x86-64 Registers

General-Purpose Registers

The x86-64 architecture provides 16 general-purpose registers, each 64 bits wide. Many of these registers have specific roles in function calls and arithmetic operations.

Register Size Purpose
RAX 64-bit Accumulator. Used for arithmetic and logical operations, and to store the function's return value.
RBX 64-bit Base register. Used as a base pointer for memory access. It is non-volatile.
RCX 64-bit Counter. Used for loop counters and shifts. Also used to pass the fourth integer argument.
RDX 64-bit Data register. Used for I/O port access, multiplication, division, and passing the third integer argument.
RSI 64-bit Source Index. Pointer to the source in data transfer operations. Also used to pass the second integer argument.
RDI 64-bit Destination Index. Pointer to the destination in data transfer operations. Also used to pass the first integer argument.
RBP 64-bit Base Pointer. Points to the current stack frame. It is non-volatile.
RSP 64-bit Stack Pointer. Points to the top of the stack. Automatically managed by push and pop.
R8–R15 64-bit General-purpose registers. Used to pass additional function arguments.

Special-Purpose Registers

Instruction Pointer (RIP)

  • Holds the memory address of the next instruction to be executed.
  • Cannot be directly modified with mov, but is updated by:
  • Jumps (jmp)
  • Calls (call) - pushes current RIP before loading new address
  • Returns (ret) — pops return address back into RIP

EFLAGS Register

The EFLAGS register is a collection of single-bit flags that track CPU state and arithmetic results.

Key flags:

  • ZF (Zero Flag): Set if result is zero.
  • CF (Carry Flag): Set on arithmetic overflow/underflow, used for multi-precision arithmetic.
  • SF (Sign Flag): Set if result is negative (MSB = 1).
  • OF (Overflow Flag): Set if signed result does not fit in destination.
  • DF (Direction Flag): Controls string operation direction (std = decrement, cld = increment).

These flags guide conditional jumps (e.g., je uses ZF after cmp).

Register Usage in Function Calls

The System V AMD64 ABI defines how registers are used in function calls.

Argument Passing

First six integer/pointer arguments are passed in registers:

  1. RDI
  2. RSI
  3. RDX
  4. RCX
  5. R8
  6. R9

Additional arguments go on the stack.

Return Value

  • Stored in RAX.
  • If >64 bits, may be returned via memory referenced by RAX.

Volatile and Non-Volatile Registers

Volatile (Caller-Saved)

Not preserved across calls. Caller must save them if needed later. Registers: RAX, RCX, RDX, RSI, RDI, R8, R9, R10, R11

Non-Volatile (Callee-Saved)

Preserved across calls. Callee must save/restore if modified. Registers: RBX, RBP, RSP, R12, R13, R14, R15

Stack Frame

The stack stores local variables, spilled registers, and extra arguments.

  • RSP → always points to the top of the stack.
  • RBP → often used as a frame pointer (fixed reference point inside stack frame).
  • Modern optimisations sometimes repurpose RBP as a general-purpose register.