The $ Symbol

Part of the Fundamentals section of Coddy's Assembly journey — lesson 29 of 45.

When you print a string using the write syscall (Chapter 5), you need to tell the CPU how many bytes to print.

You could count the characters manually:

message db 'Hello', 0x0a    ; 6 bytes (H e l l o newline)
mov rdx, 6                  ; Manual count

But manual counting is slow and error-prone. If you change the string, you must recount and update the number.

The solution: automatic length calculation using <strong>$lt;/strong>

$ is the location counter — it represents the current memory address where the next byte will be placed.

Example 1: Simple string

section .data
    message db 'Hello', 0x0a
    msgLen equ $ - message

How it works:

message is the starting address (example: 1000)
After storing 6 bytes, $ becomes 1006
$ - message = 1006 − 1000 = 6 (the length)

Note: equ stands for equate. It creates a constant — a name that represents a value.

Example 2: Multiple strings

section .data
    first db 'Hi', 0x0a
    len1 equ $ - first      ; len1 = 3 ('H','i', newline)
    
    second db 'Hello', 0x0a
    len2 equ $ - second     ; len2 = 6 ('H','e','l','l','o', newline)

Each string has its own length calculated automatically. The $ resets for each calculation because it always refers to the current position right after each string.

Challenge

Easy

Create a string called city with the value 'Paris' followed by a newline (0x0a). Then create a constant called cityLen that automatically calculates the length using $ - city.

Try it yourself

%include "data.asm"

section .text
    global _start

_start:
    mov rax, 1
    mov rdi, 1
    mov rsi, city
    mov rdx, cityLen
    syscall
    
    mov rax, 60
    mov rdi, 0
    syscall