Assembly Language Tutorial (x86)

For more detailed information about the architecture and about processor instructions, you will need access to a 486 (or 386+) microprocessor manual. The one I like is entitled The 80386 book, by Ross P. Nelson. (This book is copyright 1988 by Microsoft Press, ISBN 1-55615-138-1.) Intel processor manuals may also be found at

The GNU Assembler, gas, uses a different syntax from what you will likely find in any x86 reference manual, and the two-operand instructions have the source and destinations in the opposite order. Here are the types of the gas instructions:

    opcode                    (e.g., pushal)
    opcode operand            (e.g., pushl %edx)
    opcode source,dest        (e.g., movl %edx,%eax) (e.g., addl %edx,%eax)

Where there are two operands, the rightmost one is the destination. The leftmost one is the source.
For example, movl %edx, %eax means Move the contents of the edx register into the eax register. For another example, addl %edx,%eax means Add the contents of the edx and eax registers, and place the sum in the eax register.

Included in the syntactic differences between gas and Intel assemblers is that all register names used as operands must be preceeded by a percent (%) sign, and instruction names usually end in either "l", "w", or "b", indicating the size of the operands: long (32 bits), word (16 bits), or byte (8 bits), respectively. For our purposes, we will usually be using the "l" (long) suffix.

80386+ Register Set

There are different names for the same register depending on what part of the register you want to use. To use the first set of 8 bits of eax (bits 0-7), you would use %al. For the second set of 8 bits (bits 8-15) of eax you would use %ah. To refer to the lowest 16 bits of eax (bits 0-15) together you would use %ax. For the entire 32 bits you would use %eax (90% of the time this is what you will be using). The form of the register name must agree with the size suffix of the instruction.

Here are the important processor registers:

    EAX,EBX,ECX,EDX - "general purpose", more or less interchangeable

    EBP             - used to access data on stack
                    - when this register is used to specify an address, SS is
                      used implicitly

    ESI,EDI         - index registers, relative to DS,ES respectively

    SS,DS,CS,ES,FS,GS - segment registers
                      - (when Intel went from the 286 to the 386, they figured
                         that providing more segment registers would be more
                         useful to programmers than providing more general-
                         purpose registers... now, they have an essentially
                         RISC processor with only _FOUR_ GPRs!)
                      - these are all only 16 bits in size

    EIP            - program counter (instruction pointer), relative to CS

    ESP            - stack pointer, relative to SS

    EFLAGS         - condition codes, a.k.a. flags


We are using the 32-bit segment addressing feature of the 486. Using 32-bit addressing as opposed to 16-bit addressing gives us many advantages: You don't have to deal with any of that ugly 16-bit crud that is used in other operating systems for the PC, like DOS or OS/2; 32-bit segmentation is really a thing of beauty in comparison to that.

i486 addresses are formed from a segment base address plus an offset. To compute an absolute memory address, the i486 figures out which segment register is being used, and uses the value in that segment register as an index into the global descriptor table (GDT). The entry in the GDT tells (among other things) what the absolute address of the start of the segment is. The processor takes this base address and adds on the offset to come up with the final absolute address for an operation. You'll be able to look in a 486 manual for more information about this or about the GDT's organization.

i486 has 6 16-bit segment registers, listed here in order of importance:

  1. CS: Code Segment Register
    Added to address during instruction fetch.
  2. SS: Stack Segment Register
    Added to address during stack access.
  3. DS: Data Segment Register
    Added to address when accessing a memory operand that is not on the stack.
  4. ES, FS, GS: Extra Segment Registers
    Can be used as extra segment registers; also used in special instructions that span segments (like string copies).

The x86 architecture supports different addressing modes for the operands. A discussion of all modes is out of the scope of this tutorial, and you may refer to your favorite x86 reference manual for a painfully-detailed discussion of them. Segment registers are special, you can't do a

    movw seg-reg, seg-reg
You can, however, do
    movw seg-reg,memory
    movw memory,seg-reg
    movw seg-reg,reg
    movw reg,seg-reg
Note: If you movw %ss,%ax, then you should xorl %eax,%eax first to clear the high-order 16 bits of %eax, so you can work with long values.


Common/Useful Instructions

mov (especially with segment registers)
    - e.g.,:
        movw %es,%ax
        movl %cs:4,%esp
        movw _processControlBlock,%cs

    - note:     mov's do NOT set flags

pushl, popl       - push/pop long
pushal, popal     - push/pop EAX,EBX,ECX,EDX,ESP,EBP,ESI,EDI

call  (jumps to piece of code, saves return address on stack)
         e.g., call _cFunction

int   - call a software interrupt

ret   (returns from piece of code entered due to call instruction)
iretl (returns from piece of code entered due to hardware or software interrupt)

sti, cli - set/clear the interrupt bit to enable/disable interrupts respectively
lea  - is Load Effective Address, it's basically a direct pipeline to the address you want to do calculations on without affecting any flags, or the need of pushing and popping flags.

A simple example:

void funtction1() {
	int A = 10;
	A += 66;

compiles to...
1	pushl %ebp #
2	movl %esp, %ebp #,
3	subl $4, %esp #,
4	movl $10, -4(%ebp) #, A
5	leal -4(%ebp), %eax #, 
6	addl $66, (%eax) #, A
7	leave
8	ret
1. push ebp
2. copy stack pointer to ebp
3. make space on stack for local data
4. put value 10 in A (this would be the address A has now)
5. load address of A into EAX (similar to a pointer)
6. add 66 to A
... don't think you need to know the rest

Mixing C and Assembly Language

The way to mix C and assembly language is to use the "asm" directive. To access C-language variables from inside of assembly language, you simply use the C identifier name as a memory operand. These variables cannot be local to a procedure, and also cannot be static inside a procedure. They must be global (but can be static global). The newline characters are necessary.

unsigned long a1, r;
void junk( void )
        "pushl %eax \n"
        "pushl %ebx \n"
        "movl $100,%eax \n"
        "movl a1,%ebx \n"
        "int $69 \n"
        "movl %eax,r \n"
        "popl %ebx \n"
        "popl %eax \n"

This example does the following:

  1. Pushes the value stored in %eax and %ebx onto the stack.
  2. Puts a value of 100 into %eax.
  3. Copies the value in global variable a1 into %ebx.
  4. Executes a software interrupt number 69.
  5. Copies the value in %eax into the global variable r.
  6. Restores (pops) the contents of the temporary registers %eax and %ebx.