TMS9918

The TMS9918 is a video display controller (VDC) manufactured by Texas Instruments and introduced in 1979.^[1] In TI's manuals it's called a "Video Display Processor" (VDP). The TMS9918 and its variants are used in the Casio PV-2000, Coleco Adam, ColecoVision, CreatiVision, Hanimex Pencil II, MSX, Memotech MTX, NABU Personal Computer, PECOS, SG-1000, SC-3000, SV-318, SV-328, Sord M5, TI-99/4, Tatung Einstein, and Tomy Tutor.

VDP TMS9918A

VDP TMS9918A

The TMS9918 was an advanced design for the era. It had four graphics modes based on an underlying 256 wide by 192 high pixel layout, which could be used as a 40 by 24 character display, a 256 by 192 pixel display, or a 64 by 48 pixel display which used less memory. It had a fixed color palette with 15 colors plus transparent. It also supported 32 single-color 8x8 or 16x16 pixel sprites, numbered 0 to 31, with lower-numbered sprites appearing on top of higher-numbered ones. This allowed for layering and perspective effects. It also featured a video input and sync pin, which allowed video from another source to appear under the graphics being created by the chip. This could be used to layer the output from multiple 9918's, or to use the 9918 as a system for titling and overlays on other video sources.

To support this advanced feature set, the 9918 required fast access to memory. To allow this, it was given a dedicated 16 kbyte pool of RAM that was separate from the main memory of the computer that it was part of. TI referred to this as VRAM. This meant that the CPU could only update the video display over a separate 8-bit data bus with the VDP moving data on the bus to and from its dedicated RAM. This limited the speed at which the CPU and 9918 could communicate. It also meant that the common solution of using the video chip to refresh memory could not be used and had to be implemented in separate hardware. While these represented minor complications compared to systems that shared memory in a single pool, the result was a display that was much more colorful and offered some parity with arcade game-level displays.

Description

General information

The TMS9918 was packaged in a 40-pin dual in-line package (DIP). Power was provided as +5V on pin 33, V_CC and ground on pin 12, V_SS. For use with NTSC, a quartz crystal running at 10.7386 MHz, three times the NTSC colorburst frequency, was connected across pins 4, XT1, and 39, XT2. Video output was on pin 36, COMVID, and input, if used, on pin 35, EXTV, with their connector's shield connection connected to ground.^[2]

In order to achieve the memory performance needed to support full-color operation, the system used a dedicated area of DRAM they referred to as VRAM. To operate this memory, the system included a separate, private, set of pins for an 8-bit address bus on pins 3 (AD7) through 10 (AD0) and an 8-bit data bus on pins 25 (RD7) through 32 (RD0), supported by a read/write control on pin 11, and RAS and CAS on pins 1 and 2.^[2]

Communications with the host computer were through a separate data bus on pins 17 through 24, D7 to D0. The CPU communicates with the VDP through an 8-bit bus. Pin 13, MODE, controls whether the data on that bus is an address or data. To write to the TMS9918, the CPU first sets the mode and writes data to an internal address register, then writes the data to the bus and changes the mode to cause the data to be deposited into that address. After the data byte is read, the TMS9918 increments the pointer in the address register, allowing a series of bytes to be written in succession.^[3]

Graphics modes

The graphics of the 9918 were based on an underlying 256 wide by 192 high pixel layout.^[4] These numbers are typical for systems of the era which used analog video signals for output in a progressive scan display. This is close to the maximum resolution achieved on television sets of the era, which was around 320 by 240, due to the complexity of the NTSC radio-frequency signals.^[a]

The TMS9918 logically arranges graphics in a number of layers, 32 sprite layers on top, a graphics or text layer, a "backdrop" with a single solid color filling the screen, and finally an external video layer. The backdrop plane is slightly larger than the others, so it completely fills the screen. The system uses a 15-color palette, along with color 0 which is transparent. At any pixel location on the screen, the system sees if a particular object is in that location, and whether or not it is set to transparent. If it is transparent it looks at the next object, and so on, until it reaches the external video layer (which is often not used).^[4]

There were four ways the graphics layer could be used. Text Mode broke the screen into a 40 by 24 layout of "blocks", each 6 pixels wide and 8 high. Each block could hold a value from 0 to 255, normally representing an expanded ASCII character set. The patterns for each of the 256 characters was stored in VRAM and could be changed by loading the VRAM values using the CPU. Each line of the characters was defined as a series of "on" or "off" pixels stored in one byte per line, and 8 bytes per character. Because the blocks were 6 by 8 on the screen, it was up to the designer to lay out the characters in a 5 by 7 grid to leave space between them. The color for the "on" and "off" bits in the patterns could be any of the 15 colors available, and stored in VRAM.^[2]

Graphics I mode was essentially a modification of Text Mode that broke the screen into a 32 by 24 layout, so that each block was 8 by 8 pixels instead of 6 by 8. The pattern for any given pixel on the screen was looked up from the same 256-entry table of bit patterns as the one holding characters in Text Mode. As there were 768 possible locations and only 256 possible patterns, this mode was limited in the sorts of displays it could produce. Graphics I also added a separate section of VRAM that held the color values for the on and off pixels for any given block. This meant every block of 8 by 8 pixels could have its own two colors, but the pixels within any block all had to be the same.^[5] This is not an uncommon solution for the era, and leads to a well-known problem known as attribute clash when it is not possible to choose colors that to not result in the edges of the cells being visible.

Graphics II mode was a further modification of Graphics I, allowing separate patterns for all 768 blocks, as well as separate color settings for each line within the blocks. This meant any block could use all 15 colors, although only two per line. This display allows any image to be drawn and more colors to be displayed, at the cost of using about 12 kbytes of VRAM to hold all of the data.^[6]

Finally, Multicolor Mode broke the screen into 64 by 48, for a total of 3072 blocks. There were no patterns in this mode, the entire cell was either on or off. There was a color table entry for each block, so every block could select its own color. This produced a low-resolution but high-color display using less than 3 kbytes of VRAM.^[7]

One limitation of the system is that it did not include hardware fine scrolling support, and could only scroll along cell boundaries, normally 8 lines high. Additionally, as all of the graphics modes were essentially character modes with custom characters in each cell, patterns that spanned cells, line lines, had to calculate the pixels for each line of any given cell and upload those values into the character sets.

Sprites

A key feature of the 9918 series was its powerful sprite support. It used the same bit pattern layout as the character storage to define 8 by 8 pixel sprites, but also had a second mode with 16 by 16 pixel sprites.^[8] Additionally, either model could be magnified two times to make the spites larger on the screen.^[5] In addition to the 8 or 32 byte long entries holding the patterns, the sprite generator table (SGT), there was a separate sprite attribute table (SAT) that defined how they were used. This included 8-bit values for the horizontal and vertical position of the sprite, a pointer to the start of the data for that sprite in the SGT (which they called the name), and a color code in the lower four bits, for a total of 4 bytes per sprite for a total of 128 bytes in the SAT.^[8]

To determine what color to display at any given pixel on the screen, the system looked to see which sprites were visible at that location, and then searched down the patterns until it found the first one that held a 1 in that location, then drew that color at that pixel. This caused the sprites to be displayed in order, such that lower numbered sprites appeared on top of higher numbered ones. That meant, for instance, that sprite 0 always appeared on top. If there was no sprite on the screen at that location with a 1, it would draw the background instead.^[9]

As the sprites were drawn in order, it is sometimes necessary to change that order to produce the desired layering effects. This is the purpose of the "name" field entries in the SAT. For instance, if one wants to change the display so sprite 10 appears in front of 5, the values in the name field entries of the two can be swapped, avoiding the need to change the actual pattern data. Through careful arrangement of the sprite ordering, the system can easily produce pseudo-3D and parallax scrolling effects.^[10]

Due to performance issues, only four sprites can be displayed on any one scan line. If more are specified on a given line, they will simply not be displayed. It is up to the programmer to design their system to ensure this limit is not reached.

External video

Another key feature of the 9918 series was its ability to read an external video source and pass that signal through to the output assuming all the data above it is transparent. This allows the system to produce graphics which could then be overlaid on an analog source like a video camera or videotape. To make sure the two displays line up, the clock from the external source needs to be fed in as well, replacing the clock signal that would normally be generated by a quartz crystal.^[11]

Colors

The TMS9918 family chips used a composite video palette. Colors were generated based on a combination of luminance and chrominance values for the TMS9918A and Y, U (B-Y) and V (R-Y) values are for the TMS9928A/9929A.^[12]

Datasheet values

The TMS9918 has a fixed 16-color palette, composed of 15 displayed colors and a "transparent" color, color 0.

• When "transparent" is used for sprites, it will show the graphics plane.
• When "transparent" is used for the text/graphics plane, it will show the backdrop.
• When "transparent" is used for the backdrop, it will show the external video signal (if enabled) otherwise black.

According to "Table 2.3 - Color Assignments" on the datasheet outputs levels are the following:^[12]

Color code	Color	TMS9918A		TMS9928A/9929A
		Luminance	Chrominance	Y	U	V
1	black	0%	-	0%	47%	47%
2	medium green	53%	53%	53%	20%	7%
3	light green	67%	40%	67%	27%	17%
4	dark blue	40%	60%	40%	100%	40%
5	light blue	53%	53%	53%	93%	43%
6	dark red	47%	47%	47%	30%	83%
7	cyan	67%	60%	73%	70%	0%
8	medium red	53%	60%	53%	27%	93%
9	light red	67%	60%	67%	27%	93%
10	dark yellow	73%	47%	73%	7%	57%
11	light yellow	80%	33%	80%	17%	57%
12	dark green	46%	47%	47%	23%	13%
13	magenta	53%	40%	53%	67%	73%
14	gray	80%	-	80%	47%	47%
15	white	100%	-	100%	47%	47%

Notes: Colors are merely illustrative, and were converted from the YUV values (MS9928A/9929A) to sRGB taking into account Gamma correction. SMPTE C colorimetry was not taken into account - see the next section for alternate color conversions.

Product family

All of the ICs in this family are usually referred to by the TMS9918 name, sometimes with an "A" postfix. The "A" indicates a second version of the chip which added new features, most prominently the addition of a bitmap mode (Graphic II).

Texas Instruments TMS9918 Product Family Summary

Chip Variant	Video Out	Video In	Video Frequency	Mode 2 Support
9918	Composite	Composite	60 Hz	No
9918A / 9118	Composite	Composite	60 Hz	Yes
9928A / 9128	YPbPr	(None)	60 Hz	Yes
9929A / 9129	YPbPr	(None)	50 Hz	Yes

The key features of this chip are, as highlighted in a 1980 presentation by Karl Guttag (one of the designers):^[1]

• 256 by 192 full color pixels per screen
• 15 different colors and/or shades
• Non-interlaced color composite video output
• Direct wiring to RAS/CAS type dynamic RAMs
• Automatic refresh of dynamic RAMs
• General 8-bit memory mapped type CPU interface
• CPU accesses RAM via VDP (no need for DMA)
• 32 dynamic characters per screen
• Thirty-two 8×8 patterns per row, 24 rows per screen
• Text mode with forty 6×8 patterns per row
• Multicolor mode with 64 by 48 memory mappable color squares
• External video input and control
• Single supply +5 volt operation
• Standard N-Channel silicon gate technology

TMS9918

The TMS9918 was only used in the TI-99/4; the TI-99/4A and the other computers had the A version VDC.

TMS9918A, TMS9928A and TMS9929A

VDP TMS9928A

The TMS9918A and TMS9928A output a 60 Hz video signal, while the TMS9929A outputs 50 Hz. The difference between "1" and the "2" in "TMS9918A" and "TMS9928A" is that the "1" version outputs composite NTSC video, while the "2" versions (including the TMS9929A) outputs analog Y luminance and R-Y and B-Y colour difference signals. The need for the latter was predominant in the 50 Hz world, including Europe, due to the different video signal standards PAL and SECAM. It was more cost-effective to output Y, R-Y and B-Y and encode them into PAL or SECAM in the RF modulator, than to try to have a different console for every different color standard. The "1" version also features an external composite video input which made it a handy chip to use in video "titlers" that could overlay text or graphics on video, while the "2" version does not.

The original variants of the TMS9918 were depletion-load NMOS and manufactured on a 4.5 μm process; it was one of the first depletion load NMOS chips Texas Instruments manufactured in contrast to the TMS9900 microprocessor which used the older enhancement load NMOS process that required three supply voltages. Due to the large die size and relatively high internal speed, the TMS9918 ran warm enough to necessitate a heat sink; some devices such as the Taiwanese DINA console (a hybrid Colecovision/SG-1000) neglected to install sinks and suffered from malfunctions of the chip. By 1983 Texas Instruments had shrunk the die size to 3 μm which ran cooler and no longer required a sink; MSX machines and the Sega SG-1000 used the newer 3 μm TMS9918 while most Colecovisions had the original 4.5 μm variant (the final run of the consoles produced in 1985 had the newer model TMS9918).

TMS9118, TMS9128 and TMS9129

VDP TMP9118NL prototype

A later variant of the TMS9918 series chips, the TMS9118, TMS9128, and TMS9129, were released in the mid-late 1980s, but were never very popular. The function of one pin is changed, and the mapping of the video memory allows two 16K×4-bit chips to be used instead of the eight 16K×1-bit chips the TMS99xx needs. Otherwise the chips are completely identical to the TMS9918A, TMS9928A and TMS9929A respectively.

Graphics

The TMS9918 has two separate and distinct graphics types: characters and sprites.

Characters

Characters are typically used to create text or background images. They appear behind sprites.

Screen modes

The TMS9918 has a number of screen modes that control the characteristics of the characters.

Documented

There are four documented screen modes available in the TMS9918A (as mentioned before, the TMS9918 lacks mode Graphic 2):

• Mode 0 (Text): 240×192 pixels total, as 40×24 characters, pulled from 1 character set of 256 6×8 pixel characters. The entire character set has a 2-color limitation. This mode doesn't support sprites.
• Mode 1 (Graphic 1): 256×192 pixels total, as 32×24 characters, pulled from 1 character set of 256 8×8 pixel characters. Each group of 8 characters in the character set has a 2-color limitation. For example, the characters "0" through "7" will all have the same color attributes.
• Mode 2 (Graphic 2): 256×192 pixels total, as 32×24 characters, pulled from 3 character sets of 256 8×8 pixel characters. Each 8-pixel-wide line of a character in the character sets has a 2-color limitation. This mode provides a unique character for every character location on screen, allowing for the display of bitmapped images.
• Mode 3 (Multicolor): 256×192 pixels total, as 32×24 "semi-graphics" characters. These semi-graphics are defined in a special set of 256 characters as 2×2 "fat-pixels" per character. Each fat-pixel can have its own color (hence this mode is named "Multicolor"), so this makes 64x48 individually addressale virtual pixels. There are 4×4 pixels in each fat-pixel, but they cannot be addressed individually. This mode is very blocky and rarely used.

Screen Mode 2 details

Technically, mode 2 is a character mode with a colorful character set. The screen is horizontally divided into three 256×64 pixel areas, each of which gets its own character set. By sequentially printing the characters 0 through 255 in all three areas, the program can simulate a graphics mode where each pixel can be set individually. However, the resulting framebuffer is non-linear.

The program can also use three identical character sets, and then deal with the screen like a text mode with a colorful character set. Background patterns and sprites then consist of colorful characters. This was commonly used in games, because only 32×24 bytes would have to be moved to fill and scroll the entire screen.

The challenge of using TMS9918 mode 2 was that every 8×1 pixel area could have only two colors, foreground and background. They could be freely picked out of the 16 color palette, but for each 8×1 area, only two colors could exist. When manipulating the screen in BASIC with the LINE command, one easily could exceed the maximum 2 colors per 8×1 area and end up with "color spill".

Undocumented

Texas Instruments originally only documented the four modes listed above. However the bit that enables mode 2 is more interesting than initially let on. It is best described as a modifier bit for the other modes. Enabling it does three things:^[13]

1. Expands the color table size.
2. Divides the screen horizontally into thirds.
3. Changes two address bits of the pattern and color tables into mask bits, which controls whether each third of the screen has its own pattern and color table or not.

With this in mind, three additional modes are possible. Note that although genuine TMS9918A chips support these modes, clones and emulators may not.

• Mode 0 (Text) + Mode 2 (Graphic 2): Known as Bitmap Text Mode. This mode allows for two-color bitmap images, with no color table. This saves memory, at the expense of a slightly reduced horizontal resolution (text mode has a horizontal resolution of 240 pixels instead of 256 pixels like the graphic modes do).
• Mode 1 (Graphic 1) + Mode 2 (Graphic 2): Known as Half-Bitmap Mode. Texas Instruments actually documented this "undocumented" screen mode in their manual titled "Video Display Processors Programmers Guide SPPU004".^[14] In section 8.4.2, "Playing Games with VRAM Addressing",^[14] they discuss how this mode combines the memory savings of mode 1 with the color detail of mode 2. However, as they go on to say this mode limits the number of sprites that can be displayed to 8 instead of 32. Therefore, the term "undocumented" used to describe this mode is a misnomer. However, because this manual was not widely known, this mode is generally considered to be one of the undocumented modes. Generally, the only reason to use this mode over Mode 2 is to reduce memory consumption.
• Mode 3 (Multicolor) + Mode 2 (Graphic 2): Known as Bitmap Multicolor Mode. This mode is more of a novelty, as it offers nothing beyond what the standard Multicolor mode can already do.

Sprites

Sprites are typically used to create moving foreground objects. They appear in front of characters (tiles).

Modes 1, 2, and 3 can render sprites. There can be up to 32 monochrome sprites of either 8×8 or 16×16 pixels on screen, each sprite with its own, single color. The illusion of multicolor sprites can be created by stacking multiple sprites on top of each other.

There can be no more than 4 sprites on a single scanline; any additional sprites' horizontal pixels are dropped. Sprites with a higher priority are drawn first. The VDP reports in a status register the number of the first dropped sprite. The CPU can get around this limitation by rotating sprite priorities so that a different set of sprites is drawn on every frame; instead of disappearing entirely, the sprites will flicker. This technique is known as sprite multiplexing.

Automatic sprite movement is not handled by the VDP. Instead, in practice, the CPU will pick up on the VDP's vertical interrupt - a standard VDP output, which is triggered automatically once every 50th or 60th of a second (depending on chip variant), at the start of the VBI (vertical blanking interval). The CPU then jumps to a sprite-handling routine in the software, which in turn tells the VDP where to reposition the sprites.

The sprite collision flag is set when non-zero pattern bits of two sprites coincide, even if either sprite has transparent colour. This is useful for triggering more advanced collision detection routines inside the software which can then determine the exact location of the collision and act upon it, as the VDP is itself incapable of reporting which two sprites have collided.

CRT display

^{[original research?]}

Converting Y, R-Y and B-Y to RGB requires considering how Y originated:

 Y = R * 0.30 + G * 0.59 + B * 0.11

This leads to the following formulas:

 R = R-Y + Y
 B = B-Y + Y
 G = (Y - 0.30 * R - 0.11 * B) / 0.59

But for all colors that have no chrominance - thus black, gray and white - R-Y and B-Y are not 0% but all have an offset of 47%. So this offset has to be subtracted from all R-Y and B-Y values at first. Due to the fact that in practice this one step will never be done alone, it's no problem that some results will be negative:

Color code	Color	Y	R-Y	B-Y
1	black	0%	0%	0%
2	medium green	53%	-40%	-27%
3	light green	67%	-30%	-20%
4	dark blue	40%	-7%	53%
5	light blue	53%	-4%	46%
6	dark red	47%	36%	-17%
7	cyan	73%	-47%	23%
8	medium red	53%	46%	-20%
9	light red	67%	46%	-20%
10	dark yellow	73%	10%	-40%
11	light yellow	80%	10%	-30%
12	dark green	47%	-34%	-24%
13	magenta	53%	26%	20%
14	gray	80%	0%	0%
15	white	100%	0%	0%

Next comes the conversion to RGB. All results must be in the range from 0% to 100%:

Color code	Color	R	G	B
1	black	0%	0.0000%	0%
2	medium green	13%	78.3729%	26%
3	light green	37%	85.9831%	47%
4	dark blue	33%	33.6780%	93%
5	light blue	49%	46.4576%	99%
6	dark red	83%	31.8644%	30%
7	cyan	26%	92.6102%	96%
8	medium red	99%	33.3390%	33%
9	light red	113%	53.9492%	47%
10	dark yellow	83%	75.3729%	33%
11	light yellow	90%	80.5085%	50%
12	dark green	13%	68.7627%	23%
13	magenta	79%	36.0508%	73%
14	gray	80%	80.0000%	80%
15	white	100%	100.0000%	100%

The erroneous value of 113% for R of color "light red" might seem to come from a typo within the datasheet and there R-Y must not be greater than 80%. But measuring the output signals of the chip with an oscilloscope shows that all values in the table are correct. So the error is inside the chip and drives the red signal into saturation. For this reason this value is to be corrected to 100%.

Up to that time only cathode ray tubes had been available for computer monitors as well as for televisions, and that these CRTs had a gamma. The TMS9918 series chips had been designed to work with televisions and their CRTs had a gamma of 1.6 (remark: CRTs of Macintosh monitors had 1.8 and the CRTs of PC monitors had 2.2). Digital flat panels do not have gamma. For this reason the colors of the TMS9918 look somewhat pale here as in the first table above. The below table uses the gamma-corrected values, which are (written in hexadecimal):

Color code	Color	R	G	B
1	black	00	00	00
2	medium green	0A	AD	1E
3	light green	34	C8	4C
4	dark blue	2B	2D	E3
5	light blue	51	4B	FB
6	dark red	BD	29	25
7	cyan	1E	E2	EF
8	medium red	FB	2C	2B
9	light red	FF	5F	4C
10	dark yellow	BD	A2	2B
11	light yellow	D7	B4	54
12	dark green	0A	8C	18
13	magenta	AF	32	9A
14	gray	B2	B2	B2
15	white	FF	FF	FF

The used steps are: Round all values to two decimal places, then raise to the power of 1.6 for gamma correction and finally transform the range of values from 0...100 to 0...255.

Specifications

• Video RAM: direct wiring to 4, 8, or 16 KB
• Text modes: 40 × 24 and 32 × 24
• Resolution: 256 × 192
• Colours: 15 colours + transparent
• Sprites: 32, 1 colour, max 4 per horizontal line

Legacy

Texas Instruments' TMS9918A was succeeded by the Yamaha V9938, which added additional bitmap modes, more colorful sprites, a vertical full-screen scroll register, vertical and horizontal offset registers, a hardware blitter and a customizable palette. The V9938 was designed for the MSX2 standard of computers, and later used in a third-party upgrade to the TI-99/4A: the Geneve 9640 "computer-on-a-card".

The V9938, in turn, was succeeded by the V9958 which added some additional high-colour modes and a horizontal two-page scroll register. These chips were used in the MSX2+/turboR systems.

Toshiba made a clone called the T6950 which does not support the undocumented pattern / color table masking feature in graphics 2 mode.^{[15][better source needed]} Later, Toshiba released the T7937A MSX-Engine with a built-in VDP with working masking features. Both VDPs by Toshiba feature a palette which is slightly different (more vivid colors) from the TI VDPs.

The VDP of the Master System game console is an evolution of the TMS9918. This was further evolved into the VDP of the Genesis/Mega Drive game console, which replaces most of the graphics modes from the Master System VDP with more capable versions.

See also

• TMS34010, a 1986 microprocessor with dedicated graphics instructions
• Atari 8-bit computers have comparable graphics hardware introduced the same year
• Motorola 6845, a display controller widely used in 8-bit computers
• Motorola 6847
• Yamaha V9938
• Yamaha V9958
• List of 8-bit computer hardware graphics

Notes

1. And PAL and SECAM as well.