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collapseos/kernel/sms/vdp.asm
2020-02-26 09:43:50 -05:00

175 lines
4.1 KiB
NASM

; vdp - console on SMS' VDP
;
; Implement PutC on the console. Characters start at the top left. Every PutC
; call converts the ASCII char received to its internal font, then put that
; char on screen, advancing the cursor by one. When reaching the end of the
; line (33rd char), wrap to the next.
;
; In the future, there's going to be a scrolling mechanism when we reach the
; bottom of the screen, but for now, when the end of the screen is reached, we
; wrap up to the top.
;
; When reaching a new line, we clear that line and the next to help readability.
;
; *** Defines ***
; FNT_DATA: Pointer to 7x7 font data.
; *** Consts ***
;
.equ VDP_CTLPORT 0xbf
.equ VDP_DATAPORT 0xbe
.equ VDP_COLS 32
.equ VDP_ROWS 24
; *** Code ***
vdpInit:
ld hl, .initData
ld b, .initDataEnd-.initData
ld c, VDP_CTLPORT
otir
; Blank VRAM
xor a
out (VDP_CTLPORT), a
ld a, 0x40
out (VDP_CTLPORT), a
ld bc, 0x4000
.loop1:
xor a
out (VDP_DATAPORT), a
dec bc
ld a, b
or c
jr nz, .loop1
; Set palettes
xor a
out (VDP_CTLPORT), a
ld a, 0xc0
out (VDP_CTLPORT), a
ld hl, .paletteData
ld b, .paletteDataEnd-.paletteData
ld c, VDP_DATAPORT
otir
; Define tiles
xor a
out (VDP_CTLPORT), a
ld a, 0x40
out (VDP_CTLPORT), a
ld hl, FNT_DATA
ld c, 0x7e-0x20 ; range of displayable chars in font.
; Each row in FNT_DATA is a row of the glyph and there is 7 of them.
; We insert a blank one at the end of those 7. For each row we set, we
; need to send 3 zero-bytes because each pixel in the tile is actually
; 4 bits because it can select among 16 palettes. We use only 2 of them,
; which is why those bytes always stay zero.
.loop2:
ld b, 7
.loop3:
ld a, (hl)
out (VDP_DATAPORT), a
; send 3 blanks
xor a
out (VDP_DATAPORT), a
nop ; the VDP needs 16 T-states to breathe
out (VDP_DATAPORT), a
nop
out (VDP_DATAPORT), a
inc hl
djnz .loop3
; Send a blank row after the 7th row
xor a
out (VDP_DATAPORT), a
nop
out (VDP_DATAPORT), a
nop
out (VDP_DATAPORT), a
nop
out (VDP_DATAPORT), a
dec c
jr nz, .loop2
; Bit 7 = ?, Bit 6 = display enabled
ld a, 0b11000000
out (VDP_CTLPORT), a
ld a, 0x81
out (VDP_CTLPORT), a
ret
; VDP initialisation data
.initData:
; 0x8x == set register X
.db 0b00000100, 0x80 ; Bit 2: Select mode 4
.db 0b00000000, 0x81
.db 0b11111111, 0x82 ; Name table: 0x3800
.db 0b11111111, 0x85 ; Sprite table: 0x3f00
.db 0b11111111, 0x86 ; sprite use tiles from 0x2000
.db 0b11111111, 0x87 ; Border uses palette 0xf
.db 0b00000000, 0x88 ; BG X scroll
.db 0b00000000, 0x89 ; BG Y scroll
.db 0b11111111, 0x8a ; Line counter (why have this?)
.initDataEnd:
.paletteData:
; BG palette
.db 0x00, 0x3f, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
.db 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
; Sprite palette (inverted colors)
.db 0x3f, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
.db 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00
.paletteDataEnd:
; Convert ASCII char in A into a tile index corresponding to that character.
; When a character is unknown, returns 0x5e (a '~' char).
vdpConv:
; The font is organized to closely match ASCII, so this is rather easy.
; We simply subtract 0x20 from incoming A
sub 0x20
cp 0x5f
ret c ; A < 0x5f, good
ld a, 0x5e
ret
; grid routine. Sets cell at row D and column E to character A. If C is one, we
; use the sprite palette.
vdpSetCell:
call vdpConv
; store A away
ex af, af'
push bc
ld b, 0 ; we push rotated bits from D into B so
; that we'll already have our low bits from the
; second byte we'll send right after.
; Here, we're fitting a 5-bit line, and a 5-bit column on 16-bit, right
; aligned. On top of that, our righmost bit is taken because our target
; cell is 2-bytes wide and our final number is a VRAM address.
ld a, d
sla a ; should always push 0, so no pushing in B
sla a ; same
sla a ; same
sla a \ rl b
sla a \ rl b
sla a \ rl b
ld c, a
ld a, e
sla a ; A * 2
or c ; bring in two low bits from D into high
; two bits
out (VDP_CTLPORT), a
ld a, b ; 3 low bits set
or 0x78 ; 01 header + 0x3800
out (VDP_CTLPORT), a
pop bc
; We're ready to send our data now. Let's go
ex af, af'
out (VDP_DATAPORT), a
; Palette select is on bit 3 of MSB
ld a, 1
and c
rla \ rla \ rla
out (VDP_DATAPORT), a
ret