Down to the very core of computers, every signal sent to any part of the machine is a series of electrical pulses. If the electrical pulse is high, for example 5 volts, that means a binary 1 digit is sent. If the electrical pulse is low, for example 0 volts, that means a binary 0 digit is sent. Put 4 binary digits together and you get a nibble. Put 8 together, you get a byte. Put 16 together, you get a word. Computers understand commands in such segments, but humans would rather not. So, they developed a better way of viewing such information and that was with hexadecimal. First of all, it didn't take 16 characters to show one command and in general, it became more convenient to remember commands like:
90 - No Operation CC - Break E9 - 16 bit jump CD - Interrupt etc.
But soon, it was understood that such codes needed to be user-friendly, so they invented assembly code. With assembly code, you can represent operations with special codes that are more human-readable. For example, JMP jumped, INT interrupted, NOP did no operation, etc.
An assembler is the program that reads an assembly file (usually with the extension .asm) and converts it to a binary executable. This is very much akin to a compiler generating a executable program. However, assemblers are considered separately from compilers because compilers hide the low-level details of your system, while assemblers expose you to these details.
The source code written for any assembler is defined to be "Assembly language." But that doesn't mean much, since each assembler may require different source code for exactly the same program on an exactly identical platform.
In assembly, every instruction (or operation/operator/opcode) is followed by a comma-separated list of parameters (or operands). These operands can be either immediate values (i.e. numbers such as 1, 2, 0x16, 0101b), or registers, or a memory location. For example,
mov eax, 123
The instruction is mov, the operands are eax and 123. Here eax is a register and 123 is an immediate value.
There are two main syntaxes for assembly: the Intel syntax, and the AT&T syntax.
Generally speaking, the first operand of an operation is the destination operand, and the other operand (or operands) is the source operand. For example:
mov eax, 123
The mov instruction takes a value from the source operand, and places it in the destination operand, so the value 123 would be placed into the register eax. The original Intel syntax is that of the Intel ASM386 assembler (later licensed out to RadiSys but which eventually died off). Since then, many assemblers invented many variations of it. Also, it is used in most online assembly tutorials because the majority of those tutorials were written when TASM (which uses the Intel syntax) was the dominant assembler.
AT&T syntax is the reverse of Intel syntax. The data operated on moves from left to right. Here is the same statement as above in AT&T syntax.
mov $123, %eax
There are also some other differences with operands:
- Literal values such as 123 are prefixed with '$' (see example above).
- Memory locations have no prefix: mov 123, %eax moves the value stored at 123 to eax.
- Registers are prefixed with '%'
- When using a register as a pointer, it is put in parenthesis: mov (%eax), %ebx moves the value stored at the pointer stored in eax to ebx.
- When using a statement with memory locations (with no register present), a suffix is needed after the instruction. For example: movl $123, 123. The 'l' (stands for long) and is needed to tell the assembler that the values are 32 bits.
- Pointers can be offset by constants by prefixing them: 4(%eax) points to 4+%eax.
- Pointers can be offset by another register by writing them as: (%eax,%ebx) which points to %eax+%ebx
- Finally you can combine it all and throw in a scale if you want: 4(%eax,%ebx,2) would be 4+%eax+%ebx*2
- Inline Assembly
- Opcode syntax for more detail on AT&T syntax differences
- Learning 80x86 Assembly: list of freely available online resources to help with learning 80x86 assembly language programming
- GNU Binutils - An open source collection of software including the GNU assembler (gas)
- gas manual - Official manual for the GNU assembler (gas)
- NASM - Official site of Netwide Assembler:NASM
- x86_Assembly - A wikibook on writing assembly for x86 based PCs
- The Art of Assembly Language Programming, online book, 16-bit DOS edition. (Later editions focus on a high-level assembler dialect created by the author.)
- PC ASM, online book, 32 bit protected mode assembly on x86
- x86 Opcode and Instruction Reference, a database of the instructions and opcodes as free viewable, searchable, and downloadable XML files. They also sell a hardcopy version.