CODEWITHSUNDEEP
assemblyavr
Michael Perry
Michael Perry

Reputation: 11

RET function returning to beginning of code instead of CALL point

I am new to assembly, but am working on a recursive factorial subassembly.

It must use the stack only to store values between iterations, and upon completion push the factorial onto the stack to POP after returning to CALL.

Below is my code, which works, except it returns to 'RJMP main' instead of returning to the 'CALL' after finishing.

Suggestions?

.def n = R16
.def result = R17
.def tmp =r20

.org 0x0000 ; next instruction will be written to address 0x0000
            ; (the location of the reset vector)
rjmp main   ; set reset vector to point to the main code entry point

main:       ; jump here on reset

; initialize the stack (RAMEND = 0x10FF by default for the ATmega128A)
ldi R16, HIGH(RAMEND)
out SPH, R16
ldi R16, low(RAMEND)
out SPL, R16
LDI n, 5; load a value into n
PUSH n ; push n on the stack
CALL factN ; calculate the factorial of n
POP result ; pop n! from the stack, save in result
here: RJMP here ; loop forever

factN: 
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
; Recursive Subroutine to Calculate Factorial of n
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
push n  ; push n onto the stack
pop n   ; pop n off the stack
TST R0  ; test if R0 is <= 0
BRNE notZero ; if RO is >0 jump to nonZero
ldi tmp,1 ; load 1 into tmp
mul tmp,tmp ; multiply tmp by itself to populate R0 with 1
push R0 ; push R0 onto the stack
notZero:
pop R0  ; pop R0
mul R0, n ; mulitply R0 by n
dec n ; decrement n
push n ; push n onto stack
push R0 ; push r0 onto stack
CPI n, 1 ; does n == 1
BRNE factN ;loop factN until n == 1
ret ; return to CALL

Upvotes: 1

Views: 838

Answers (1)

AterLux
AterLux

Reputation: 4654

Keep in mind CALL and RET instructions use the same stack as PUSH and POP. If RET does not return back that means the stack was somehow altered.

Each PUSH operation has to have one and only one corresponding POP in each possible flow path between the routine entry point and RET.

In your example PUSHes and POPs are unbalanced. For example:

push n  ; push n onto the stack
pop n   ; pop n off the stack

Why it popped right after it have been pushed? What the reason?

BRNE notZero ; if RO is >0 jump to nonZero
...
notZero:
pop R0  ; pop R0

if BRNE happens, this pop extracts data, which was never pushed, i.e. moves the stack pointer beyond the return address.

push n ; push n onto stack
push R0 ; push r0 onto stack

what those two pushes for?

etc. In fact there are many issues with your code. Just check carefully every possible flow.

And also an advice: never use recursion for what could be done in a simple loop. Such as factorial calculations.

Upvotes: 2

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