a string length function in AVR assembler

strlen.asm

/*
 * strlen.asm
 *
 *  Created: 28/03/2012 6:00:53 PM
 *   Author: Luke
 */ 


 /*
 * strlen.asm
 *
 *  Created: 28/03/2012 4:24:50 PM
 *   Author: Luke
 */ 


; A strlen function
.include "m64def.inc"

.def zero = r16

.cseg
jmp main
shortString:.db "hello"
longString:.db "supsupsupsupsupsupsupsupsupsupsup sup sup sup sup sup sups"

main:
	;init the stack (main has no stack frame as it doesn't store anything)
	ldi YL, low(RAMEND)
	ldi YH, high(RAMEND)
	out SPH, YH
	out SPL, YL

	;create our zero constant
	ldi zero, 0 ;hehehe

	;strlen uses r21:r20 as its input parameter (arbitrarily chosen)
	;remember that the return value will be in r25:r24

	;calculate the length of shortString (should be five)
	ldi r21, low(shortString)
	ldi r20, high(shortString)
	rcall strlenn

end:
	rjmp end

strlenn:
	;some variable names
	.def count = r2
	.def char = r3

	;prologue
	;push conflict registers
	push ZH ;we use Z as our current pointer into program memory
	push ZL
	push YL
	push YH
	push char	;could have made this a local, but didn't bother since it's temp

	;reserve space for locals and parems
	;locals: int8 count. parems: char *str. = three bytes
	;I might make count an int16 at some stage
	sbiw Y, 3		;subtracts 3 from Y
	out SPH, YH
	out SPL, YL

	;store parameter on the stack
	std Y+1, r20
	std Y+2, r21

	;store count on the stack
	std Y+3, zero

	;body
	ldd count, Y+3	;load our count from the stack into r2
	ldd ZH, Y+1		;load our string pointer from the stack
	ldd ZL, Y+2

	;a post-check loop
	loop:
		;load the next char from flash
		lpm char, Z+	;increment Z as well since we always want to load the next char
		cp char, zero
		breq loopend
		inc count
		rjmp loop
	loopend:
	
	;epilogue
	;store the result in r25:r24
	mov r25, count

	;deallocate locals and params
	adiw Y, 3
	out SPH, YH
	out SPL, YL

	;restore conflict registers
	pop char
	pop YH
	pop YL
	pop ZL
	pop ZH
	ret

This is very helpful. Thank you for this.