Accidental overwrite of general register

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I am working on a homework assignment where we have to write comments to describe what some assembly code is doing. I am new to assembly so I can't really tell if the following code has a mistake.

I am using i686 architecture (not entirely sure what that means TBH)

This program determineGrade.s is being called by another program cs3843p3Driver.o

############################## determineGrade #####################
# Purpose:
#   determineGrade determines the grade of a student based on a
#   1000 pt scale.
# Parameters:
#   i 8(%ebp)  int iG1       Grade 1
#   i 12(%ebp) int iG2       Grade 2
#   i 16(%ebp) int iG3       Grade 3
#   i 20(%ebp) int iG4       Grade 4
#   i 24(%ebp) int iG5       Grade 5
#   i 28(%ebp) int iG6       Grade 6
#   i 32(%ebp) int iG7       Grade 7
#   i 36(%ebp) int iG8       Grade 8
#   i 40(%ebp) int iG9       Grade 9
#   i 44(%ebp) int iG10      Grade 10
# Locals:
#   -4(%ebp)  int iqTot
#   -8(%ebp)  int iTot
# Notes:
#   Grade is determined by
#       ??
# Return Value:
#   total grade on a 1000 pt scale
# Register Usage
#   %eax - mostly working register until after .L7 then ??
  .file "determineGrade.s"
  .text
.globl determineGrade
  .type determineGrade, @function
determineGrade:
  pushl %ebp
  movl  %esp, %ebp
    pushl %ebx                # Save the caller's %ebx since we are using %ebx
  subl  $20, %esp
  movl  $0, -4(%ebp)          # Overrides the caller's %ebx?

It looks as if the caller's %ebx value is stored in the same place as iqTot, which is defined as -4(%ebp). Was the caller's %ebx value overwritten by accident?

att x86 assembly
1 Answers

I double checked with the professor, and yes, the stored value of %ebx was overwritten by iqTot. The function which calls determineGrade did not depend on %ebx to work properly, so the professor decided to leave the mistake to see if any of us would notice. Apparently I was the only student to call it out, so he gave me extra credit.

It should instead be:

  • iqTot is stored at -8(%ebp)
  • iTot is stored at -12(%ebp)
user_48795930
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