; frames.s - generated-program software frames (E04-S02).
;
; All routines run with CR=$0E and IRQs masked. FRAME_SP/FRAME_FP live at
; $FB-$FE; the frame stack occupies $B800-$BFFF and grows downward from
; $C000. See src/common/frames.inc for the layout and calling convention.
;
; frame_enter is transactional on failure: if the stack bound would be
; crossed, SP and FP are restored to their entry values and C=1 with
; frame_error=FRAME_ERR_STACK. No partial frame remains.

.setcpu "6502"
.include "../common/frames.inc"

.export frame_init, frame_enter, frame_leave, frame_discard
.export frame_push_byte, frame_push_word, frame_error

.segment "CODE"

; Empty software stack: SP=$C000, FP=0, no error.
frame_init:
        lda #<FRAME_STACK_TOP
        sta FRAME_SP
        lda #>FRAME_STACK_TOP
        sta FRAME_SP+1
        lda #$00
        sta FRAME_FP
        sta FRAME_FP+1
        sta frame_error
        clc
        rts

; A/X = byte count (low/high). C=0 and SP updated on success; C=1 with
; FRAME_ERR_STACK and SP unchanged when the new SP would fall below the base.
frame_reserve:
        sta frame_size
        stx frame_size+1
        lda FRAME_SP
        sec
        sbc frame_size
        sta frame_new_sp
        lda FRAME_SP+1
        sbc frame_size+1
        sta frame_new_sp+1
        lda frame_new_sp+1
        cmp #>FRAME_STACK_BASE
        bcc @fail
        bne @ok
        lda frame_new_sp
        cmp #<FRAME_STACK_BASE
        bcc @fail
@ok:    lda frame_new_sp
        sta FRAME_SP
        lda frame_new_sp+1
        sta FRAME_SP+1
        clc
        rts
@fail:  lda #FRAME_ERR_STACK
        sta frame_error
        sec
        rts

; Push one argument byte (A). C=1 on stack overflow.
frame_push_byte:
        sta frame_byte
        lda #1
        ldx #0
        jsr frame_reserve
        bcs @out
        ldy #0
        lda frame_byte
        sta (FRAME_SP),y
        clc
@out:   rts

; Push one argument word: A=low, X=high. C=1 on stack overflow.
frame_push_word:
        sta frame_word
        stx frame_word+1
        lda #2
        ldx #0
        jsr frame_reserve
        bcs @out
        ldy #1
        lda frame_word+1
        sta (FRAME_SP),y
        dey
        lda frame_word
        sta (FRAME_SP),y
        clc
@out:   rts

; Enter a callee frame. Arguments are already on the stack above FP+2.
;   A/X = local byte count (low/high)
;   C=0: old FP pushed, FP set, locals reserved
;   C=1: no frame was created; SP/FP are unchanged
frame_enter:
        sta frame_locals
        stx frame_locals+1
        tsx
        cpx #FRAME_HW_MIN
        bcs @hw_ok
        lda #FRAME_ERR_STACK
        sta frame_error
        sec
        rts
@hw_ok: lda FRAME_SP
        sta frame_saved_sp
        lda FRAME_SP+1
        sta frame_saved_sp+1
        lda FRAME_FP
        sta frame_saved_fp
        lda FRAME_FP+1
        sta frame_saved_fp+1

        lda FRAME_FP
        ldx FRAME_FP+1
        jsr frame_push_word
        bcs @out                       ; old-FP push overflow: nothing changed
        lda FRAME_SP
        sta FRAME_FP
        lda FRAME_SP+1
        sta FRAME_FP+1

        lda frame_locals
        ldx frame_locals+1
        jsr frame_reserve
        bcc @ok
        lda frame_saved_sp              ; locals overflow: undo the old FP push
        sta FRAME_SP
        lda frame_saved_sp+1
        sta FRAME_SP+1
        lda frame_saved_fp
        sta FRAME_FP
        lda frame_saved_fp+1
        sta FRAME_FP+1
        sec
@out:   rts
@ok:    clc
        rts

; Leave the current frame. SP=FP, pop the previous FP, and leave SP pointing
; at arg0. The caller still owns the argument bytes.
frame_leave:
        lda FRAME_FP
        sta FRAME_SP
        lda FRAME_FP+1
        sta FRAME_SP+1
        ldy #0
        lda (FRAME_SP),y
        sta FRAME_FP
        iny
        lda (FRAME_SP),y
        sta FRAME_FP+1
        lda FRAME_SP
        clc
        adc #2
        sta FRAME_SP
        bcc @done
        inc FRAME_SP+1
@done:  rts

; Pop argument bytes after a return. A/X = byte count (low/high).
frame_discard:
        clc
        adc FRAME_SP
        sta FRAME_SP
        txa
        adc FRAME_SP+1
        sta FRAME_SP+1
        rts

.segment "BSS"
frame_size:     .res 2
frame_new_sp:   .res 2
frame_byte:     .res 1
frame_word:     .res 2
frame_locals:   .res 2
frame_saved_sp: .res 2
frame_saved_fp: .res 2
frame_error:    .res 1