Question

int getFirstDayOfMonth(int mon, int year)
{

   int adjusted_month = (mon + 9) % 12 + 1;
  
   int adjusted_year = year - 2000;
   if(adjusted_month > 10)
       adjusted_year--;

   // declare day, set to 1
   int day = 1;
   // declare century, set to 20
   int century = 20;
   // declare output, set to
  
   // (13 * mon - 1) / 5 + year / 4 + century / 4 + day + year - 2 * century;
   int output = (13 * adjusted_month - 1) / 5 + adjusted_year / 4 + century / 4 + day + adjusted_year - 2 * century;
  

   // mod output by 7
   output = output%7;
   // add 7 to output
   output = output + 7;
   // mod output by 7 again
   output = output%7;

   // return output

   return output;

}

Please convert above C program to x86. Please post the correct solution. TIA,

Answer 1

Given, Asked to convert the given c code to x86 assembly code.

Explanation: I have used x86 GCC compiler to check the method result and it is working properly. Please verify the code and let me know if it works for you.

Given Code:

int getFirstDayOfMonth(int mon, int year)
{
int adjusted_month = (mon + 9) % 12 + 1;
int adjusted_year = year - 2000;
if(adjusted_month > 10)
adjusted_year--;
int day = 1;
int century = 20;
int output = (13 * adjusted_month - 1) / 5 + adjusted_year / 4 + century / 4 + day + adjusted_year - 2 * century;
output = output%7;
output = output + 7;
output = output%7;
return output;
}

x86 Code:

getFirstDayOfMonth:
pushl %ebp
movl %esp,%ebp
subl $28,%esp
pushl %ebx
pushl %esi
pushl %edi
movl 8(%ebp),%eax
addl $9,%eax
movl %eax,-24(%ebp)
movl $12,%ecx
cltd
idivl %ecx
movl %edx,-28(%ebp)
movl %eax,-24(%ebp)
incl %edx
movl %edx,-4(%ebp)
movl $1,-8(%ebp)
movl $20,-12(%ebp)
movl 12(%ebp),%eax
addl $-2000,%eax
movl %eax,-16(%ebp)
cmpl $10,-4(%ebp)
jle .L2
decl -16(%ebp)
.L2:
imull $13,-4(%ebp),%eax
movl %eax,-24(%ebp)
decl %eax
movl %eax,-24(%ebp)
movl $_divsi3,-28(%ebp)
pushl $5
pushl %eax
call _divsi3
leal 8(%esp),%esp
movl %eax,%esi
movl -16(%ebp),%edi
cmpl $0,%edi
jge .L3
leal 3(%edi),%edi
.L3:
sarl $2,%edi
leal (%edi,%esi),%esi
movl -12(%ebp),%edi
cmpl $0,%edi
jge .L4
leal 3(%edi),%edi
.L4:
sarl $2,%edi
leal (%edi,%esi),%ebx
addl -8(%ebp),%ebx
addl -16(%ebp),%ebx
movl -12(%ebp),%esi
sall $1,%esi
movl %ebx,%eax
subl %esi,%eax
movl %eax,-20(%ebp)
movl $7,%ecx
cltd
idivl %ecx
movl %edx,%esi
movl %eax,%ebx
movl %esi,-20(%ebp)
addl $7,-20(%ebp)
movl -20(%ebp),%eax
cltd
idivl %ecx
movl %edx,%esi
movl %eax,%ebx
movl %esi,-20(%ebp)
movl -20(%ebp),%eax
jmp .L1
.L1:
leal -40(%ebp),%esp
popl %edi
popl %esi
popl %ebx
leave
ret

Please let me know if you need more help in this.