What F18A mode is

There is a second video chip inside your ACE, and it has been switched off since the day the machine was built.

The card in the video slot is a Pico9918. It spends its life pretending to be a TMS9918A, and it is very good at it — 40 columns of text, three graphics modes, 32 sprites, 16 colors, all exactly as the 1979 chip did them. But the Pico9918 also carries a second personality called F18A mode, and when you turn that on the same card gains a second tile layer, hardware scrolling, 64 programmable colors out of 4096, sprites that flip and never flicker, a bitmap layer, and a small processor of its own.

None of it is available until you ask. That is deliberate, and the reason is the best kind of engineering story — it is in Turning it on.

This one needs the real thing

F18A mode exists on hardware and nowhere else. The emulator is a faithful TMS9918A, and a faithful TMS9918A has eight registers, so every instruction in this section either does nothing there or does something you did not intend.

That is survivable, and every program here survives it: the first thing each one does is ask the card what it is, and the plain answer is a perfectly good answer. Write it that way and one program runs on both machines.

What you actually get

The TMS9918A is a chip you program around. Almost every limit below is one that shaped how games looked on it, and F18A mode lifts it.

The chip you have	What unlocking gives you
Four sprites per scan line, and the fifth vanishes	Up to 31, so nothing flickers
One color per sprite	Up to seven, out of a palette chosen per sprite
One color pair per group of eight patterns	One palette per tile, four flags per tile
16 fixed colors	64 registers, any of 4096 colors, changed at any time
One tile layer	Two, each with its own name table and scroll
Scrolling means redrawing the screen	Two registers, one pixel at a time, no redraw
24 rows	24 or 30
Sprites face one way	Flip horizontally, vertically, or both, for free
A frame interrupt	A frame interrupt and one at any scan line you name
The 6502 does all the work	A processor inside the card that does some of it

What that means if you are writing a game

Concretely, in the order you would probably want them.

Scrolling stops being the hard part. On a stock 9918A a side-scroller means rewriting a column of the name table every few frames and living with 8-pixel steps. Here you write one register per frame and the screen moves one pixel. Set the horizontal page size and the card scrolls onto a second name table, so you are writing new tiles into a column that is off-screen and nobody sees the seam. Scrolling and layers does this properly.

Your sprites stop disappearing. The four-per-line limit is the single most recognizable artifact of this chip — it is why the shots in a shoot-em-up strobe. Register 30 turns it off. Set it once, at startup, and forget about sprite multiplexing forever.

Sprites get colors. Not one color each: up to seven, from a palette you chose per sprite. And that choice is one byte, so recoloring an enemy for a second level costs nothing. Colors has the whole scheme.

A status bar stops costing you a layer. Tile layer 2 sits over the top of tile layer 1 with its own scroll registers, which you simply never write. The score, the lives and the mini-map stay still while the world moves under them.

Palette animation comes back. 64 color registers you can write mid-frame means cycling water, flashing a hit, fading a screen to black, and doing it in a handful of writes rather than by rewriting patterns.

A scan-line interrupt splits the screen. Name a line, get an interrupt there, change the scroll register in the handler, and the bottom half of the screen scrolls at a different speed to the top. That is parallax, on a machine from 1979.

Something else can do the work. The card contains a TMS9900 that can be told to run once a frame, or once a scan line, with direct access to VRAM, the palette and every register. Anything you would have burned 6502 cycles on — clearing a buffer, plotting into the bitmap layer, moving a sprite table — can happen inside the card instead. The GPU.

The trade you are making

A program that uses any of this only runs on a machine that has it. That is a smaller problem than it sounds, because detection is six bytes of code and the answer arrives before you have drawn anything, but it is a decision to make on purpose:

• Detect and degrade. Run in plain mode everywhere, and light up the extras where they exist. More work, and the way to write something you can hand to anybody.
• Detect and refuse. Say so and stop. Fair for something built around 30-row mode and a bitmap layer, where there is no lesser version.
• Detect and lie to yourself. Skip the check, write the registers, and hope. On a stock card the unlock attempt alone blanks the screen. Don't.

Find out what you have

Type this in. It is the whole detection procedure, in BASIC, and it will tell you which kind of machine you are sitting at.

10 D = 39936 : R = 39937
20 GOSUB 200
30 GOSUB 300
40 GOSUB 400
50 GOSUB 500
60 IF P = 0 THEN 90
70 PRINT "PLAIN TMS9918A. NO HIDDEN MODE HERE."
80 END
90 GOSUB 600
100 PRINT "F18A MODE IS IN THERE."
110 IF (I AND 8) THEN PRINT "AND IT IS A PICO9918."
120 END
200 REM UNLOCK - $1C TO REGISTER 57, TWICE
210 POKE R,28 : POKE R,185
220 POKE R,28 : POKE R,185
230 RETURN
300 REM SIX BYTES OF GPU CODE TO VRAM $3F00
310 POKE R,0 : POKE R,127
320 FOR N = 1 TO 6 : READ B : POKE D,B : NEXT N
330 RETURN
400 REM POINT THE GPU AT $3F00 AND START IT
410 POKE R,63 : POKE R,182
420 POKE R,0 : POKE R,183
430 RETURN
500 REM READ THE FIRST BYTE BACK, THEN TIDY UP
510 POKE R,0 : POKE R,63
520 P = PEEK(D)
530 POKE R,0 : POKE R,134
540 POKE R,31 : POKE R,135
550 POKE R,208 : POKE R,129
560 RETURN
600 REM ASK THE CARD WHAT IT IS
610 POKE R,1 : POKE R,143
620 I = PEEK(R)
630 POKE R,0 : POKE R,143
640 RETURN
900 DATA 4, 224, 63, 0, 3, 64

On a plain machine, or in the emulator:

PLAIN TMS9918A. NO HIDDEN MODE HERE.

On an ACE whose card has the enhanced firmware:

F18A MODE IS IN THERE.
AND IT IS A PICO9918.

What every line of that is doing is the next chapter.

Where this comes from

The F18A is Matthew Hagerty's FPGA replacement for the TMS9918A, designed for the TI-99/4A and adopted by most of the machines that used the original chip. The Pico9918 is Troy Schrapel's re-creation of a 9918A on a Raspberry Pi Pico, and it implements the F18A's enhanced feature set as well.

Two things follow from that, and they are worth knowing before you trust anything on these pages.

The Pico9918 is not an F18A. It reports itself differently on purpose, it adds registers and about 40 KB of memory the F18A never had, and it leaves out the F18A's serial flash chip entirely. Where the two differ, these chapters say so, and describe the card an ACE actually has.

The documentation for this is genuinely thin. What exists is a register spreadsheet, a set of forum posts from the years the F18A was being designed, and the Pico9918's own reference. They do not entirely agree — several registers moved late in the F18A's development, and the forum posts describe a few features that never shipped. Where the sources conflict, these chapters say which one they followed and why, rather than picking one quietly.

The chapters

Turning it on	The unlock, the detection probe, and the damage both do to a card that turns out to be ordinary
Colors	64 palette registers, and the bitplane scheme that gets more than two colors into a tile
Sprites	Flipping, sizing, coloring, and the end of the four-sprite limit
Scrolling and layers	Two tile layers, pixel scrolling, pages, and how a split screen is done
The bitmap layer	A pixel-addressed layer that floats over the tiles
The GPU	The TMS9900 in the video card
Every register	All of them, with their bits