In order to calculate the length of a string, we'll first need to know what determines the end of a given string.
Strings in memory are represented as a pointer. The location pointed too is a byte of data representing a character followed by additional characters contiguous in memory. The important point is that this sequence of bytes is terminated by the byte 0x00. This is called the zero-termination character. This is what determines the end of a string.
For example, the string "HELLO WORLD!" might look like this in memory:
| H | E | L | L | O | W | O | R | L | D | ! | zero-termination character | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0X48 | 0X45 | 0X4C | 0X4C | 0X4F | 0X20 | 0X57 | 0X4F | 0X52 | 0X4C | 0X44 | 0X21 | 0x00 |
So the length can be determined by looping through each byte of memory in a string until the zero-termination character is reached. Here is an example of how I implemented this:
; expects * char array in $rdi
.strlen:
mov rax, 1 ; initialize strlen counter
.loop:
add rdi, 1 ; increment char * to next character
add rax, 1 ; increment strlen counter
cmp byte [rdi], 0x00 ; if value at [rdi] is 0x00 return
jne .loop ; loop if not at end of string
ret