tiny16/asm/tutorial/32_stack_frame.asm at main · rafael84/tiny16 · GitHub

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; Exercise 32: Stack-Based Parameters with Frame Pointer
; Level: 4 - Stack & Subroutines
;
; Goal: Pass three arguments via the stack to a function
;       Function uses frame pointer to access arguments, computes sum
;       Also demonstrates local variables with negative offsets
;       Call sum(5, 10, 15) and get R0 = 30
;
; Instructions to use: LOADI, PUSH, MOVSPR, LOAD, STORE, ADD, CALL, RET, HALT
; Expected result: R0 = 30
;
; Hint: Push arguments in reverse order: arg3, arg2, arg1
; Hint: Function uses MOVSPR to get frame pointer (SP into R6:R7)
; Hint: Access arguments via POSITIVE offset: [R6:R7 + offset]
; Hint: Access local variables via NEGATIVE offset: [R6:R7 - offset]
; Hint: Signed offsets range from -128 to +127
;
; Stack layout after MOVSPR and local allocation:
;   Higher addresses
;   [FP + 5] = arg1 (5)  - first pushed
;   [FP + 4] = arg2 (10)
;   [FP + 3] = arg3 (15) - last pushed
;   [FP + 2] = return address high byte
;   [FP + 1] = return address low byte
;   [FP + 0] = SP points here after CALL <- FP saved here
;   [FP - 1] = local1 (temporary sum)    <- pushed after MOVSPR
;   Lower addresses

section .code

; Push arguments in order: arg1, arg2, arg3
loadi   r0, 5
push    r0          ; arg1
loadi   r0, 10
push    r0          ; arg2
loadi   r0, 15
push    r0          ; arg3
call    sum_three
halt

sum_three:
    movspr  r6:r7               ; R6:R7 = SP (frame pointer)
    ; Allocate space for local variable
    loadi   r0, 0
    push    r0                  ; local1 at [FP - 1]

    ; Load arguments using positive offsets
    load    r0, [r6:r7 + 5]     ; arg1 (5) at FP+5 (first pushed)
    load    r1, [r6:r7 + 4]     ; arg2 (10) at FP+4
    add     r0, r1              ; R0 = 5 + 10 = 15

    ; Store intermediate result in local variable using negative offset
    store   r0, [r6:r7 - 1]     ; Store 15 in local1

    ; Load local and add final argument
    load    r0, [r6:r7 - 1]     ; Load local1 (15)
    load    r2, [r6:r7 + 3]     ; arg3 (15) at FP+3 (last pushed)
    add     r0, r2              ; R0 = 15 + 15 = 30

    ; Clean up local variable
    pop     r1
    ret