module
12
Graphics 4 gave every dot its own colour. The 256 has three more bitmap modes, each trading something for something else: more dots across, more colours, or more lines. All of them cost video memory, and some of them cost it twice over. This section shows you each one and what it costs, so you can choose.
All four are set by register 0 and laid out the same way: line after line, from the start of the page.
| Mode | R#0 | Dots across | Colours | Dots a byte | Bytes a line |
|---|---|---|---|---|---|
| Graphics 4 | 06h | 256 | 16, from the palette | 2 | 128 |
| Graphics 5 | 08h | 512 | 4, palette entries 0-3 | 4 | 128 |
| Graphics 6 | 0Ah | 512 | 16, from the palette | 2 | 256 |
| Graphics 7 | 0Eh | 256 | 256, fixed | 1 | 256 |
The 512-dot modes fit twice as many dots into the same width of screen. Each dot is half as wide: fine detail, small text, thin lines.
Each byte holds four dots, the leftmost in bits 7-6, then 5-4, 3-2 and 1-0. Each pair is a colour number 0 to 3, and those are palette entries 0 to 3. The dot at (x, y) is in byte
y*128 + x/4
Bytes a line and bytes a screen are the same as Graphics 4: you trade colours for dots. Four colours from 512 is enough for a sharp, hi-res playfield in a chosen scheme, and S6's palette chooses them. Plotting a dot is S11's read-modify-write with a 2-bit mask: AND out the dot's two bits, OR in the colour shifted to them.
Its pages are 32K, as Graphics 4's are: R#2 = 3Fh shows the page at 08000h, and 5Fh the one at 10000h.
Graphics 6 is Graphics 4 with twice the dots. Two dots a byte, the left dot in the high nibble, but 256 bytes a line:
y*256 + x/2
S11's plot works with one change: the address. y is the whole high byte and x/2 the low byte, which is even simpler.
A Graphics 6 screen is twice the size of a Graphics 4 one, so its pages are 64K: R#2 = 1Fh shows 00000h and 3Fh shows 10000h. There are two of them in the 128K. The page at 00000h covers MOS's screen, so a Graphics 6 game of your own uses the page at 10000h.
In Graphics 7 each byte is one dot, and the byte is the colour. There is no palette:
bit: 7 6 5 4 3 2 1 0
G G G R R R B B
Green and red have three bits each, levels 0 to 7, shown as 0, 36, 73, 109, 146, 182, 219 and 255. Blue has two bits, shown as levels 0, 2, 4 and 7: 0, 73, 146 and 255. So E0h is full green, 1Ch full red, 03h full blue, FFh white and 92h a mid grey (146, 146, 146). Writing the palette port changes nothing on a Graphics 7 screen.
The border follows suit: in Graphics 7, register 7 is read as a colour byte too, and so is every dot of colour 00h, which shows the border instead of black. Write 00h to register 7 and both are black.
Plotting is a plain write: one dot is one byte, so there is nothing to read first. The address is y*256 + x. The pages are 64K, as in Graphics 6: 1Fh and 3Fh show 00000h and 10000h.
Graphics 7 is for pictures: title screens, shaded backgrounds, a sky shaded from one colour to another without touching the palette. The price is a screen twice the size of Graphics 4's for the same 256 dots across.
Register 9's LN bit (bit 7, S11) works in all four modes. With it, each shows 212 lines, the extra 20 being lines 192 to 211 of the page.
The 256 can weave two pages into one picture of twice the height. In one field it shows the even page, in the next field the odd page, each line of one falling between two lines of the other. In Graphics 6 that is 512 x 424.
Three settings do it:
LD A, 3Fh
LD B, 2
CALL setreg ; R#2: the odd page, 10000h
LD A, 8Eh ; LN + IL + EO + 50 Hz
LD B, 9
CALL setreg
Each line of the picture comes from the two pages in turn. The cost is both 64K pages: all 128K of the main video memory. MOS's screen in the even page goes with it, so a program using interlace leaves MOS to rebuild its screen with BE.
| 192 lines | 212 lines | Page size | Pages in 128K | Plotting a dot | |
|---|---|---|---|---|---|
| Graphics II (TC-01) | 12,288 | - | - | 1 (16K) | read-modify-write a bit |
| Graphics 4 | 24,576 | 27,136 | 32K | 4 | read-modify-write a nibble |
| Graphics 5 | 24,576 | 27,136 | 32K | 4 | read-modify-write 2 bits |
| Graphics 6 | 49,152 | 54,272 | 64K | 2 | read-modify-write a nibble |
| Graphics 7 | 49,152 | 54,272 | 64K | 2 | one write |
| Graphics 6 interlaced | 98,304 | 108,544 | both | 1 | read-modify-write a nibble |
Bytes are also time. Every full redraw sends all of a screen's bytes through port 8, so a Graphics 6 or 7 screen takes twice as long to clear or redraw as a Graphics 4 one. The pages matter too: S13 flips between two pages to hide drawing, and S17 keeps graphics in spare pages. Graphics 4 and 5 leave room for both; Graphics 6 and 7 have exactly two pages and nothing spare.
MOS can set up Graphics 6 for you, and go on printing on it. SETMOD, MCAL 92h, takes a number in HL and sets the screen:
| HL | Screen |
|---|---|
| 0 | Graphics II, 32 columns |
| 1 | Graphics II, 40 columns (MOS's usual screen) |
| 2 | Text 2, 80 columns (S18) |
| 3, 4, 5, 6 | Graphics 6, 32, 40, 64 or 80 columns |
It clears the screen. A number above 6 returns with carry set and changes nothing. At the > prompt the command S0 to S6 does the same.
In Graphics 6, MOS lays out the 64K page at 00000h like this:
| Address | What MOS keeps there |
|---|---|
| 0000h-BFFFh | the bitmap: 192 lines of 256 bytes. MOS draws its text here |
| E800h | a copy of the screen's text, as characters |
| F000h | the font, which is also the sprite patterns |
| F800h | the sprite colour table |
| FA00h | the sprite attribute table |
| FA80h | the function-key table |
The bitmap is an ordinary Graphics 6 bitmap. You can draw in it with the formula above and MOS will print over your drawing, so a program can mix MOS's text with lines, bars and charts. Keep to 0000h-BFFFh, and leave everything from E800h up to MOS.
One trap. After setting Graphics 6, MOS leaves register 15 at 2, so a read of port 9 returns status register 2, not 0. The frame interrupt handler in frame.asm reads port 9 to clear the frame flag. With R#15 at 2 it reads the wrong register and the interrupt is never cleared. Set R#15 to 0 before you turn the interrupt on.
The first program shows every Graphics 7 colour at once, as a 16 x 16 grid, on the page at 10000h.
; S12: all 256 colours of Graphics 7 at once.
;
; Graphics 7: one byte a dot, 256 bytes a line. The byte IS the colour,
; GGGRRRBB, and the palette plays no part. Pages are 64K: R#2 = 3Fh shows
; the one at 10000h, above MOS's screen.
;
; A 16 x 16 grid of blocks, 16 dots by 12 lines each, 192 lines in all.
; Block (column c, row r) is colour r*16 + c: across a row the low four
; bits change (blue, and red's two low bits), down the grid the high four
; (red's top bit, and green). Shown for 250 frames, then back to MOS.
; MOS's BE puts R#7 back.
ORG 256
DI
LD A, 0Eh
LD B, 0
CALL setreg ; R#0 = 0Eh: Graphics 7 - before writing the page
LD A, 3Fh
LD B, 2
CALL setreg ; R#2 = 3Fh: show the 64K page at 10000h
XOR A
LD B, 7
CALL setreg ; R#7 = 00h: in G7 the border is a colour byte too,
; and dots of colour 00h show it
; --- the sprites' own attribute table at 1FA00h (line 250 of
; the page, below the picture): only Y = 216
LD A, 7
LD B, 14
CALL setreg
XOR A
OUT (9), A
LD A, 3Ah + 40h
OUT (9), A
LD A, 216
OUT (8), A
LD A, 0F7h
LD B, 5
CALL setreg
LD A, 03h
LD B, 11
CALL setreg
; --- the grid, written as one run from 10000h: 192 lines
LD A, 4
LD B, 14
CALL setreg ; R#14 = 4: 10000h
XOR A
OUT (9), A
LD A, 40h
OUT (9), A ; address 0000h, for writing
LD D, 0 ; D = the colour at the start of this row of blocks
grow: LD E, 12 ; 12 lines a block
gline: LD C, D ; this line starts at colour D
LD H, 16 ; 16 blocks across
gblock: LD B, 16 ; 16 dots a block, a byte each
LD A, C
gdot: OUT (8), A
DJNZ gdot
INC C ; next colour along
DEC H
JR NZ, gblock
DEC E
JR NZ, gline
LD A, D
ADD A, 16 ; next row of blocks: 16 colours on
LD D, A
JR NZ, grow ; 256 colours = 16 rows, then D wraps to 0
; --- show it for 250 frames, then MOS's screen back
CALL irqon
LD A, 0E0h
LD B, 1
CALL setreg ; display on, frame interrupt on
EI
LD B, 250
hold: PUSH BC
CALL waitf
POP BC
DJNZ hold
LD A, 0C0h
CALL irqoff
CALL tomos
RET
onframe: RET
INCLUDE "frame.asm"
Build and run
Save it as s12-256-colours.asm next to frame.asm. Build and run it as in S2.
What you should see
A black border round a grid of 256 coloured blocks, 16 across and 16 down, filling the 192-line picture. The top-left block is black. Along each row the colour steps through four blues, then repeats with a touch more red, four times over. Down the grid, more red and then more green come in, so the bottom-right block is white. No two blocks are the same colour.
After five seconds, MOS's screen comes back.
The second program asks MOS for its Graphics 6 screen, prints on it, draws on it, prints again, and puts MOS's usual screen back.
; S12: MOS's own Graphics 6 screen - 512 dots across, and MOS still prints.
;
; SETMOD (MCAL 92h) with HL = 4 sets Graphics 6 with 40 columns of text
; and clears it. MOS keeps its bitmap at 0000h-BFFFh: 256 bytes a line,
; two dots a byte, left dot in the high nibble. The program prints, draws
; in the same bitmap, prints again, holds the picture for 250 frames and
; puts MOS's usual screen back with SETMOD HL = 1.
ORG 256
LD HL, 4
RST 8
DEFB 92h ; SETMOD: Graphics 6, 40 columns, cleared
LD HL, title
CALL print
DI
; --- a white line across all 512 dots, on line 20
LD D, 20
LD E, 0FFh ; colour 15 in both nibbles
CALL gline
; --- 16 bars, one per colour, 32 dots (16 bytes) wide, lines 40-79
LD D, 40
bars: CALL gaddr ; the start of line D
LD C, 0 ; colour 0 ...
bar1: LD A, C
RLCA
RLCA
RLCA
RLCA
OR C ; ... in both nibbles
LD B, 16
bar2: OUT (8), A
DJNZ bar2
INC C
BIT 4, C
JR Z, bar1 ; colours 0-15
INC D
LD A, D
CP 80
JR NZ, bars
; --- a band of single dots: white, black, white, black ...
LD D, 90
fine: LD E, 0F1h ; one white dot, one black dot
CALL gline
INC D
LD A, D
CP 100
JR NZ, fine
XOR A
LD B, 14
CALL setreg ; R#14 back to 0
EI
LD HL, more
CALL print ; MOS prints on, over our dots
; --- SETMOD left R#15 = 2. Set it to 0 BEFORE the frame interrupt:
; the handler reads port 9, and must read S#0 to clear the flag.
DI
XOR A
LD B, 15
CALL setreg
CALL irqon
LD A, 0E0h
LD B, 1
CALL setreg ; display on, frame interrupt on
EI
LD B, 250
hold: PUSH BC
CALL waitf
POP BC
DJNZ hold
LD A, 40h
CALL irqoff ; frame interrupt off, MOS's table back
LD HL, 1
RST 8
DEFB 92h ; SETMOD: MOS's usual Graphics II, 40 columns
RET
onframe: RET
; --- gaddr: write address of the start of line D (0-191) of MOS's
; Graphics 6 bitmap: D*256 = R#14 (D/64), then (D mod 64)*256
gaddr: LD A, D
RLCA
RLCA
AND 3 ; D / 64
LD B, 14
CALL setreg ; R#14
XOR A
OUT (9), A ; low byte 0
LD A, D
AND 3Fh
OR 40h
OUT (9), A ; high six bits, for writing
RET
; --- gline: fill line D with byte E (256 bytes = 512 dots)
gline: CALL gaddr
LD B, 0 ; 256 times
gl1: LD A, E
OUT (8), A
DJNZ gl1
RET
; --- print: the text at HL, up to a 0, through MOS
print: LD A, (HL)
OR A
RET Z
RST 8
DEFB 158
INC HL
JR print
title: DEFB "MOS'S OWN GRAPHICS 6: 512 DOTS ACROSS", 13, 10, 0
more: DEFB "AND MOS STILL PRINTS", 0
INCLUDE "frame.asm"
Build and run
Save it as s12-mos-graphics-6.asm next to frame.asm. Build and run it as in S2.
What you should see
On MOS's blue, two lines of MOS's text at the top:
MOS'S OWN GRAPHICS 6: 512 DOTS ACROSS
AND MOS STILL PRINTS
Under them, a thin white line right across the picture. Below that, sixteen bars in the sixteen colours, from colour 0 on the left (the same blue as the border) to white on the right. Below the bars, a band ten lines deep that looks grey: up close it is white and black dots, alternating every single dot, 512 of them across the line.
After five seconds the usual 40-column screen comes back, cleared, with the register display and the prompt.
s12-256-colours.asm, change R#0 from 0Eh to 0Ah. The bytes are the same. What does each block look like now, and why is the picture twice as wide in dots?s12-mos-graphics-6.asm, change SETMOD's HL from 4 to 6. What happens to the text, and to your drawing?| Symptom | Cause |
|---|---|
| In Graphics 7 the dots you meant to be black come out the border's colour | Colour 00h shows register 7. Set R#7 to 00h for a black border and black dots |
You know the four bitmap modes and what each trades: Graphics 5 and 6 for 512 dots across, Graphics 7 for 256 fixed colours, interlace for 424 lines. You know what each screen costs in bytes and in pages, and you can draw on MOS's own Graphics 6 screen while MOS prints on it.
S13, Flicker-Free: two pages, one shown while the other is drawn.
