Registers Reference

• Register is a temporary storage built into the CPU itself. So it can contains 64-bit data, (can be a value or a memory address).

General-purpose registers

• Callee-saved (%rbx, %rbp, %r12-%r15): when procedure P calls procedure Q, Q must preserve the values of these registers, ensuring that they have the same values when Q returns to P as they did when Q was called.
• Caller-saved (all other except %rsp): they can be modified by any function

Important

An instruction generating a 4-byte value with a register as the destination will fill the upper 4 bytes with zeros

Special registers

• Flag register (rFlags): used for status and CPU control information. The rFlag register is updated by the CPU after each instruction and not directly accessible by programs. This register stores status information about the instruction that was just executed. Of the 64-bits in the rFlag register, many are reserved for future use.

  Name	Symbol	Bit	Use
  Carry	CF	0	Used to indicate if the previous operation results in a carry
  Parity	PF	2	Used to indicate if the last byte has an even number of 1’s (even parity)
  Adjust	AF	4	Used to support Binary Coded Decimal
  Zero	ZF	6	Used to indicate if the previous operation resulted in a zero result
  Sign	SF	7	Used to indicate if the previous operation resulted in a 1 in the MSB
  Direction	DF	10	Used to specify the direction (increment or decrement) for some string operations
  Overflow	OF	11	Used to indicate if the previous operation resulted in an overflow

• Instruction Pointer Register (%rip): used by the CPU to point to the next instruction to be executed.

Sizes of C data types in x86-64