How should I write such an if statement in assembly?

if ((a == b AND a > c) OR c == b) { ...

Platform: Intel 32-bit machine, NASM syntax.

Update

For variable types and value, use whatever is more easy to understand. Integers would works fine for me, I guess.

2

2 Answers

In generic assembly, it will be basically something like this (a in ax, b in bx, c in cx):

 cmp bx, cx jeq istrue cmp ax, cx jle isfalse cmp ax, bx jeq istrue
isfalse: ; do false bit jmp nextinstr
istrue: ; do true bit
nextinstr: ; carry on

If there's no false bit, it can be simplified to:

 cmp bx, cx jeq istrue cmp ax, bx jne nextinstr cmp ax, cx jle nextinstr
istrue: ; do true bit
nextinstr: ; carry on

0

You will need to break the if statement into a series of comparisons and jumps. In the same way that in C you could write:

int test = 0;
if (a == b) { if (a > c) { test = 1; }
}
// assuming lazy evaluation of or:
if (!test) { if (c == b) { test = 1; }
}
if (test) { // whole condition checked out
}

Which breaks the complex expression into constituent parts your asm will do likewise, although you can write it more cleanly in asm by jumping to the parts that are still relevant.

Assuming a, b and c are being passed to you on the stack (if they're not load them from elsewhere obviously)

 mov eax, DWORD PTR [ebp+8] cmp eax, DWORD PTR [ebp+12] ; a == b? jne .SECOND ; if it's not then no point trying a > c mov eax, DWORD PTR [ebp+8] cmp eax, DWORD PTR [ebp+16] ; a > c? jg .BODY ; if it is then it's sufficient to pass the
.SECOND: mov eax, DWORD PTR [ebp+16] cmp eax, DWORD PTR [ebp+12] ; second part of condition: c == b? jne .SKIP
.BODY: ; .... do stuff here jmp .DONE
.SKIP: ; this is your else if you have one
.DONE:

0

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