TMS9918

TMS9918 chip

General Information

The Texas Instruments TMS9918 videochip was used in systems like MSX, Coleco Vision, TI-99 and Sega SG-1000/SC-3000. There are several variants called TMS9918A, TMS9928A and TMS9929A, where the 'A' indicates a second version of the chip which added new features, most prominently the addition of the Graphic II mode. The non-A version was only used in the TI-99/4, the TI-99/4A and the other computers did have the A version VDP. The TMS9918A and TMS9928A output NTSC, while the TMS9929A outputs a PAL signal. The difference between the TMS9918A and TMS9928A is that the '1' version outputs NTSC composite, while the '2' versions (including TMS9929A) basically outputs a form of RGB. All these chips are usually generically referred to as TMS9918 (sometimes with an A postfix).

The TMS9918A got its successor in the form of the Yamaha v9938, which added bit map modes, more colorful sprites, a vertical scroll register and a customizable palette. The v9938 in turn was succeeded by the v9958, which added some high colour modi and finally a horizontal scroll register as well. These chips were only used on the MSX 2 and MSX 2+/turboR systems, although rumor says the v9958 was also used in a generation of the Photo Play arcades. Yamaha also produced a v9990, which is considered the follow-up of the v9958 by some, but it is not backwards compatible. A graphic chip extension utilizing the v9990 exists for the MSX in the form of the 'Graphics9000' cartridge by Sunrise (http://www.msx.ch/).

Interface

The TMS9918 has its own 16kB of video memory, outside the address space of the CPU. Data is transferred by writing bytes to a port using the "OUT" command. As a byte is written, the TMS9918 increments its internal address register. The disadvantage is that for random access, the CPU has to write 2 additional bytes to set the address register. Some advantages of this approach are that the video framebuffer reads don't slow the system memory, and that the video address space is not subtracted from your valuable 8 bit CPU 64 kB addressing space, so you can have a 80 kB setup without getting into banking issues. When copying a block of data, all the CPU has to do is use two initial "OUT" instructions to set the video memory access address of the VDP. So no additional CPU address register is required, nor the incrementation thereof. Data transfer is roughly 2 kB per PAL video frame (100 kB/s).

Screen Modi

There are 4 different screen modi available in the TMS9918A (as mentioned before, the TMS9918 lacks mode Graphic II):

Text: 40x24 characters monochrome. As the display is 256 pixels width, the character set is only 6 pixels wide. This mode doesn't support sprites, nor a separate border colour setting.

Graphic I: 32x24 characters (256x192 bitmap), where for each 8 characters in the character set the foreground and background colour can be set. The chars "0"-"7" for example all have the same attributes.

Graphic II: 32x24 characters (256x192 bitmap), with a 2-color limitation for each 8 pixel wide line inside a character. More about mode 2 below.

Multicolor: 64x48 mode, very blocky and rarely used. Each 'pixel' can have its own colour defined though, hence the name. Its sprites still have the same resolution as in screen modes 1 and 2.

The TMS9918 has a 16 color palette, which is hardwired. e.g. colour 4 is always dark blue.

Sprites

There can be 32 monochrome sprites of either 8x8 or 16x16 pixels on screen, each of which can have its own color. However sprite bandwidth is limited to max. 4 sprites per scanline. I.e. if there are more than 4 sprites next to each other horizontally, then the rest will disappear. In that case only the sprites with the highest priority (lowest position in the spritelist) are shown. If necessary, games often attack this problem by rotating sprite priorities. This way, every video frame a different set of sprites disappears in areas where there are more than 4 per scanline. Instead of disappearing entirely, the sprites will flicker (sprite multiplexing). This is a common technique, for example the Atari 2600 had max. 2 sprites per scanline and still the Pac-Man faced 4 ghosts (which would start to flicker as soon as more than one was on the same scanline. One hardware sprite was used for the ghosts, the other for the Player, which didn't flicker).

There is a status bit "5th sprite" that tells that one or more sprites disappeared due to bandwidth overload. By checking this bit, software can determine whether it needs to enable the abovementioned technique.

There also is a sprite collision flag, which becomes set when two sprites overlap. This is useful for triggering more advanced collision detection routines inside the software which can then determine the exact location and act upon it (for instance show 'Game Over' because Pac-Man got eaten by a ghost).

Konami games such as Nemesis had the priority rotation always active (as can be seen when getting an "option", flying into it (with no further sprites positioned in the affected scanlines), and then pressing F1 for Pause). Method: A sprite position ring buffer is held in main memory. Up to 32 of the leading entries are copied to TMS9918 memory. Next video frame, the start position of the ringbuffer is incremented by 4 entries (this value leads to minimum flicker on a chip that can do 4 sprites per scanline). If the ringbuffer got more than 32 entries, then even without 5th sprite condition (more than 4 sprites per scanline), sprites start to flicker. But even more than 32 sprites are visible, i.e. the max 32 sprites onscreen limit is broken the same way as the max 4 sprites per scanline limit. It can happen that rotation of the list is nessesary even when no 5th sprite condition occurred (when there are more than 32 enemies), and it can happen that objects collide without the videochip noting it (because the sprite multiplexer displayed them in distinct video frames), which is why an advanced game engine does not use those hardware flags.

Screen Mode 2 Detail

Technically mode 2 is a character mode whose charset is colorful. The screen is vertically divided into three areas which each got its own character set. Each such area is 256x64 pixels size. I.e. 256 characters of 8x8 pixels exactly cover one such area. By sequentially printing the characters 0 through 255 in every area, you get a graphics mode where each pixel can be set individually.

However, you can also use three identic character sets, and then deal with the screen like a text mode with a colorful character set. Background patterns and big enemies then consist of colorful characters. This was commonly used in games, because to fill/scroll the entire screen, only 32x24 bytes had to be moved. Games on other home computers such as the Commodore64 also worked on a character basis. The graphics have to be drawn such that the 8x8 pixel borders are not too obvious, an art where Konami was particularly well known for their excellence.

This is the TMS9918 screen mode 2 challenge: every 8x1 pixel area has two colors, foreground and background. They may be freely picked out of the 16 color palette. But within each 8x1 pixel area, only two different colors can exist. When manipulating the screen in BASIC with the LINE command, one easily could exceed the limit of max 2 colors per 8x1 area and end up with "color spill". It is a big challenge to the graphics artist. Cheap game conversions from other computers usually lead to monochrome games, whilst Konami, 'the master of 9918', pixeled wonderfully around the 8x1 limitation. In turn, you faced a 256x192 16 color screen, which looked closer to "16 bit" home computer backgrounds than the usual blocky 160x200 resolution of "8 bit" homecomputers.

In comparison, the Commodore 64 limit was 4 colors per 4x8 fat-pixel area. So there was less local color pressure, but more global color pressure: only one of the 4 colors actually could be freely picked out of 16, the other 3 had to be the same over the entire screen. Globally

the graphics were less colorful (although the 3 colors could be redefined every scanline to make a rainbow effect ("copper bars") and similar).

So the TMS9918 was suited for colorful 256x192 resolution backgrounds, where a high fraction of the 16 colors were actually used. But it had one major drawback: it was missing scroll registers as found in C64. The screen had to be scrolled in 8 pixel jumps, or by manipulating the character tables.

Tradeoffs Used in Games

Some games tried to get around the 8 pixel scroll limitation, by scrolling the character set itself (and because this is not possible speed-wise, actually the character set was filled with 8 times the graphics, each one in a different position, which cut down the number of actually usable characters by a factor of eight).

Circus Charlie (MSX) scrolled horizontally, and hence bumped into the maximum of 2 colors per 8x1 area limit. The graphics were "monochrome-ish" and there were some glitches halfheartedly covered by sprites. Yet, a game smoothly scrolling horizontally on the TMS9918 is quite exceptional.

Pippols (MSX) scrolled vertically. Because this isn't affected by the 8x1 area limit, you walk along smoothly scrolling colorful flowers etc. However there aren't many distinct objects onscreen, because the character set is cut down by factor 8. Pippols seems to be even below that limit by some factor. Unfortunately Konami never further explored this topic. By using a background pattern of a 8x4 size, you could even have a parallax scroller.

When you are at full speed in Road Fighter (MSX, vertically scrolling racing game), the screen moves 8 pixels each frame. This results in a smooth scroller in spite of the 8 pixel jumps. For many games this scrolling speed is not feasible, though. A creative approach could be seen in Ghost'n'Goblins (Schneider/Amstrad CPC): as you walk near the border, the scene scrolls quickly (8 pixels/frame) until the player is near the other side of the border. Then you walk for a while with no scrolling at all. A perfect solution for this type of game.

In Knightmare (MSX), the scene scrolls vertically so slowly that the 8 pixel jump doesn't disturb much. Zanac scrolls vertically fairly fast, but due to the soft backgrounds which you never collide with it is a minor issue. It is most problematic in Nemesis ("R-Type style" horizontal space shooter). On the other hand, the Nemesis 2 backgrounds are really of "16 bit" beauty.

Last but not least there is Penguin Adventure (MSX). It does scroll "3D", "Outrun style" (the scene aproaches from ahead). The character set is defined to hold objects in multiple zoom levels. The lack of scroll registers plays no role here, and what goes on on screen is incredible considering an "8 bit" computer. Here, the TMS9918 is at its best. Penguin Adventure also is very much worth playing due to Konami gameplay/humour, but that is another story.