak, %sp ! set initial stack ptr

mov %sp, %fp ! set initial frame ptr! STARTUP SEQUENCE to call subchr()

sub %sp, 16, %sp ! move stack ptr set A_m, %r1 ! A is passed by reference

st %r1, [%sp+4] ! push address on stack set B_m, %r1 ! B is passed by value ld [%r1], %r1 ! get value of B st %r1, [%sp+8] ! push parameter B on stack set C_m, %r1 ! C is passed by value ld [%r1], %r1 ! get value of C st %r1, [%sp+12] ! push parameter C on stack

! SUBROUTINE CALL call subchr ! make subroutine call nop ! branch delay slot! CLEANUP SEQUENCE ld [%sp], %r1 ! pop return value off stack

add %sp, 16, %sp ! pop stack set R_m, %r2 ! get address of R st %r1, [%r2] ! store R . . . ! the rest of the program

Return value

b

stack:

%sp ->

%fp ->

addr (a)

c

CIS360 175

Stack-based Linkage 7! SUBROUTINE PROLOGUEsubchr: sub %sp, 32, %sp ! open 8 words on stack

st %fp, [%sp+28] ! Save old frame pointer add %sp, 32, %fp ! old sp is new fp st %r15, [%fp-8] ! save return address

st %r8, [%fp-12] ! Save gen. Register … ! Save r9-r13, omitted

! SUBROUTINE BODYld_reg: ld [%fp+4], %r8 ! “pop” (load) addr of A

ld [%fp+8], %r9 ! “pop” (load) value of B ld [%fp+12], %r10 ! “pop” (load) value of C clr %r12 ! count clr %r13 ! index

loop: ldub [%r8+%r13], %r11 ! load a string chr cmp %r11, 0x0 ! is chr=null? be done ! then go to done cmp %r11, %r9 ! is chr<>B? (branch delay) bne inc ! then go to inc nop ! branch delay slot stb %r10, [%r8+%r13] ! change chr to C add %r12, 1, %r12 ! increment count

inc: add %r13, 1, %r13 ! increment index ba loop ! do next chr nop ! branch delay slot

done: st %r12, [%fp+0] ! “push” (store) count on stack

! EPILOGUE … ! Restore r9-r13, omitted ld [%fp-12], %r8 ! Restore r8 ld [%fp-8], %r15 ! get saved return address

ld [%fp-4], %fp ! Get old value of frame ptr add %sp, 32, %sp ! Restore stack pointer retl ! return to caller nop ! branch delay slot

cb

addr (a)

%sp ->

%fp ->

return addrold frame ptrReturn value

...%r9%r8

CIS360 176

Stack-based Linkage 8 General Guidelines

– Keep Startups, Cleanups, Prologues, and Epilogues standard (but not necessarily identical); easy to cut, paste, and modify.

– Caller: leave space for return value on the TOP of the stack.

– Callee: always save and restore locally used registers.

– Pass data structures and arrays by reference, all others by value (efficiency).

CIS360 177

Traps and Exceptions 1 Traps, Exceptions, and Extended Operations

– Other side of low level programming -- the interface between applications and peripherals

– OS provides access and protocols

CIS360 178

Traps and Exceptions 2– BIOS: Basic Input/Output System

Subroutines that control I/O No need for you to write them as application programmer OS interfaces application with BIOS through traps (extended

operations (XOPs))

B I O S

K eyboar d S cr een M ouse D isk

A pplicat ionssoftwar e

CIS360 179

Traps and Exceptions 3– Where are OS traps kept? Two approaches:

Transient monitor: traps kept in a library that is copied into the application at link-time

Resident monitor: always keep OS in main memory; applications share the trap routines.

OS routines monitor devices. Frequently used routines kept resident; others loaded as needed.

O S r tns

A ppl 1

O S r tns

A ppl 2

O S r tns

A ppl 3

O S r tns

A ppl 4

O S r tnsA ppl 1

A ppl 2

A ppl 3

A ppl 4

A ppl 5

A ppl 6

CIS360 180

D ispatcherA ppl icat ion

B I O S 1

B I O S 1

B I O S n

Traps and Exceptions 4– (Assuming a res. monitor) How to find I/O routines?

Store routines in memory, and make a call to a hard address. E.g., call 256

– When new OS is released, need to recompile all application programs to use different addresses.

Use a dispatcher– Dispatcher is a subroutine that takes a parameter (the trap

number). Dispatcher knows where all routines actually are in memory, and makes the branch for you. Dispatcher subroutine must always exist in the same location.

2

CIS360 181

Traps and Exceptions 5 Use vectored linking

– Branch table exists at a well known location. The address of each trap subroutine is stored in the table, indexed by the trap number.

– On RISC, usually about 4 words reserved in the table. If the trap routine is larger than 4 words, can call the actual routine.

A ddr of t r ap 0

A ddr of t r ap 1

A ddr of t r ap 2

A ddr of t r ap n

100

104

108

100+4n

100

116

132

100+16n

CIS360 182

Traps and Exceptions 6– Levels of privilege

Supervisor mode - can access every resource User mode - limited access to resources OS routines operate in supervisor mode, access is determined

by bit in PSW (processor status word). XOP (book’s notation) can always be executed, sets privilege

to supervisor mode (ta) RTX (book’s notation) can only be executed by the OS, and

returns privilege to user mode (rett)

– Exceptions Caused by invalid use of resource. E.g., divide by zero,

invalid address, illegal operation, protection violation, etc.

CIS360 183

Traps and Exceptions 7 Control transferred automatically to exception handler routine.

Similar to trap or XOP transfer. Exceptions vs. XOPs

– XOPs explicit in code, exceptions are implicit– XOPs service request and return to application; exceptions print

message and abort (unless masked).

– Trap example: non-blocking read ta 3 If there is nothing in the keyboard buffer, return with a

message that nothing is there. Otherwise, put the character into register 8.

CIS360 184

Traps and Exceptions 8 Status of the keyboard is kept in a memory location, as is the

(one-character) keyboard buffer. Memory mapped devices.

On SPARC, trap table has 256 entries. 0-127 are reserved for exceptions and external interrupts. 128-255 are used for XOPs. Trap table begins at address 0x0000. Each entry is 4 instructions (16 bytes) long.

! ta 3 returns character if one is there, otherwise! it returns 0x8000000 into %r8 set 0x8000000, %r8 ! set default return val set KbdStatus, %r1 ! KbdStatus is memory loc ld [%r1], %r1 ! read status (1 is ready) andcc %r1, 1, %r1 ! check status be rtn ! can’t read anything set KbdBuff, %r1 ! KbdBuff is memory loc ld [%r1], %r8 ! get characterrtn: rett ! return to caller

CIS360 185

Traps and Exceptions 9 Trap execution: ta 3

– Calculate trap address: 3 * 16 + 0x0800 = 16 * (3 + 0x080)– Save nPC and PSW to memory

• SPARC uses register windows• Assumes local registers are available

– Set privilege level to supervisor mode– Update PC with trap address (and make nPC = PC + 4) (jumps to

trap table)– Trap table has instruction ba ta3_handler– rett

• Restores PC (from saved nPC value) and PSW (resets to user mode)

• Returns to application program

CIS360 186

Programmed I/O 1 Programmed I/O

– Early approach: Isolated I/O Special instructions to do input and output, using two

operands: a register and an I/O address. CPU puts device address on address bus, and issues an I/O

instruction to load from or store to the device.

CIS360 187

Programmed I/O 2

C P U

M em or y

I /O

addr busdata bus

r ead/wr ite

addr busdata bus

r ead/wr ite

Isolated I/O

CIS360 188

Memory Mapped I/O No special I/O instructions. Treat the I/O device like a

memory address. Hardware checks to see if the memory address is in the I/O device range, and makes the adjustment.

Use high addresses (not “real” memory) for I/O memory maps. E.g., 0xFFFF0000 through 0xFFFFFFFF.

CPU

Memory

I/O

addr busdata bus

read/write

memor y

unused

I /O

unused

CIS360 189

Programmed I/O 3– Advantages of each

Memory mapped: reduced instruction set, reduced redundancy in hardware.

Isolated: don’t have to give up memory address space on machines with little memory

CIS360 190

Programmed I/O - UARTs UARTs

– Universal Asynchronous Receiver Transmitter

– Asynchronous = not on the same clock.– Handshake coordinates communication between two

devices.– A kind of programmed I/O.

Keyboard UART

0110 CPU..0

01101010serial

parallel

CIS360 191

UARTs 1 UART registers

– Control: set up at init, speed, parity, etc.

– Status: transmit empty, receive ready, etc.

– Transmit: output data– Receive: input data– All four needed for bi-

directional communications, – Status/control, transmit /

receive often combined. Why?

Control Reg

Status Reg

Transmit Reg

Receive Reg

TransmitLogic

ReceiveLogic

Control bus

Address bus

Data bus

CIS360 192

UARTs 2 Memory mapped UARTs

– Both memory and I/O “listen” to the address bus. The appropriate device will act based on the addresses.

– Keyboards and Printers require three addresses (when addresses are not combined).

– Modems require four.– (why?)

UART 1 data

UART 1 status

UART 1 control

UART 2 xmit

UART 2 recv

UART 2 status

UART 2 control

UART 3 xmit

FFFF 0000

FFFF 0004

FFFF 0008

FFFF 000C

FFFF 0010

FFFF 0014

FFFF 0018

FFFF 001C

CPUMemory UART1 UART2

Control busAddress bus

Data bus

and so on

CIS360 193

Programmed I/O 4 Programmed I/O Characteristics:

– Used to determine if device is ready (can it be read or written).

– Each device has a status register in addition to the data register.

– Like previous trap example, must check status before getting data.

– Involves polling loops.

CIS360 194

Programmed I/O – PollingEx.: ta 2 handler (blocking keyboard input)

Can’t afford to wait like this. Computer is millions of times faster than a typist. Also, multi-tasking operating systems can’t wait.

Special purpose computers can wait. E.g., microwave oven controllers.

Must have a better way! Interrupts are the answer!

ta_2_handler: set KbdBuff, %r1 ! get addr of kbd buffer set KbdStatus, %r9 ! get addr of kbd statuswait: ld [%r9], %r10 ! get status andcc %r10, 1, %r10 ! check if ready be wait ! loop until ready nop ! branch delay ld [%r1], %r8 ! get data rett ! return from trap

Are you ready?...Are you ready

now?...How about NOW?...

Nope ..Not yet..Hang on..

CIS360 195

Interrupts and DMA transfers 1 Programmed (polled) I/O used busy waiting.

– Advantages: simpler hardware– Disadvantages: wastes time

Interrupts (IRQs on PCs)– I/O device “requests” service from CPU.– CPU can execute program code until interrupted.

Solves busy waiting problems.– Interrupt handlers are run (like traps) whenever an

interrupt occurs. Current application program is suspended.

CIS360 196

Interrupts and DMA transfers 2 Servicing an interrupt

– I/O controller generates interrupt, sets request line “high”.

– CPU detects interrupt at beginning of fetch/execute cycle (for interrupts “between” instructions).

– CPU saves state of running program, invokes intrpt. handler.

– Handler services request; sets the request line “low”.

– Control is returned to the application program.

Application Program::*Interrupt Detected*::

InterruptHandlerService Request::ClearInterrupt

CIS360 197

Interrupts and DMA transfers 3 Changes to fetch/execute cycle Problems

– Requires additional hardware in Timing & Control.

– Queuing of interrupts– Interrupting an interrupt handler

(solution: priorities and maskable interrupts)

– Interrupts that must be serviced within an instruction

– How to find address of interrupt handler

Interrupt Pending?

Save PCSave PSW

PSW=new PSWPC=handler_addr

PC -> busload MARINC to PCload PC

Y N

CIS360 198

Interrupts and DMA transfers 4 Example: interrupt driven string output

– Want to print a string without busy waiting.– Want to return to the application as fast as

possibleI’m

ready!

CIS360 199

Trap handler implementation Install trap handler into trap table

– Buffer is like circular queue– only outputs, at most, one character

disp_buf: .skip 256 ! buffers string to printdisp_frnt: .byte 0 ! offset to front of queuedisp_bck: .byte 0 ! offset to back of queue

ta_6_handler:! Copy str from mem[%r8] to mem[disp_buf+disp_bck]! Disp_back = (disp_back+len(str)) mod 256! If display is ready! If first char is not null, then output it! Disp_frnt = (disp_frnt+1) mod 256rett ! Return from trap

Disp_buf:

disp_frnt

disp_bck

newestbyte

Undisplayedbyte

Oldestbyte

CIS360 200

Interrupt handler implementation

This too outputs only one character at most, but when display becomes ready again, it generates another interrupt which invokes this routine!

display_IRQ_handler:! Save any registers used! If disp_frnt != disp_bck (queue is not empty)! Get char at mem[disp_frnt]! If char is not null, then output it! Disp_frnt = (disp_frnt+1) mod 256! Restore registers and set the request line “low”

rett ! Return from trap

Uses the UART for transmission.

I’m ready!

CPUMemory

CIS360 201

Interrupts and DMA transfers 5 Problems with interrupt driven I/O

CPU is involved with each interrupt Each interrupt corresponds to transfer of a single byte Lots of overhead for large amounts of data (blocks of 512 bytes)

Memory CPU Device Controller

Execute 10s or 100sof instructions per byte

Transfer oneword of data

InterruptTransfer one byte of data

CIS360 202

Interrupts and DMA transfers 6 DMA (Direct Memory Access)

Want I/O without CPU intervention Want larger than one byte data transfers Solution: add a new device that can talk to both I/O devices

and memory without the CPU; a “specialized” CPU strictly for data transfers.

Memory

CPU

Device Controller

DMA Controller

CIS360 203

Interrupts and DMA transfers 7 Steps to a DMA transfer

– CPU specifies a memory address, the operation (read/write), byte count, and disk block location to the DMA controller (or specify other I/O device).

– DMA controller initiates the I/O, and transfers the data to/from memory directly

– DMA controller interrupts the CPU when the entire block transfer is completed.

Problem– Conflicts accessing memory. Can either arbitrate

access or get a more expensive dual ported memory system.

CIS3601 CIS 360: Introduction to Computer Systems Course Notes Mary Beth Zak Lohse Rick.

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Transcript of CIS3601 CIS 360: Introduction to Computer Systems Course Notes Mary Beth Zak Lohse Rick.