Colors
The TMS9918A has sixteen colors and you may not have any others. They are cast into the silicon, and every machine that used the chip — the MSX, the ColecoVision, the TI-99/4A — looks a particular shade of green because of it.
F18A mode replaces the lot with 64 registers, each holding any of 4096 colors.
The palette
64 entries, twelve bits each: four bits of red, four of green, four of blue. Every one is writable at any time, including while the screen is drawing.
They are grouped into four palettes of sixteen, and at power-on they hold:
| Palette | What is in it |
|---|---|
| 0 | The TMS9918A's own sixteen |
| 1 | The same sixteen, arranged for 1-bit color — Every even entry black, every odd entry one of the stock colors, so a two-color tile is one color on black without touching the palette. |
| 2 | IBM CGA |
| 3 | CGA arranged for 1-bit color — The same trick as palette 1, over the CGA colors. |
Palette 0 is the reason a locked card looks right: it is the 9918A's own sixteen colors, in the 9918A's own order, so every program written for the original chip gets exactly what it asked for.
$000$000$2C3$5D6$54F$76F$D54$4EF$F54$F76$DC3$ED6$2B2$C5C$CCC$FFFAnd palette 2, which is there because somebody wanted CGA and it costs nothing to include:
$000$00A$0A0$0AA$A00$A0A$A50$AAA$555$55F$5F5$5FF$F55$F5F$FF5$FFFWriting one
The card has one data port, and normally everything you send it lands in VRAM. Register 47 redirects that port at the palette instead.
lda #$84 ; bit 7 = palette mode, entry 4
ldx #47
jsr SetVdpReg
lda #$00 ; red nibble
sta $9C00
lda #$0F ; green and blue nibbles
sta $9C00Two bytes make one entry. After the second, the port goes back to writing VRAM by itself, which is exactly what you want when you are changing one color.
Set bit 6 as well and it does not: the entry number steps on after each pair and you can pour in a whole palette without touching register 47 again. It stays in palette mode until you clear it, or until the entry number rolls past 63, or until anything reads a status register. That last one is a fail-safe, and it is also a trap — a frame interrupt that reads status will quietly drop you back into VRAM mode partway through a palette load. Do palette loads with interrupts off.
Which byte goes first
The two sources this chapter is built from describe the byte order the opposite way around from each other. Hagerty's own worked example sends the red byte first; the Pico9918's reference describes the green-and-blue byte first.
There is no way to settle it from here — this is a mode that only exists on hardware. Write four entries you will recognize, look at the screen, and swap the order if the colors come out wrong. It is a one-line fix and you will know within a second which way your card wants them.
Palettes outlive a reset
Changed colors survive a soft reset. Reset the machine and the palette you left behind is still there, which is a real surprise the first time BASIC comes back up in the wrong colors. Only a power cycle, or writing $C0 to register 50, restores the defaults.
Getting more than two colors into a tile
The palette is only half of it. The other half is how a pattern picks an entry.
On a stock 9918A the pattern bits are not colors. A 1 bit means ink here and a 0 bit means paper here, and the two actual colors come from somewhere else — the color table, one entry per group of eight patterns, which is why 9918A graphics have that particular blocky, two-tone look.
The enhanced color modes change what a pattern bit means. It stops saying where the ink goes and starts being part of a color number.
| Mode | Bits per pixel | Planes | Colors per tile | In a sprite | Pattern table | Palette becomes |
|---|---|---|---|---|---|---|
| Original | — | 1 | 2 | 1 | 2 KB | 4 of 16 |
| ECM1 | 1 | 1 | 2 | 1 | 2 KB | 32 of 2 |
| ECM2 | 2 | 2 | 4 | 3 | 4 KB | 16 of 4 |
| ECM3 | 3 | 3 | 8 | 7 | 6 KB | 8 of 8 |
Tiles and sprites choose independently — register 49 has a field for each — so you can leave your tiles alone and give only the sprites more colors.
Why a sprite shows one color fewer
Color 0 in a sprite is always transparent. It is not an option and there is no bit to change it. So a three-bit sprite has eight values available and seven of them are colors.
Tiles get to choose. Each tile's attribute byte has a transparency bit that says whether index 0 is a real color or a hole.
Bitplanes
Where do the extra pattern bits come from? Not from packing more bits into a byte — from more copies of the pattern table.
The original 2 KB pattern table stays exactly where it is and keeps meaning exactly what it meant. A second one sits 2 KB further along, a third 2 KB after that. To get a pixel's color you take one bit from the same position in each table and stack them:
plane 3 0 0 0 0 1 1 1 1 the most significant bit
plane 2 0 0 1 1 0 0 1 1
plane 1 0 1 0 1 0 1 0 1 the original pattern table
─────────────────
color 0 1 2 3 4 5 6 7Two consequences, and they are the reason it was built this way.
Your existing patterns still work. Plane 1 is the pattern table you already had. Add a second plane and every tile that used to be two colors is now four, with the shapes unchanged. You can convert artwork one tile at a time.
Three bits per pixel does not fit in a byte. Eight pixels at three bits each is three bytes, and a pixel would straddle a byte boundary. As planes it stays one byte per eight pixels per plane, which is simple in hardware and simple in your drawing code.
The cost is that authoring is harder — you are drawing in separated bit layers rather than in colors — and you will want a tool. The TMS9918 editor understands the original pattern format, which gets you plane 1.
The gap is adjustable
2 KB between planes is the default, not a rule. Register 29 sets it to 2 KB, 1 KB, 512 or 256 bytes, separately for tiles and sprites. If you are only using 64 patterns, 512-byte spacing packs three planes into the space one used to take.
Which of the 64 a pixel lands on
Six bits address a palette register. They come from three places, and the split shifts as the color mode changes:
| Mode | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|
| Original | VR24 PS0 | VR24 PS1 | attr 0 | attr 1 | attr 2 | attr 3 |
| 1-bit (ECM1) | VR24 PS0 | attr 0 | attr 1 | attr 2 | attr 3 | pattern 0 |
| 2-bit (ECM2) | attr 0 | attr 1 | attr 2 | attr 3 | pattern 1 | pattern 0 |
| 3-bit (ECM3) | attr 0 | attr 1 | attr 2 | pattern 2 | pattern 1 | pattern 0 |
Read down the table and the trade is obvious. The more color a pattern carries, the less of the address is left for choosing which palette — so the 64 registers behave as four palettes of sixteen at one end and eight palettes of eight at the other.
- attr is the tile's or sprite's attribute byte: four bits that belong to that one tile or that one sprite.
- pattern is the bitplane data for that pixel.
- VR24 is a global two-bit palette select per layer, and it drops out entirely once the pattern supplies two bits or more.
The practical read: in two-bit color, a tile's attribute byte is a palette number, 0 to 15, and the tile's four colors are that group of four. Change one byte in the attribute table and the whole tile recolors. That is the mechanism behind recolored enemies, damage flashes and level palettes, and it costs one write.
What to do with it
Recolor by writing one byte. Same patterns, different attribute byte, different palette group. An enemy sprite turns from green to red without a single pattern byte moving.
Flash a hit. Write white into the palette register that sprite is using, then write it back three frames later. Two writes, no artwork, and it affects exactly the thing you meant.
Fade the screen. Walk all 64 registers down toward zero over eight frames. Nothing else on the machine changes.
Cycle a color. Rotate a few entries and water moves, a portal swirls, a power-up pulses — the classic 8-bit effect, and here it is nearly free because the palette is registers rather than pixels.
Use palette 1 on purpose. In one-bit color every tile picks a pair of colors out of 64, and the default palette 1 is arranged as black-and-something. Point a layer at it and every tile is one color on black without you setting up anything at all.