Two's complement in Assembly

neg flips sign; signed jumps read it correctly.

; Two's complement: neg flips sign, signed jumps read it right.
        global _start

        section .text
_start:
        mov rax, 25
        neg rax                 ; -25

        cmp rax, 0
        jl .below               ; signed less-than sees the sign
        mov rdi, 0
        jmp .exit
.below:
        neg rax                 ; back to 25
        lea rdi, [rax + rax]    ; 50
.exit:
        mov rax, 60             ; exit code 50
        syscall

How it works

1. neg rax computes 0 minus the value.
2. jl is the *signed* less-than jump.
3. Exit codes are unsigned — return the positive 50.

Keywords and builtins used here

• cmp
• global
• jl
• jmp
• lea
• mov
• neg
• rax
• rdi
• section
• syscall

The run, in numbers

Lines

18

Characters to type

360

Tokens

46

Three-star pace

55 tpm

At the three-star pace of 55 tokens a minute, this run takes about 50 seconds.

Type this snippet

Step 3 of 3 in Arithmetic, step 6 of 31 in Language basics.

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