Registers

x86_64 Registers

x86_64 processors have 16 general-purpose 64-bit registers. Each register can also be accessed as smaller sub-registers:

Important Rule

When you write to a 32-bit register (like eax), the upper 32 bits of the corresponding 64-bit register (rax) are automatically zeroed. This does not happen with 8-bit or 16-bit writes.

Using mov to Set Registers

The mov instruction copies data:

mov rax, 42       ; rax = 42
mov rbx, rax      ; rbx = rax (copy)
mov rcx, 0xFF     ; rcx = 255 (hex)

Your Task

Write a program that:

1. Stores the value 72 in rax (ASCII for 'H')
2. Stores 105 in rbx (ASCII for 'i')
3. Stores 10 in rcx (newline)
4. Writes the character 'H' to stdout, then 'i', then a newline

Hint: To write a single byte, you can store it in memory on the stack and point rsi at it.