Introduction to SPIM Simulator

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SPIM Simulator

SPIM is a software simulator that runs programs written for MIPS R2000/R3000 processors

SPIM’s name is just MIPS spelled backwards SPIM can read and immediately execute MIPS

assembly language files or MIPS executable files

SPIM contains a debugger and provides a few operating system-like services

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MIPS Processors

MIPS is a load-store architecture, which means that only load and store instructions access memory

Computation instructions operate only on values in registers

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MIPS Registers

Name Number Usage$zero 0 constant 0$at 1 reserved for assembler$v0~$v1 2~3 return value of a function$a0~$a3 4~7 arguments$t0~$t7 8~15 temporary (not preserved across call)$s0~$s7 16~23 saved temporary (preserved across call)$t8~$t9 24~25 temporary (not preserved across call)$k0~$k1 26~27 reserved for OS kernel$gp 28 pointer to global area$sp 29 stack pointer$fp 30 frame pointer$ra 31 return address

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Addressing Modes

Format Address computationregister contents of registerimm immediateimm(register) contents of (immediate + contents of register)label address of label

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Load, Store and Move Instructions

li rd, imm rd immla rd, label rd labellw rd, imm(rs) rd imm(rs)sw rd, imm(rs) imm(rs) rdmove rd, rs rd rs

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Arithmetic and Logical Instructions

add rd, rs, rt rd rs + rtsub rd, rs, rt rd rs – rtmul rd, rs, rt rd rs * rtdiv rd, rs, rt rd rs / rtrem rd, rs, rt rd rs % rtneg rd, rs rd - rsand rd, rs, rt rd rs & rtor rd, rs, rt rd rs | rtnot rd, rs rd ! rs

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Branch and Jump Instructions

beq rs, rt, label branch to label if rs == rtbne rs, rt, label branch to label if rs != rtbgt rs, rt, label branch to label if rs > rtbge rs, rt, label branch to label if rs >= rtblt rs, rt, label branch to label if rs < rtble rs, rt, label branch to label if rs <= rtb label branch to label jal label jump to label,

save the next address in $rajr rs jump to the address in rs

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Assembler Syntax

Comments in assembler files begin with a sharp sign (#) and continue to the end of the line

Identifiers are a sequence of alphanumeric characters, underbars (_), and dots (.) that do not begin with a number

Opcodes are reserved words that cannot be used as identifiers

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Assembler Syntax

Labels are declared by putting them at the beginning of a line followed by a colon

Strings are enclosed in doublequotes (“). Special characters in strings: newline \n, tab \t, quote \“

Numbers are base 10 by default. If they are preceded by 0x, they are interpreted as hexadecimal

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Memory Layout

7ffffff

10000000

400000Reserved

Text segment

Data segment

Stack segment

Static data

Dynamic data

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Assembler Directives

.globl sym Declare that label sym is global

.text <addr> Subsequent items are put in the user text segment. These items may only be instructions. If the optional addr is present, theitems are stored starting at address addr

.data <addr> Subsequent items are stored in the data segment. If the optional addr is present, theitems are stored starting at address addr

.word w1, …, wn Store the n 32-bit values in successive memory words

.asciiz str Store the string str in memory and null-terminate it

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Procedure Call Convention

…Argument 6Argument 5

$a3$a2$a1$a0$ra$fp

Saved registersLocal variables

$fp

$sp

Higher address

Lower address

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Frame Size

The minimum frame size is 24 bytes. It can hold $a0~$a3, $ra, and $fp

Frame size must be double word aligned

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Caller

Pass arguments. The first four arguments are passed in $a0~$a3. The remaining arguments are passed in frame

Save caller-saved registers $t0~$t9 Execute a jal instruction, which jumps to the cal

lee’s first instruction and saves the return address in $ra

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Entry of Callee

Allocate the frame by subtracting the frame size from the stack pointer

Save callee-saved registers. $s0~$s7, $fp, and $ra

Establish the frame pointer by adding the frame size minus four to $sp and storing the sum in $fp

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Exit of Callee

If the callee is a function that returns a value, place the returned value in $v0

Restore all callee-saved registers that were saved upon procedure entry

Pop the stack frame by adding the frame size to $sp

Return by jumping to the address in $ra

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An Example

int main(){ int m; m = fact(10);}

int fact(int n){ int m; if (n <= 1) return 1; else { m = fact(n – 1); return (n * m); }}

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An Example

$a3$a2$a1$a0$ra$fpm

$fp

$sp48

12

-4-8

-12-16

-20-24-28

16202428

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An Example

.text

.globl mainmain:

li $t0, 32sub $sp, $sp, $t0sw $ra, 12($sp)sw $fp, 8($sp)li $t0, 28add $fp, $sp, $t0li $a0, 10jal fact

lw $ra, 12($sp) lw $fp, 8($sp)li $t0, 32add $sp, $sp, $t0jr $ra

.textfact: li $t0, 32

sub $sp, $sp, $t0sw $ra, 12($sp)sw $fp, 8($sp)li $t0, 28add $fp, $sp, $t0

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An Example

sw $a0, -12($fp)lw $t0, -12($fp)bgt $t0, $zero, L1li $v0, 1b L2

L1: lw $t0, -12($fp)li $t1, 1sub $t0, $t0, $t1move $a0, $t0jal factsw $v0, -24($fp)

lw $t0, -12($fp)lw $t1, -24($fp)mul $t0, $t0, $t1move $v0, $t0

L2: lw $ra, 12($sp) lw $fp, 8($sp)li $t0, 32add $sp, $sp, $t0jr $ra

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System Calls

SPIM provides a small set of operating system-like services through the system call (syscall) instruction

To request a service, a program loads the system call code into register $v0 and arguments into registers $a0~$a3

System calls that return values put their results in register $v0

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System Call Code

Service System call code Arguments Resultprint_int $v0=1 $a0=intergerprint_string $v0=4 $a0=stringread_int $v0=5 integer in $v0exit $v0=10

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An Example

.datastr:

.asciiz “the answer = ”

.textla $a0, strli $v0, 4syscallli $a0, 5li $v0, 1syscall

“the answer = 5”

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An Example

int main(){ int m;

print_string(“The factorial of 10 is ”); m = fact(10); print_int(m);}

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An Example

.text

.globl mainmain: li $t0, 32

sub $sp, $sp, $t0sw $ra, 12($sp)sw $fp, 8($sp)li $t0, 28add $fp, $sp, $t0la $a0, LCli $v0, 4syscallli $a0, 10jal fact

sw $v0, -24($fp)lw $a0, -24($fp)li $v0, 1syscalllw $ra, 12($sp) lw $fp, 8($sp)li $t0, 32add $sp, $sp, $t0jr $ra

.data LC: .asciiz “The factorial of

10 is ”