Registers

Memory is great, especially as there is (usually) a lot of it. However, memory is also slow. How slow? Well: If one CPU cycle would be 1 second, then a DRAM memory access would take around 6 minutes.

In the lectures about the memory subsystem, you will learn that there are mechanisms to avoid or reduce the impact that memory accesses have on the performance (e.g., through caching). The typical trade-off is: speed vs. size, whereas storage with a lot of space is usually slow but fast storage is very space-limited. The "final" stage of this trade-off is reached with registers. Registers are the smallest but fastest unit of storage available to the CPU. A CPU only has a limited number of registers, some of which are designated to specific functionality. There are also a number of general-purpose registers, which are used, among others, for calculations, storing temporary values, keeping track of the stack, or passing information to and from subroutines.

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General-Purpose Registers

Below is a list of the x86-64 general-purpose registers. Note that the list gives the name of the 64-bit version of each register. The "Widths" tab shows the names and sizes (and locations) of the associated smaller registers.

Purposes

Register	Usage
rax	return value, number of vector registers used for functions with variable arguments, (lower) result of multiplication and division
rbx	optionally used as base pointer
rcx	4th argument, commonly used as loop counter (see branching instructions)
rdx	3rd argument, upper 64 bits of 128-bit return value, (upper) result of multiplication and division
rsi	2nd argument
rdi	1st argument
rsp	stack pointer
rbp	base pointer
r8	5th argument
r9	6th argument
r10	(used for passing a function's static chain pointer)
r11
r12-r15

Green registers are callee-saved (their value is/has to be preserved during subroutine calls), while orange registers are caller-saved. More on this will be explained on the page about Calling Subroutines.

Widths