Replace all

; rdi - buffer, rsi - string, rdx - symbol to replace, rcx - replacement
replace_all:
xor r9, r9
_loop:
mov al, byte[rsi + r9]
cmp al, dl
jne no_rep
mov [rdi+r9], cl
jmp continue
no_rep:
mov [rdi+r9], al
continue:
inc r9
cmp al, 0
je replace_all_exit
jmp _loop

replace_all_exit:
mov rax, rdi
ret