; vi:filetype=z80:
fb_addr equ $4000
attr_addr equ $5800
ula_port equ $fe

main:
	call init
	ld c, 16
	ld b, 12
	call calc_cell_addr
	ld a, 'F'
	call putchar
	inc de
	ld a, 'O'
	call putchar
	inc de
	ld a, 'O'
	call putchar

.looptop:
	jr .looptop

init:
	; setup attributes
	ld hl, attr_addr
	ld a, $47
	ld b, 24
.attry:	ld c, 32
.attrx:	ld (hl), a
	inc hl
	dec c
	jr nz, .attrx
	dec b
	jr nz, .attry

	; clear screen
	ld hl, fb_addr
	xor a
	ld b, 192
.clry:	ld c, 32
.clrx:	ld (hl), a
	inc hl
	dec c
	jr nz, .clrx
	dec b
	jr nz, .clry

	; set border
	out (ula_port), a
	
	ret

; expects X -> c, Y -> b, returns in de
calc_cell_addr:
	push af
	sla b
	sla b		; change from blocks to pixels
	sla b
	; construct low address byte -> e
	ld a, b		; start with Y
	sla a
	sla a
	and $e0		; keep top 3 bits
	ld e, a		; move into e
	ld a, c		; a <- X
	and $1f		; keep low 5 bits
	or e		; combine with Y bits
	ld e, a		; move the result back to e
	; construct high address byte -> d
	ld a, b		; start with Y again
	sra a
	sra a
	sra a
	and $18		; keep bits 3 and 4
	ld d, a		; keep it in d
	ld a, b		; grap Y one more time
	and $7		; keep low 3 bits of Y in a
	or d		; combine with Y6-Y7
	ld bc, fb_addr
	or b		; combine with high byte of fb address
	ld d, a		; move result back to d
	pop af
	ret


	; de: dest
	; a: char
putchar:
	sub 'A'
	ret c
	cp 27
	ret nc
	push de
	sla a
	sla a
	sla a
	ld h, 0
	ld l, a
	ld bc, glyph_a
	add hl, bc
	ld c, 8
.chartop:
	ld a, (hl)
	ld (de), a
	inc d
	inc hl
	dec c
	jr nz, .chartop
	pop de
	ret

	include glyphs.inc