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Lecture 10: Assembly Language

Lecture 10: Assembly Language. Computer Engineering 211 Spring 2002. Stack pointer vs Frame Pointer. Same variable referred with different offset with $sp inside the same procedure. SUM: sw $ra, -4($sp) sw $s0, -8($sp) sw $s1, -12($sp) addi $sp, $sp, -12

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Lecture 10: Assembly Language

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  1. Lecture 10: Assembly Language Computer Engineering 211 Spring 2002

  2. Stack pointer vs Frame Pointer Same variable referred with different offset with $sp inside the same procedure. SUM: sw $ra, -4($sp) sw $s0, -8($sp) sw $s1, -12($sp) addi $sp, $sp, -12 lw $s0, 12($sp) lw $s1, 16($sp) # load F add $v0, $a0, $a1 add $t0, $a2, $a3 add $t1, $s0, $s1 add $t0, $t0, $t1 add $v0, $v0, $t0 lw $s0, 4($sp) lw $s1, 0($sp) lw $ra, 8($sp) addi $sp, $sp, 12 jr $ra Where do the local variables of a procedure reside? Stack. Frame pointer makes the local variable access easier.

  3. Local Variables int add-array (int *A, int lo, int Hi) { int i, temp, N; N=(Hi – lo +1) if (N < 10){ temp = 0; for (i=lo; i<lo+Hi; i++) temp = temp +A[i]; } else {temp = add-array (A, lo, (lo+[N/2])); temp = add-array(A, (lo+[N/2]+1), Hi) +temp;} return temp; }

  4. After Add-array(A, 0, 49) After Add-array(A, 50, 99) Data N 100? 50? temp i Text OS Local Variables contd. Add-array(A, 0, 99): Add-array(A, 0, 49) + Add-array(A, 50, 99)

  5. Another Example Temp: 1 1 1 -- -- 1 1 factorial (10) factorial (9) factorial (8) --- --- factorial (1) factorial (0) factorial (N) { int temp; if (N==0) return 1; else { temp = N; return (temp * factorial(N-1)); } }

  6. Factorial assembly factorial: sw $ra, -4($sp) # $s0 for temp addi $sp, $sp, 8 sw $s0, -8($sp) jr $ra addi $sp, $sp, -8 # var temp is located at # next address bne $a0, $zero, recurse move $v0, 1 j exit recurse: move $s0, $a0 sw $s0, -4($sp) #un-necessary addi $a0, $a0, -1 jal factorial mult $v0, $v0, $s0 exit: lw $s0, 0($sp) lw $ra, 4($sp) $ra $s0 temp

  7. Bottom of the stack frame: $fp $sp Frame Pointer Top of the stack frame: $sp $ra All the information relevant to a call to factorial. $s0 temp

  8. factorial(10) factorial(9) factorial(10) stack frame/AR $ra $fp $s0 $ra temp $s0 factorial(9) AR $sp temp Frame Pointer, Contd. How do we pop factorial(9) AR? $sp changes continuously. $fp moves in big steps.

  9. $fp sw $fp, -4($sp) $ra addi $sp, $sp, -4 move $fp, $sp $s0 temp $sp $ra old $fp 0x7fff0000 sw $ra, -4($sp) sw $s0, -8($sp) #create temp addi $sp, $sp, -12 $s0 temp Frame Pointer, Contd. Addr: 0x7fff0000

  10. $ra #restore old $fp lw $fp, 0($fp) addi $sp, $sp, 16 $s0 temp $ra old $fp 0x7fff0000 $s0 temp Frame Pointer, Contd. lw $ra, 8($sp) lw $s0, 4($sp) $fp $sp

  11. Factorial assembly with FP factorial: sw $fp, -4($sp) addi $fp, $sp, -4 lw $fp, 0($fp) sw $ra, -8($sp) addi $sp, $sp, 16 sw $s0, -12($sp) jr $ra addi $sp, $sp, -16 bne $a0, $zero, recurse move $v0, 1 j exit recurse: move $s0, $a0 addi $a0, $a0, -1 jal factorial mult $v0, $v0, $s0 exit: lw $s0, 4($sp) lw $ra, 8($sp)

  12. old $fp $fp $ra $s0 temp $sp FP advantage: same var. name Each variable same offset from $fp throughout program! factorial: sw $fp, -4($sp) addi $fp, $sp, -4 lw $fp, 0($fp) sw $ra, -4($fp) addi $sp, $sp, 16 sw $s0, -8($fp)jr $ra addi $sp, $sp, -16 bne $a0, $zero, recurse move $v0, 1 j exit recurse: move $s0, $a0 addi $a0, $a0, -1 jal factorial mult $v0, $v0, $s0 exit: lw $s0, -8($fp) lw $ra, -4($fp)

  13. Example Program SUM: sw $ra, -4($sp) # read E & F into $s0 & $s1 add $v0, $v0, $t0 sw $s0, -8($sp) #restore $s0 & $s1 sw $s1, -12($sp) lw $s0, 4($sp) addi $sp, $sp, -12 lw $s1, 0($sp) lw $s0, 12($sp) # load E # restore $ra lw $s1, 16($sp) # load F lw $ra, 8($sp) add $v0, $a0, $a1 addi $sp, $sp, 12 add $t0, $a2, $a3 jr $ra add $t1, $s0, $s1 add $t0, $t0, $t1

  14. $t0 $t1 F 10000 E $sp $ra 9996 $s0 9988 $s1 Example Program Contd. Calling Context: #save any temp registers sw $t0, -4($sp) move $a2, $s0 sw $t1, -8($sp) move $a3, $s1 #stack parameters jal SUM sw $s3, -12($sp) move $s4, $v0 sw $s4, -16($sp) addi $sp, $sp, -16 #reg parameters move $a0, $t0 move $a1, $t1 Return address: 2000

  15. SUM: sw $fp, -4($sp) addi $fp, $sp, -4 $fp 9996 sw $ra, -4($fp) $t0 sw $s0, -8($fp) $t1 sw $s1, -12($fp) F 10000 11000 addi $sp, $sp, -16 E $sp Old $fp=11000 $ra=2000 $s0 9984 $s1 AR(Caller) AR(SUM) Example Program Contd. --- --- lw $s1, -12($fp) lw $s0, -8($fp) lw $ra, -4($fp) lw $fp, -0($fp) addi $sp, $sp, 16 jr $ra

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