Elsewhere: Calling Functions and Branching in Assembly

CS 301 Lecture, Dr. Lawlor

Reminder: Arithmetic In Assembly

Here's how you add two numbers in assembly:
Here's the C/C++ equivalent:
int a = 3;
int c = 7;
a += c;
return a;
And finally here's the assembly code:
mov eax, 3
mov ecx, 7
add eax, ecx
(executable NetRun link)

Here are the x86 arithmetic instructions.  Note that they *all* take just two registers, the destination and the source. 
add eax,ecx
sub eax,ecx
imul eax,ecx
idiv eax,ecx
and eax,ecx
or eax,ecx
xor eax,ecx
not eax

Be careful doing these!  Assembly is *line* oriented, so you can't say:
    add (sub eax,ecx),edx
but you can say:
    sub eax,ecx
    add eax,edx

Calling Functions from Assembly

You use the "call" instruction to call functions.  You can actually call C++'s "cout" if you're sufficiently dedicated, but the builtin NetRun functions are designed to be a little easier to call.  You first need to tell the assembler that "read_input" is an external function.  All you do is say "extern read_input".  Then you run that function, with "call read_input", and the CPU will execute the read_input function until it returns.  Before read_input returns, it puts the read-in value into eax, where you can grab it.

So this assembly program reads an integer and returns it:
extern read_input
call read_input
(executable NetRun link)

Be careful, though!  The read_input function can and will use all the other registers for its own purposes.  In particular, it's tricky to call read_input twice to read two numbers, since you need to stash the first number somewhere other than registers during the second call!


A jump instruction, like "jmp", just switches the CPU to executing a different piece of code.  It's the assembly equivalent of "goto", but unlike goto, jumps are not considered shameful in assembly.

You say where to jump to using a "jump label", which is just any string with a colon after it.  (The same syntax is used in C/C++)
Assembly jump
C++ goto
	mov eax,3
jmp lemme_outta_here
mov eax,999 ; <- not executed!

(Try this in NetRun now!)

	int x=3;
goto quiddit;
return x;

(Try this in NetRun now!)

In both cases, we return 3, because we jump right over the 999 assignment.  Jumping is somewhat useful for skipping over bad code, but it really gets useful when you add conditional jumps...

Conditional Jumps: Branching in Assembly

In assembly, all branching is done with two types of instruction:
Here's how to use compare and jump-if-equal ("je"):
	mov eax,3
cmp eax,3 ; how does eax compare with 3?
je lemme_outta_here ; if it's equal, then jump
mov eax,999 ; <- not executed *if* we jump over it

(Try this in NetRun now!)

Here's compare and jump-if-less-than ("jl"):
	mov eax,1
cmp eax,3 ; how does eax compare with 3?
jl lemme_outta_here ; if it's less, then jump
mov eax,999 ; <- not executed *if* we jump over it

(Try this in NetRun now!)

The C++ equivalent to compare-and-jump-if-whatever is "if (something) goto somewhere;".

Also, check out the machine code generated for the conditional jump--the jump destination is encoded as the number of bytes of machine code to skip over.  For example, the "jl" above gets encoded in machine code like this:
   0:	b8 01 00 00 00       	mov    eax,0x1
5: 83 f8 03 cmp eax,0x3
8: 7c 05 jl f <foo+0xf>
a: b8 e7 03 00 00 mov eax,0x3e7
f: c3 ret
The distance to jump, shown in red above, is five bytes, because the code we're skipping over is five bytes long.  Note that a jump label doesn't show up in machine code at all--it's just used by the assembler to figure out how far to jump.