CS2.201 - Computer Systems Organization | CSO Lecture 6 Notes

Assembly code

Consider the below simple code in C.

int accum = 0;

int sum(int x, int y)
{
	int t = x + y;
	accum += t;
	return t;
}

If we compile it to assembly code with gcc -O1 -S code.c, we get the following.

sum:
	pushl	%ebp
	movl	%esp, %ebp
	movl	12(%ebp), %eax
	addl	8(%ebp), %eax
	addl	%eax, accum
	popl	%ebp
	ret

All these assembly instructions take up different numbers of bytes in the actual binary encoding, depending on how many arguments they have and how commonly they are used.

Processor State

The condition code registers can hold information that tell us some of the more crucial information about the most recent operation, such as if its result was a zero.

Data formats

For x86 Intel, the term “word” usually refers to 16-bits, with “double words” being 32-bits, and “quad words” being 64-bits.

Accessing Information - IA32

In assembly, 8 registers are commonly used: eax, ecx, edx, ebx, esi, esp, ebp. The last two of these have specific functions, they are the stack pointer and the frame pointer respectively. The first six are general purpose for the most part.

eax points to 32 bits somewhere in the memory, and we can point just to the last half, 16 bits with ax. These 2 bytes can be further broken down into ah and al if we want to access the second last and the last byte.

 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x
31                            15             8 7               0
eax                           ax
                              ah              al

Accessing Information - x86-64

In 64-bit architecture, since we have access to 64 bit registers, we have rax, rbx etc., whose second halves can be pointed to by eax, ebx and so on.

We have 16 total registers here, 8 are the same as in 32-bit, and the rest 8 are new general purpose registers, names r8 through to r15.

Operand specifiers

Many instructions have one or more operands, used as source and destination references. They can be of 3 types - constant (immediate), register, memory.