Registers and operand widths

The idea

Enter with binary integers and systems programming experience. Every example in this course uses x86-64 NASM-style Intel syntax, with the destination operand first. These are conceptual instruction traces for a Linux setting, not programs that run in the browser. Registers hold bit patterns; the instructions and your interpretation determine whether those patterns represent numbers, addresses, or other data.

RAX names a sixty-four-bit register, EAX its low thirty-two bits, and AL its low eight bits. Writing EAX clears the upper half of RAX. Writing AL changes only that byte and preserves the remaining bits. Operand width is therefore part of the computation. Track the complete register when mixing widths, and report final state rather than inventing output from a snippet with no output operation.

; Conceptual trace: no I/O
mov rax, -1
mov eax, 5
mov al, 9

Final RAX = 9; no console output.

The EAX write zeroes RAX’s upper thirty-two bits. The AL write then changes its low byte from five to nine.