; codegen.s - documented scalar instruction emitter (E04-S03).
;
; The emitter writes into cg_code, a 256-byte RAM buffer. cg_len is the byte
; count (0-254); every failure sets cg_err and returns C=1. It does not emit a
; whole function yet: parser/codegen integration is E04-S04.
;
; API:
;   cg_init
;   cg_put          A = byte
;   cg_lda_imm      A = value
;   cg_ldx_imm      A = value
;   cg_ora_imm      A = value
;   cg_lda_abs      A/X = address
;   cg_ldx_abs      A/X = address
;   cg_sta_abs      A/X = address
;   cg_stx_abs      A/X = address
;   cg_adc_abs      A/X = address
;   cg_sbc_abs      A/X = address
;   cg_cmp_abs      A/X = address
;   cg_jsr_abs      A/X = address
;   cg_clc, cg_sec, cg_rts
;   cg_branch_fwd   A = branch opcode; emits opcode + placeholder, returns
;                   C=0 and A = fixup index
;   cg_patch_here   A = fixup index; target is the current end of buffer
;   cg_here         returns A = current length
;
; Only forward relative branches within +0..+127 bytes are accepted. This is
; enough for the scalar shapes and gives E04-S04 the checked primitive it
; needs for a later long-branch expansion.

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

.export cg_init, cg_put, cg_here, cg_branch_fwd, cg_patch_here
.export cg_lda_imm, cg_ldx_imm, cg_ora_imm, cg_lda_abs, cg_ldx_abs
.export cg_sta_abs, cg_stx_abs, cg_adc_abs, cg_sbc_abs, cg_cmp_abs
.export cg_jsr_abs, cg_clc, cg_sec, cg_rts
.export cg_code, cg_len, cg_err

.segment "CODE"

cg_init:
        lda #$00
        sta cg_len
        sta cg_err
        sta cg_fix_count
        clc
        rts

cg_here:
        lda cg_len
        rts

; A = byte to append. C=1 on overflow.
cg_put:
        ldy cg_len
        cpy #CG_MAX
        bcs @full
        sta cg_code,y
        inc cg_len
        clc
        rts
@full:  lda #CG_ERR_FULL
        sta cg_err
        sec
        rts

; Immediate forms.
cg_lda_imm:
        pha
        lda #$A9
        jsr cg_put
        bcs @out
        pla
        jmp cg_put
@out:   pla
        sec
        rts

cg_ldx_imm:
        pha
        lda #$A2
        jsr cg_put
        bcs @out
        pla
        jmp cg_put
@out:   pla
        sec
        rts

cg_ora_imm:
        pha
        lda #$09
        jsr cg_put
        bcs @out
        pla
        jmp cg_put
@out:   pla
        sec
        rts

; Absolute-address forms. Each wrapper supplies the opcode in Y; the shared
; routine stores the address before Y is reused by cg_put.
cg_lda_abs:
        ldy #$AD
        jmp cg_emit_addr
cg_ldx_abs:
        ldy #$AE
        jmp cg_emit_addr
cg_sta_abs:
        ldy #$8D
        jmp cg_emit_addr
cg_stx_abs:
        ldy #$8E
        jmp cg_emit_addr
cg_adc_abs:
        ldy #$6D
        jmp cg_emit_addr
cg_sbc_abs:
        ldy #$ED
        jmp cg_emit_addr
cg_cmp_abs:
        ldy #$CD
        jmp cg_emit_addr
cg_jsr_abs:
        ldy #$20
        jmp cg_emit_addr

cg_emit_addr:
        sta cg_addr
        stx cg_addr+1
        tya
        jsr cg_put
        bcs @out
        lda cg_addr
        jsr cg_put
        bcs @out
        lda cg_addr+1
        jmp cg_put
@out:   sec
        rts

cg_clc:
        lda #$18
        jmp cg_put
cg_sec:
        lda #$38
        jmp cg_put
cg_rts:
        lda #$60
        jmp cg_put

; A = branch opcode; forward target must be patched later.
cg_branch_fwd:
        sta cg_op
        lda cg_fix_count
        cmp #CG_FIX_MAX
        bcs @fix_full
        sta cg_idx
        lda cg_op
        jsr cg_put
        bcs @out
        lda cg_len
        sta cg_pos
        ldy cg_idx
        sta cg_fixpos,y
        inc cg_fix_count
        lda #$00
        jsr cg_put
        bcs @out
        lda cg_idx
        clc
        rts
@fix_full:
        lda #CG_ERR_FIX
        sta cg_err
        sec
        rts
@out:   sec
        rts

; A = fixup index. Patch its placeholder so the branch targets cg_len.
cg_patch_here:
        cmp cg_fix_count
        bcs @bad
        tax
        lda cg_len
        sec
        sbc cg_fixpos,x
        sec
        sbc #1
        cmp #128
        bcs @range
        ldy cg_fixpos,x
        sta cg_code,y
        clc
        rts
@bad:   lda #CG_ERR_FIX
        sta cg_err
        sec
        rts
@range: lda #CG_ERR_RANGE
        sta cg_err
        sec
        rts

.segment "BSS"
cg_code:        .res 256
cg_len:         .res 1
cg_err:         .res 1
cg_fix_count:   .res 1
cg_fixpos:      .res CG_FIX_MAX
cg_addr:        .res 2
cg_op:          .res 1
cg_idx:         .res 1
cg_pos:         .res 1