Registers

In the previous post, we explored the memory architecture of a computer system examining its various components and how access speed differs between them. Now, let’s take a deeper dive into registers, the fastest memory component in this hierarchy and the one closest to the CPU.

You can think of registers as a special kind of memory built right into the CPU that is very small, but extremely fast to access. There are many different kinds of registers in x86-64, and for now we'll only focus on the so-called general-purpose registers, of which there are sixteen. Each of them is 64 bits wide, and for each of them the lower byte, word and double-word can be addressed individually (incidentally, 1 "word" = 2 bytes, 1 "double-word" = 4 bytes, in case you haven't heard this terminology before).

Register	Lower byte	Lower word	Lower dword
rax	al	ax	eax
rbx	bl	bx	ebx
rcx	cl	cx	ecx
rdx	dl	dx	edx
rsp	spl	sp	esp
rsi	sil	si	esi
rdi	dil	di	edi
rbp	bpl	bp	ebp
r8	r8b	r8w	r8d
r9	r9b	r9w	r9d
r10	r10b	r10w	r10d
r11	r11b	r11w	r11d
r12	r12b	r12w	r12d
r13	r13b	r13w	r13d
r14	r14b	r14w	r14d
r15	r15b	r15w	r15d

Additionally, the higher 8 bits of rax, rbx, rcx and rdx can be referred to as ah, bh, ch and dh.

Some instructions can only be used with certain registers, and some registers have special meaning for certain instructions. In particular, rsp holds the stack pointer (which is used by instructions like push, pop, call and ret), and rsi and rdi serve as source and destination index for "string manipulation" instructions. Another example where certain registers get "special treatment" are the multiplication instructions, which require one of the multiplier values to be in the register rax, and write the result into the pair of registers rax and rdx.

In addition to these registers, we will also consider the special registers rip and rflags. rip holds the address of the next instruction to execute. It is modified by control flow instructions like call or jmp. rflags holds a bunch of binary flags indicating various aspects of the program's state, such as whether the result of the last arithmetic operation was less, equal or greater than zero. The behavior of many instructions depends on those flags, and many instructions update certain flags as part of their execution. The flags register can also be read and written "wholesale" using special instructions.