Assembly Language (at long last!)

CS 301 Lecture, Dr. Lawlor

OK, so in the last two weeks, we've looked at bits, bit operations, hexadecimal, tables, and finally machine code (in excruciating detail).  Together, these are everything you need to know in order to understand assembly language.  Assembly language is, simply, a line-by-line copy of machine code transcribed into human-readable words.

For example, we've been using the "move 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
ret

(Try this in NetRun now!)

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    
   6:	c3                   	ret
Note the middle column contains the same 0xb8 and so on that in HW2, we wrote by hand.  (The duplicate "ret" instructions are because 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,
	         5
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!

Assembly Instructions

A list of all possible x86 instructions can be found in: The really important opcodes are listed in my cheat sheet.  Most programs can be writen with mov, the arithmetic instructions (add/sub/mul), the function call instructions (call/ret), the stack instructions (push/pop), and the conditional jumps (cmp/jmp/jl/je/jg/...).   We'll learn about these over the next few weeks!

Assembly Registers

Registers are where you store data in assembly language--there aren't any variables, so everything has to either go in registers or somewhere in memory.

Here are the commonly-used x86 registers:
"Scratch" registers you're allowed to overwrite and use for anything you want.  "Preserved" registers serve some important purpose somewhere else, so you have to put them back ("save" the register) if you use them.

Each of these registers is available in several sizes:
Notes
 64-bit
32-bit
 16-bit
 8-bit
Values are returned from functions in this register.  Multiply instructions put the low bits of the result here too.
 rax
 eax
 ax
 ah and al
Typical scratch register.  Some instructions use it as a counter (such as SAL or REP).
 rcx
 ecx
 cx
 ch and cl
Scratch register.  Multiply instructions put the high bits of the result here.
rdx
 edx
 dx
 dh and dl
Preserved register: don't use it without saving it!
 rbx
 ebx
 bx
 bh and bl
The stack pointer.  Points to the top of the stack.
 rsp
 esp
 sp
 spl
Preserved register.  Sometimes used to store the old value of the stack pointer, or the "base".
 rbp
 ebp
 bp
 bpl
Scratch register.  Also used to pass argument #2 in 64-bit mode (on Linux).
 rsi
 esi
 si
 sil
Scratch register.  Argument #1.
 rdi
 edi
 di
 dil
Scratch register.  These were added in 64-bit mode, so the names are slightly different.
 r8
 r8d
 r8w
 r8b
Scratch register.
 r9
 r9d
 r9w
 r9b
Scratch register.
 r10
 r10d
 r10w
 r10b
Scratch register. r11
 r11d
 r11w
 r11b
Preserved register.
 r12
 r12d
 r12w
 r12b
Preserved register. r13 r13d r13w r13b
Preserved register. r14 r14d r14w r14b
Preserved register. r15 r15d r15w r15b

Curiously, you can write a 64-bit value into rax, then read off the low 32 bits from eax, or the low 16 bitx from ax, or the low 8 bits from al--it's just one register, but they keep on extending it!

rax: 64-bit
eax: 32-bit
ax: 16-bit
ah
al

For example,
mov rcx,0xf00d00d2beefc03; load 64-bit constant
mov eax,ecx; pull out low 32 bits
ret

(Try this in NetRun now!)

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
ret
(executable NetRun link)

Here are the x86 arithmetic instructions.  Note that they *all* take just two registers, the destination and the source. 
Opcode
 Does
 Example
add
 +
 add eax,ecx
sub
 -
 sub eax,ecx
imul
 *
 imul eax,ecx
idiv
 /
 idiv eax,ecx
and
 &
 and eax,ecx
or
 |
 or eax,ecx
xor
 ^
 xor eax,ecx
not
 ~
 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

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

Dissasembly at Runtime

You can just typecast a function pointer over to an unsigned char, and play with the bytes of a function's machine code:
int bar(void) { /* some random function: we look at bar's machine code below! */
	return 2;
}

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 */
		std::cout<<"0x"<<std::hex<<(int)data[i]<<"\n";
	return 0;
}

(Try this in NetRun now!)

This prints out *exactly* the machine code we would write:

0xb8    Opcode for "mov eax,"
0x2     Constant to load into eax (4 bytes)
0x0
0x0
0x0
0xc3    Opcode for "ret"

You can also click "Disassemble" in NetRun to see the assembly and machine code the compiler or assembler produces.

Hierarchy of "REALLY"

Thus, the notion that Facebook is interesting is REALLY an illusion.