What Does a RAX Register Do?

In an x86_64 CPU architecture, the RAX register is one of the general-purpose registers. Here’s what it does:

🔹 What is RAX?

  • RAX stands for Register A Extended.
  • It is the 64-bit extension of the EAX register, which itself is the 32-bit version of the original AX register from the 16-bit x86 architecture.

🔹 Primary Uses of RAX

  1. Accumulator Register:
    • Traditionally used for arithmetic operations like multiplication and division.
    • For example, in MUL or DIV instructions, RAX is often implicitly used.
  2. Function Return Values:
    • In the System V AMD64 ABI (used by Linux and many Unix-like systems), RAX is used to store the return value of a function.
  3. System Calls:
    • In Linux, when making a system call, the system call number is placed in RAX.

🔹 Register Breakdown

  • RAX (64-bit)
  • EAX (lower 32 bits of RAX)
  • AX (lower 16 bits of EAX)
  • AH and AL (high and low 8 bits of AX)

This layered access allows for flexible manipulation of data sizes.

Updated: