Machine Code & Assembly Language

CS 301 Lecture, Dr. Lawlor

x86 Machine Code

Here's a table-driven program, but using x86-compatible table numbers:
const unsigned char table[]={
0xb0, /*set x = ... */
73, /* ... this byte */
0xc3 /* exit */

int foo(void) {
int x=0; /* our "register" (temporary storage, and return value) */
int i=0; /* our location in the table */
while (1) { /* always keep looping through the table */
int instruction=table[i++];
if (instruction==0xb0) { /* set-x instruction */
x=table[i++]; /* next byte is the new value for x */
else if (instruction==0xc3) {
return x; /* stop looping through the table */
else {
cout<<"Illegal instruction:" <<std::hex<<instruction<<"\n";
return -999;

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What's amazing is that I can tell the CPU to execute the bytes above, and those bytes act like a function that returns 73--the CPU is just table-driven hardware!  For example, the byte "0xc3" tells an x86 CPU to return from the current function.  The byte "0xb0" is followed by a one-byte parameter to load up for return.  So this code actually works!

(Don't worry about the hideous C++ syntax for function pointer stuff.)
const char commands[]={
0xb0,73, /* load a value to return */
0xc3 /* return from the current function */
int foo(void) {
typedef int (*fnptr)(void); // pointer to a function returning an int
fnptr f=(fnptr)commands; // typecast the command array to a function
return f(); // call the new function!

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Machine Code and Disassembly

These raw byte commands that the CPU executes are called "machine code".  "assembly language" is just a human-readable translation of machine code.  An "assembler", like NASM, reads assembly language and writes executable machine code.  A "disassembler", like PE Explorer or IDA Pro (for Windows), or objdump (for Linux or Mac OS X), reads an executable and writes assembly language (in NetRun, hit "Disassemble" checkbox under "Options").

If you just want to look at the machine code inside a function, you can just do some pointer typecasting from function to array (the opposite of what we did above!) and start printing bytes of machine code:

int bar(void) { /* some random function: we look at bar's machine code below! */
return 3;

int foo(void) {
const unsigned char *data=(unsigned char *)(&bar);
for (int i=0;i<10;i++) /* print out the bytes of the bar function */
return 0;

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This prints out the same bytes inside bar that you see in the "Disassembler" tab.  Which instructions is the compiler using?  0xb8 is the 32-bit version of the load-a-constant instruction 0xb0 above, so the next four bytes are all representing the constant 3 (0x00000003 is stored as 0x03 0x00 0x00 0x00).  0xc3 is just the return instruction, like we used above.

Assembly Language

For example, we've been using the "move a constant into register 0" instruction (0xb8) a lot.  In an assembler, you can emit the same machine code with this little assembly language program:
mov eax,5

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The assembler (NASM, in this case) will then spit out the following machine code:
00000000 <foo>:
0: b8 05 00 00 00 mov eax,0x5
5: c3 ret
Note the middle column contains the same 0xb8 and so on that the compiler generates, or we could even write by hand.  (NetRun always puts in a spare "ret" instruction at the end, in case you forget.)

The big advantage of using an assembler is that you don't need to remember all the funky arcane numbers, like 0xb8 or 0xc3 (these are "opcodes").  Intead, you remember a human-readable name like "mov" (short for "move").  This name is called an "opcode mnemonic", but it's always the first thing in a CPU "instruction", so I usually will say "the mov instruction" rather than "the instruction that the mov opcode mnemonic stands for".

There are several parts to this line:
Unlike C/C++, assembly is line-oriented, so the following WILL NOT WORK:
	mov eax,
Yup, line-oriented stuff is indeed annoying.  Be careful that your editor doesn't mistakenly add newlines to long lines of text!  I usually leave off the semicolons for lines without comments, because otherwise I find myself tempted to do this:
	mov ecx, 5;  mov eax, 3;   Whoops!
It doesn't look like it, but the semicolon makes that second instruction A COMMENT!

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 ecx    <- Warning!  Weirdness!  (see below)
and eax,ecx
or eax,ecx
xor eax,ecx
not eax

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

In assembly, arithmetic has to be broken down into one operation at a time!

Note that "idiv" is really weird. Basically, "idiv bot" divides eax by bot (the eax is hardcoded).  But it also treats edx as high bits above eax, so you have to set them to zero first.

  idiv bot
  top = eax+(edx<<32)
  eax = top/bot
  edx = top%bot

Here's an example:
mov eax,73; top
mov ecx,10; bottom
mov edx,0 ; high bits of top
idiv ecx ; divide eax by ecx
; now eax = 73/10, edx=73%10

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What a strange instruction!