SCASB - x86-64 instruction reference

scasbScan string for a byte 
Compares al with the byte at [rdi] and sets the flags accordingly.
8086 and later
Intel's reference: SCAS/SCASB/SCASW/SCASD 
Used in Arrays, strings and the string instructionsx86-64 assembly
Length of a stringx86-64 assembly
 
CFPFAFZFSFDFOF ●●●●●–●
● set from the result
– not changed
Reads DF
Forms  scasb  
The b suffix selects an element of 1 byte. After each element, the instruction increases rdi by 1 when DF = 0, or decreases rdi by 1 when DF = 1. In 64-bit mode the default addresses use rdi. The a32 address-size prefix instead uses edi; 16-bit string addressing is unavailable.
repe / repz repeats while ZF = 1, and repne / repnz repeats while ZF = 0. Each stops when the counter reaches zero; a nonzero initial counter permits the first comparison before testing its result. The counter is rcx by default or ecx with a32. If it starts at zero, a repeated instruction accesses no element. The data width is independent of this address and counter width.
 global _start

section .data
source: db 1, 2, 3, 4, 5, 6, 7, 8
destination: times 8 db 0

section .text
_start:
        cld                 ; count upwards
        mov rsi, source
        mov rdi, destination
        mov rax, 7
        scasb               ; compares al with [rdi] and sets the flags

        mov rax, 60         ; syscall 60: exit
        xor rdi, rdi
        syscall