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Assembly Code Example

Selection Sort. Assembly Code Example. Swap Function in C. Swap(int & num1, int & num2) // post: values of num1 and num2 have // been swapped { int temp; temp = num1; num1 = num2; num2 = temp; }. Swap Function in Assembly Code. Swap: lw $t0, 0($a0) # t0 gets num1

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Assembly Code Example

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  1. Selection Sort Assembly Code Example

  2. Swap Function in C • Swap(int & num1, int & num2) • // post: values of num1 and num2 have • // been swapped • { • int temp; • temp = num1; • num1 = num2; • num2 = temp; • }

  3. Swap Function in Assembly Code • Swap: lw $t0, 0($a0) # t0 gets num1 • lw $t1, 0($a1) # t1 gets num2 • sw $t0, 0($a1) # num2 gets t0 • sw $t1, 0($a0) # num1 gets t1 • jr $ra

  4. Selection Sort function in C • void SelSort(int v[], int length) • { • int indexMin; • for(int s = 0; s < length-1; s++){ • indexMin = s; • for(int k = s+1; k < length; k++){ • if(v[k] < v[indexMin]) • indexMin = k; • } • swap(v[s], v[indexMin]); • } • }

  5. Selection Sort in Assembly Code • Do we make a subsidiary function call? • Yes, must save state, use s registers • What must go into s registers? • Variables whose values must be retained across function calls • v base, length, length-1, s • Other variables can use t registers • k, minIndex, temps for address/branch computations • Construct code by units (if, for, for)

  6. Selection Sort in Assembly Code • # register assignments • # v base in $s0 (move from $a0) • # length in $s1 (move from $a1) • # length-1 in $s2 (compute from n) • # s in $s3 (initialize to 0) • # minIndex in $t0 • # k in $t1

  7. Selection Sort in Assembly Code • # if(v[k] < v[minIndex]) { minIndex = k } • # • sll $t3, $t1, 2 # t3 = 4 * k • add $t3, $s0, $t3 # t3 = address of v[k] • sll $t4, $t0, 2 # t4 = 4 * minIndex • add $t4, $s0, $t4 # t4 = addr of v[minIndex] • lw $t5, 0($t3) # t5 = v[k] • lw $t6, 0($t4) # t6 = v[minIndex] • slt $t2, $t5, $t6 # v[k] < v[minIndex]? • beq $t2, $zero, endif # if not skip if part • add $t0, $t1, $zero # minIndex = k • endif:

  8. Selection Sort in Assembly Code • # for(k = s+1; k < length; k++) • # { if …} • # • addi $t1, $s3, 1 # k = s + 1 • fork: slt $t2, $t1, $s1 # k < length ? • beq $t2, $zero, endfork # if not, exit • # { if…} • addi $t1, $t1, 1 # k++ • j fork • endfork:

  9. Selection Sort in Assembly Code • # for(s = 0; s < length-1; s++) • # minIndex = s; for k …; swap(v[s], v[minIndex]); • # • add $s3, $zero, $zero # s = 0 • fors: slt $t2, $s3, $s2 # s < length-1 ? • beq $t2, $zero, endfors # if not, exit loop • add $t0, $s3, $zero # minIndex = s • # for k ... • sll $t3, $s3, 2 # compute array addresses • add $a0, $s0, $t3 # and store as parameters • sll $t3, $t0, 2 • add $a1, $s0, $t3 • jal Swap # call swap function • addi $s3, $s3, 1 # s++ • j fors • endfors:

  10. Selection Sort in Assembly Code • # save state # restore state • addi $sp, $sp, -20 • sw $ra, 16($sp) lw $ra, 16($sp) • sw $s3, 12($sp) lw $s3, 12($sp) • sw $s2, 8($sp) lw $s2, 8($sp) • sw $s1, 4($sp) lw $s1, 4($sp) • sw $s0, 0($sp) lw $s0, 0($sp) • addi $sp, $sp, 20 • # initialize • add $s0, $a0, $zero # return • add $s1, $a1, $zero jr $ra • addi $s2, $s1, -1

  11. Assemble / Link / Load / Run • Assemble • translates assembly code to object code (machine lang) • pseudo instructions are replaced by actual machine instructions • e.g. move $t0, $s1 becomes add $t0, $s1, $zero • addresses are set relative to start of code segment • relocation done by linker • internal addresses which need to be resolved are kept in a symbol table • external addresses which are needed are listed • e.g. function calls not part of this segment

  12. Assemble / Link / Load / Run • Link puts together different object code segments • resolves addresses with function calls across segments • resolves final addresses of labels • places data in data segment and resolves addresses • Load place code into main memory • Run start at label main

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