CODEWITHSUNDEEP
cassemblyx86att
João Pedro de Sousa
João Pedro de Sousa

Reputation: 1

Cmp instruction mismatch

I have to write a function in assembly to complete the following c code. 

int main(){

int hist[26]={0};
int i;

charHistogram("This is a string", hist);

for (i=0; i<26; i++ )
printf("%c:%d ", i+’a’, hist[i] );
printf("\n");
}

return 0;
}

And this is the assembly code I wrote:

_charHistogram:

push %ebp
movl %esp,%ebp
subl $0x10,%esp
movl $0x0,-0x4(%ebp) 
movl $0x41,-0x8(%ebp)

condFor1:

movl  -0x4(%ebp),%edx
movl  0X8(%ebp),%eax
cmp  (%eax,%edx,1), $0
je  exit

condFor2:

cmpl $0x5a,-0x8(%ebp)
jg   condFor1

if1:

movl -0x8(%ebp), %ecx
cmp (%eax,%edx,1), %ecx
jne if2 
subl $0x41,%ecx
movl 0xc(%ebp), %ebx
add $0x1, (%ebx,%ecx,4)
add 0x20,%ecx
inc %ecx
inc %edx
jmp condFor1

if2:

add 0x20,%ecx
cmp (%eax,%edx,2), %ecx
jne condFor1
subl $0x41,ecx
movl 0xc(%ebp), %ebx
add $0x1, (%ebx,%ecx,4)
add 0x20,%ecx
inc %ecx
inc %edx
jmp condFor1

exit:

leave
ret

Basically the function written in assembly has to count the number of occurences of a letter on a given string and store it at the int array hist. So I thought it could compare each char value to its ascii value starting at 65 to 90 and from 97 to 122. But when I begin to compile the assembly it keeps getting the error "operand size mismatch for 'cmp'" for the instruction cmp (%eax,%edx,1), $0. Can you help me out?

Upvotes: 0

Views: 2842

Answers (1)

Sep Roland
Sep Roland

Reputation: 39296

cmp  (%eax,%edx,1), $0

You need to reverse the operands.

cmp  $0, (%eax,%edx,1)

Did you notice that you have programmed endless loops?
You setup a variable with movl $0x0,-0x4(%ebp) but you are forgetting to change its value throughout the code!

Since your input string is ASCIIZ shouldn't you compare with CL in stead of comparing with ECX?

Upvotes: 3

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