Third is the little sibling of Forth

third.6502

_start:
	jmp main


; functions with fixed addresses go here

; User compiled code starts at address $2000
initCode:
	lda #$00
	sta $00
	lda #$20
	sta $01
	rts

; Compile a byte stored on the accumulator
compileByte:
	ldx #$00
	sta ($00,x)
	; increment user code address
	inc $00
	bne compileByteRet
	inc $01
compileByteRet:
	rts

; compile a rts instruction (opcode 60)
compileRts:
	lda #$60
	jsr compileByte
	rts


; To compile instructions with addresses,
; we need a way to store an address on registers.
;
; We store a 2-byte address as follows:
;   - store the lower byte on the X register
;   - store the higher byte on the Y register

; compile the address stored in registers X and Y
compileAddr:
	txa
	jsr compileByte
	tya
	jsr compileByte
	rts

compileJsr:
	lda #$20
	jsr compileByte
	rts

compileJmpAbs:
	lda #$4c
	jsr compileByte
	rts

compileJmpInd:
	lda #$6c
	jsr compileByte
	rts

compileLdaImm:
	lda #$a9
	jsr compileByte
	rts

compileLdxImm:
	lda #$a2
	jsr compileByte
	rts

compileLdyImm:
	lda #$a0
	jsr compileByte
	rts

compileLabel:
	ldx $00
	ldy $01
	jsr compileLdxImm
	txa
	adc #$08
	bcc endIny
	iny
endIny:	jsr compileByte
	jsr compileLdyImm
	tya
	jsr compileByte
	jsr compileJsr
	; address of `compileAddr`
	ldx #$1d
	ldy #$06
	jsr compileAddr
	jsr compileRts
	rts

compileStaAbs:
	lda #$8d
	jsr compileByte
	rts

; User dictionary starts at address $4000
initDict:
	lda #$00
	sta $02
	lda #$40
	sta $03
	rts
	
main:
	; initialize scaffold
	jsr initCode
	jsr initDict

	; dot label: address $2000
	jsr compileLabel

	; dot: address $2008
	jsr compileLdaImm
	lda #$01
	jsr compileByte
	jsr compileStaAbs
	ldx #$00
	ldy #$02
	jsr compileAddr
	jsr compileRts

	; subroutine that calls dot: address $200e
	jsr compileJsr
	jsr $2000 ; run dot label -> compile dot address
	jsr compileRts
	
	jsr $200e
	brk