Scroller.z80

{AB1}
org $8000

; Simple vertical scrolltext for ZX Spectrum Bot
; @P_Malin https://twitter.com/P_Malin
; https://gist.github.com/pmalin/c97f01e0338de762ca30d04239bd9cba

ATTR_DATA: equ 16384+32*192

;border 0
ld a,0
ld hl,$5C8D
ld (hl),a
out (254),a

;cls
call $d6b

mainLoop:

halt
halt

; scroll attributes upwards
ld de,ATTR_DATA
ld hl,ATTR_DATA+32
ld bc,32*(24-1)
ldir

; write bottom line

; clear bottom line
ld hl,ATTR_DATA+32*24-1
ld de,ATTR_DATA+32*23
ld a,1*9
ld b,32
loopx:
ld (de),a
inc de
djnz loopx


; get character bit pattern in c

; get string character
ld a,(charIndex)
ld l,a
ld h,0
ld de,msg
add hl,de

; get char value
ld a,(hl)
ld l,a
ld h,0

add hl,hl ; x2
add hl,hl ; x4
add hl,hl ; x8

ld d,$3d-1
ld a,(chary)
ld e,a
add hl,de

ld a,(hl)
ld c,a
ld a,0*9

; dest
ld de,ATTR_DATA+32*23+13

call draw

pop bc

ld de,ATTR_DATA+32*23+12
ld a,(hl)
ld c,a
push bc 
ld a,5*9
call draw

; increment string positions
ld hl,chary
ld a,(hl)
inc a
and 7
ld (hl),a

jr nz,noSkip

ld hl,charIndex
ld a,(hl)
inc a
and 15
ld (hl),a

noSkip:

jr mainLoop

draw:


ld b,8

bitloop:
rl c
jr nc,noDraw

ld (de),a

noDraw:

inc de
djnz bitloop

ret

msg: db "ZX Spectrum!!!  "

chary: db 0
charIndex: db 0