Read Assembly

Some notes to facilitate the reading of assembly code in IA32.

IA32 Registers

Machine code views the memory as a large, byte-addressable array. Aggregated data types in C are represented in machine code as contiguous collections of bytes. A pointer in C just saves the address of the first byte of the block.

An IA32 CPU contains 8 registers storing 32-bit values. They have peculiar names and usage. X86-assembly/Registers gives a synthesis on them.

Name	Usage	Comment
%eip	Program counter	Indicates the address in memory of the next instruction to be executed.
%esp	Stack pointer	Holds the address of the top element in the stack (as the stack grows downward, this is the bottom)
%eip	Frame pointer	Holds the address of the first element in the current stack frame

Access

Three types of values accesses in assembly

• Immediate value is for constant value, e.g. $4 is to take the value 4
• register denotes the content of of one of the register, e.g. %eax takes the 32 bits data in the register denoted by %eax
• Memory reference designates a value stored in memory, it is like pointer in C, e.g. (%eax) is to take the value at the address saved in %eax, this is called as argument in a command, it always comes with size of bytes to read.

The third case may can be more complicated than the other two in practice, since it can involve memory access both to registers and to memory. For example, for movl (%eax),%edx, we first locate the byte at the address %eax in memory, then copy 4 bytes from address %eax to %eax+3 to the register %edx. By the way, we cannot have both source and destination be memory reference.

Note: It worth noticing that when we add values of one register to another like addl %eax,%edx, it is one single operation. In contrast, addl %eax,(%edx) yields multiple operations: we first load the value at the address %edx from memory to CPU register, then add the value %eau, finally store back the result into memory. The former instruction has much higher performance.

Another commun format to access memory is to use offset and scaling factor. Like movl 4(%esp),%eax is to copy the 4 bytes value starting from the address %esp+4 in memory to the register %eax.

Note: Pointer Dereferencing a pointer in C involves copying that pointer into a register, and then use this register to take the value in memory. As the access to register is much faster than to memory, this understanding can help us to further optimize our code with compound types.

Instructions

All the lines beginning with . are directives to guide the assembler and linker.

Suffix

• b = 1 byte, e.g. char
• w = 2 bytes, word, e.g. short
• l = 4 bytes, double words, e.g. int
• q = 8 bytes, quad words, e.g. double

Suffix are often attached to command like movw (move word).

Instruction	Format	Meaning
mov	movl S,D	copy 4 bytes source value to destination
push	pushl S	decrement %esp by 4 and copy the 4 bytes source value to (%esp)
pop	popl D	copy the 4 bytes value stored at %esp to D and increment %esp by 4

Stack Frame Structure

IA32 use program stack to support procedure calls. The stack is used to pass proceduree arguments and to store return information etc. The portion of the stack allocated for a single procedure call is called a stack frame, delimited by %ebp and %esp.