Variables in memory and constants

height and area name places in memory. WIDTH names a number for the assembler to use. This program multiplies a height of 5 by a width of 12 and stores the area.

default rel
global _start

WIDTH equ 12               ; a number named during assembly; no storage

section .data
height: dq 5                ; eight bytes, starting at 5
area:   dq 0                ; eight bytes, starting at 0

section .bss
scratch: resq 1             ; eight bytes, starting at 0 when the program loads

section .text
_start:
    mov rax, [height]       ; read 5 from memory
    imul rax, WIDTH         ; multiply by 12
    mov [area], rax         ; store 60 in memory

    mov qword [scratch], 99 ; store the eight-byte value 99 in scratch

    mov rax, 60
    xor rdi, rdi
    syscall

Run it, then enter 402000 in the memory panel's address box. In this playground, height begins at 0x402000 and area follows eight bytes later. Their first bytes should be 05 and 3C: 3C is hexadecimal for 60. Each is followed by seven 00 bytes because x86 stores the lowest byte first. scratch has its own address in .bss; its zeroes are only its starting value. The program replaces them with 99. WIDTH has no memory address to inspect. In imul rax, WIDTH, the assembler uses 12 as the instruction's immediate operand.

Both area: dq 0 and scratch: resq 1 give the running program eight bytes. The difference is in the executable file: dq 0 puts eight initial zero bytes in .data, while .bss reserves space that the loader starts at zero without storing those bytes in the file. That can save a lot of file space for a large buffer.

The qword in mov qword [scratch], 99 says to write an eight-byte value. The address [scratch] and the immediate number 99 do not tell NASM the memory size; if you omit qword here, NASM reports that the operation size is not specified. When storing an immediate into memory, state the size you intend. In mov [area], rax, rax already tells NASM the size: it is a 64-bit register, so that store writes eight bytes.

Your turn

Complete the program below. Read height, multiply it by WIDTH, and store the answer in area. Then put 42 in the eight-byte stamp variable using an immediate store with an explicit size. Before running, predict the three qwords. After running, the memory panel at 402000 should show height = 7, area = 63 (3F in its first byte), and stamp = 42 (2A in its first byte). Use Test to check both writes.

default rel
global _start

WIDTH equ 9

section .data
height: dq 7
area:   dq 0
stamp:  dq 0

section .text
_start:
    ; Store height * WIDTH in area, then store 42 in the eight-byte stamp.

    mov rax, 60
    xor rdi, rdi
    syscall

default rel
global _start

WIDTH equ 9

section .data
height: dq 7
area:   dq 0
stamp:  dq 0

section .text
_start:
    mov rax, [height]
    imul rax, WIDTH
    mov [area], rax
    mov qword [stamp], 42

    mov rax, 60
    xor rdi, rdi
    syscall