INSTRUCTIONS

Instruction Set Architecture (ISA) - an abstract interface between the hardware and the

lowest level software of a machine that encompasses all the information necessary to write a machine language program that will run correctly, including instructions, registers, memory access, and

I/O.

(source: Computer Organization & Design, Patterson & Hennessy)

in short: it defines the set of resources (e.g., registers) and

operations (e.g., instructions) that must be directly supported in a microprocessor

Microprocessors are implementations of ISA

they are semiconductor integrated circuits that are implemented in conformity to the

specification of a given ISA

there is no single way of doing this, many distinct implementations may be provided for a given ISA

e.g., the MIPS R2000 microprocessor is an implementation of the MIPS ISA

e.g., the Intel and AMD microprocessors in your PC all implement the Intel x86 ISA

MIPS R2000 encodes the instructions as 32 bit sequences

a textual representation of the binary instructions, called MIPS R2000 assembly, is provided to aid the development, debugging and optimisation of code

the MIPS R2000 ISA defines how these 32 bits sequencesare broken into fields, the information that lies within

and how it should be interpreted

there are only 3 distinct ways to break the instructions into fields of different length

(corresponding to 3 instruction-types)

there are only 5 distinct ways to interpret the information within these fields

(corresponding to 5 addressing modes)

original (source) code in C

MIPS R2000 assembly code

the Intel x86 assembly code for the

same program is rather different

MIPS R2000 provides instructions for :

• arithmetic and logic operations • comparison of values• memory access• program control flow

instructions are executed sequentially by default

control flow instructions change the sequential flow

an internal, non programable, register ($pc, the program counter) keeps the memory address of

the next instruction to be executed

some information is stored inside the microprocessor in small memory cells called registers

the MIPS R2000 features 32 registers, each with a capacity of 32 bits; registers are known internally as $0, $1, $2 …, $31 (but they have names too)

registers $ra (return address), $sp (stack pointer), $gp (global pointer) and $fp (frame pointer) hold

important memory addresses for the program

registers $t0 to $t9 and $s0 to $s7 are used to keep the values of program variables and temporary

values for expressions

registers $a0-$a3 are used to pass arguments to functions

MIPS R2000 REGISTERS32 registers with distinct designations and semanticsnote: 1 = $at (assembler), 26-27 = $k0, $k1 (OS)

SPECIAL REGISTERS

MIPS R2000 ASSEMBLY

The MIPS R2000 ISA defines 3 instruction types, i.e., 3 different ways of dividing the 32 bits of an instruction

MIPS R2000 BINARY

it is easy to translate binary sequences into instructions and vice-versa

for example, in the MIPS R2000 instruction set, the 32-bit sequence

000000 10001 10010 01000 00000 100000

represents the type-R instruction

add $t0, $s1, $s2

how to interpret the data in the fields?

the MIPS ISA defines 5 addressing modes:

immediate addressingregister addressing base addressing PC relative addressingPC absolute addressing

the simplicity of the architecture is typical of a Reduced (complexity!) Instruction Set Computer (RISC)

design, namely:

• many registers• same-sized instructions• few instruction types• few addressing modes• load/store architecture• pipelining• branch prediction

indeed, MIPS R2000 was the first commercial RISC processor and hit the market in 1986