Arm to mips

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Introduction

In this page there two code example for ARM and MIPS architecture.Main idea is to see the difference between ARM and MIPS. Here only the MIPs assembly is commented. A good read about MIPS Architecture and Assembly Language

MIPS

The MIPS instruction set acknowledges 32 general-purpose registers in the register file. For most processors implementing the MIPS instruction set architecture, each register is 32 bits in size. Registers are designated using the “$” symbol. For all practical purposes, three of these registers have special functionality ($0,29,$31). Also, it should be noted that these registers can be accessed via standardized naming conventions in software. The C/C++ library file “regdef.h” is an implementation of one such naming convention.


Example1 code

int binary(int a, int b)
{
  return a + b;
}

void stk(void)
{
  binary(binary(binary(1, 2), binary(3, 4)), binary(binary(5, 6), binary(7, 8)));
}

Example1 Mips

binary:
        j       $31                     # return
        addu    $2,$4,$5                # r2 = a + b

stk:
        subu    $sp,$sp,32              # allocate space for local vars & 4 slots
        li      $4,0x00000001           # 1
        li      $5,0x00000002           # 2
        sw      $31,24($sp)             # store return address on stack
        sw      $17,20($sp)             # preserve r17 on stack
        jal     binary                  # call binary(1,2)
        sw      $16,16($sp)             # preserve r16 on stack

        li      $4,0x00000003           # 3
        li      $5,0x00000004           # 4
        jal     binary                  # call binary(3,4)
        move    $16,$2                  # r16 = binary(1,2)

        move    $4,$16                  # r4 = binary(1,2)
        jal     binary                  # call binary(binary(1,2), binary(3,4))
        move    $5,$2                   # r5 = binary(3,4)

        li      $4,0x00000005           # 5
        li      $5,0x00000006           # 6
        jal     binary                  # call binary(5,6)
        move    $17,$2                  # r17 = binary(binary(1,2), binary(3,4))

        li      $4,0x00000007           # 7
        li      $5,0x00000008           # 8
        jal     binary                  # call binary(7,8)
        move    $16,$2                  # r16 = binary(5,6)

        move    $4,$16                  # r4 = binary(5,6)
        jal     binary                  # call binary(binary(5,6), binary(7,8))
        move    $5,$2                   # r5 = binary(7,8)

        move    $4,$17                  # r4 = binary(binary(1,2), binary(3,4))
        jal     binary                  # call binary(binary(binary(1,2), binary(3,4)), binary(binary(5,6), binary(7,8)))
        move    $5,$2                   # r5 = binary(binary(5,6), binary(7,8))

        lw      $31,24($sp)             # restore return address from stack
        lw      $17,20($sp)             # restore r17 from stack
        lw      $16,16($sp)             # restore r16 from stack
        addu    $sp,$sp,32              # remove local vars and 4 slots
        j       $31                     # return
        nop


Example1 ARM

binary(int, int):
        push    {r7}
        sub     sp, sp, #12
        add     r7, sp, #0
        str     r0, [r7, #4]
        str     r1, [r7, #0]
        ldr     r2, [r7, #4]
        ldr     r3, [r7, #0]
        adds    r3, r2, r3
        mov     r0, r3
        add     r7, r7, #12
        mov     sp, r7
        pop     {r7}
        bx      lr
stk():
        push    {r4, r5, r7, lr}
        add     r7, sp, #0
        mov     r0, #1
        mov     r1, #2
        bl      binary(int, int)
        mov     r4, r0
        mov     r0, #3
        mov     r1, #4
        bl      binary(int, int)
        mov     r3, r0
        mov     r0, r4
        mov     r1, r3
        bl      binary(int, int)
        mov     r4, r0
        mov     r0, #5
        mov     r1, #6
        bl      binary(int, int)
        mov     r5, r0
        mov     r0, #7
        mov     r1, #8
        bl      binary(int, int)
        mov     r3, r0
        mov     r0, r5
        mov     r1, r3
        bl      binary(int, int)
        mov     r3, r0
        mov     r0, r4
        mov     r1, r3
        bl      binary(int, int)
        pop     {r4, r5, r7, pc}

Example2 code

 int fib(int n) {
 if (n == 0)
   return 0;
 else if (n == 1)
   return 1;
 return fib(n - 1) + fib(n - 2);
  }


Example2 ARM gcc 4.5.3

fib(int):
        push    {r4, r7, lr}
        sub     sp, sp, #12
        add     r7, sp, #0
        str     r0, [r7, #4]
        ldr     r3, [r7, #4]
        cmp     r3, #0
        bne     .L2
        mov     r3, #0
        b       .L3
.L2:
        ldr     r3, [r7, #4]
        cmp     r3, #1
        bne     .L4
        mov     r3, #1
        b       .L3
.L4:
        ldr     r3, [r7, #4]
        add     r3, r3, #-1
        mov     r0, r3
        bl      fib(int)
        mov     r3, r0
        mov     r4, r3
        ldr     r3, [r7, #4]
        sub     r3, r3, #2
        mov     r0, r3
        bl      fib(int)
        mov     r3, r0
        adds    r3, r4, r3
.L3:
        mov     r0, r3
        add     r7, r7, #12
        mov     sp, r7
        pop     {r4, r7, pc}


Example2 MIPs gcc 5.4

$LFB0 = .
fib(int):
        addiu   $sp,$sp,-40
        sw      $31,36($sp)
        sw      $fp,32($sp)
        sw      $16,28($sp)
        move    $fp,$sp
        sw      $4,40($fp)
        lw      $2,40($fp)
        bne     $2,$0,$L2
        nop

        move    $2,$0
        b       $L3
        nop

        lw      $3,40($fp)
        li      $2,1                        # 0x1
        bne     $3,$2,$L4
        nop

        li      $2,1                        # 0x1
        b       $L3
        nop

        lw      $2,40($fp)
        addiu   $2,$2,-1
        move    $4,$2
        jal     fib(int)
        nop

        move    $16,$2
        lw      $2,40($fp)
        addiu   $2,$2,-2
        move    $4,$2
        jal     fib(int)
        nop

        addu    $2,$16,$2
        move    $sp,$fp
        lw      $31,36($sp)
        lw      $fp,32($sp)
        lw      $16,28($sp)
        addiu   $sp,$sp,40
        j       $31
        nop


Example2 MIPs Stack

   
   .text
main:
# Prompt user to input non-negative number
la $a0,prompt   
li $v0,4
syscall

li $v0,5    #Read the number(n)
syscall

move $t2,$v0    # n to $t2

# Call function to get fibonnacci #n
move $a0,$t2
move $v0,$t2
jal fib     #call fib (n)
move $t3,$v0    #result is in $t3

# Output message and n
la $a0,result   #Print F_
li $v0,4
syscall

move $a0,$t2    #Print n
li $v0,1
syscall

la $a0,result2  #Print =
li $v0,4
syscall

move $a0,$t3    #Print the answer
li $v0,1
syscall

la $a0,endl #Print '\n'
li $v0,4
syscall

# End program
li $v0,10
syscall

fib:
# Compute and return fibonacci number
beqz $a0,zero   #if n=0 return 0
beq $a0,1,one   #if n=1 return 1

#Calling fib(n-1)
sub $sp,$sp,4   #storing return address on stack
sw $ra,0($sp)

sub $a0,$a0,1   #n-1
jal fib     #fib(n-1)
add $a0,$a0,1

lw $ra,0($sp)   #restoring return address from stack
add $sp,$sp,4


sub $sp,$sp,4   #Push return value to stack
sw $v0,0($sp)
#Calling fib(n-2)
sub $sp,$sp,4   #storing return address on stack
sw $ra,0($sp)

sub $a0,$a0,2   #n-2
jal fib     #fib(n-2)
add $a0,$a0,2

lw $ra,0($sp)   #restoring return address from stack
add $sp,$sp,4
#---------------
lw $s7,0($sp)   #Pop return value from stack
add $sp,$sp,4

add $v0,$v0,$s7 # f(n - 2)+fib(n-1)
jr $ra # decrement/next in stack

zero:
li $v0,0
jr $ra
one:
li $v0,1
jr $ra

.data
prompt: .asciiz "This program calculates Fibonacci sequence with recursive functions.\nEnter a non-negative number: "
result: .asciiz "F_"
result2: .asciiz " = "
endl: .asciiz "\n"