Texas Instruments TI-99/4A - Technical Specifications

Technical Specifications

• CPU: TI TMS9900, 3.0 MHz, 16-bit, 64 pin DIP. (Note: the TMS9900 was also available in ceramic housing, but this contributor has never seen it outside of military computers.)
• Memory: 16 kB VDP RAM (Video Display Processor RAM) (expandable to 192 kB with the use of Yamaha V9938 - this was not a standard upgrade option but was a user-designed modification), 256 bytes CPU fast "scratchpad RAM" intended for the TMS9900 processor to maintain register "workspaces". It was also possible to add an 8 kB "supercart" or 32 kB "superspace" cartridge via the cartridge slot which also included the Editor/Assember GROM. This used the cartridge ROM space.
• Video: TI TMS9918A VDP (TMS9918 in the earlier 99/4, TMS9929/9929A in PAL versions, 40 pin DIP. Distinct in being the only chip on the TI motherboard which had a heat sink on all models. Early models also had a heat sink on the clock generator, the TMS9904.)
  • 32 single-color sprites in defined layers allowing higher-numbered sprites to transparently flow over lower-numbered sprites. Sprites were available at 8×8 pixels or 16×16 pixels, with a 'magnify' bit that doubled all sprites' size but not their resolution. A single bit was available in hardware for coincidence (collision detection), and the console supported automatic movement via an interrupt routine in the ROM. There could be no more than 4 visible sprites per horizontal scanline.
  • 16 fixed colors (15 visible, one color reserved for 'transparent' which merely showed the background color. Transparent was intended for the 9918's genlock functionality used in conjunction with TI's Video Controller Card. This feature was demonstrated in October 1999 at an international TI meeting near Stuttgart, Germany. (This would have required a hardware modification to the console itself, as the video input line is not routed on the motherboard.)
  • Text mode: 40×24 characters (256 6×8 user-definable characters, no sprites, foreground and background color only, not accessible in BASIC)
  • Graphics mode: 32×24 characters (256 8×8 user-definable characters, full 15 color palette + transparent (available in groups of 8 through the character table) and 32 sprites (The only mode available in BASIC. Extended BASIC is required for sprites, and can only access 28 of them.)
  • Bitmap mode: 256×192 pixels (no more than two colors in an eight pixel row, full 15 color palette + transparent, all 32 sprites available but interrupt-based motion through the ROM routine is not due to the memory layout, not available to BASIC or the original 9918). Bitmap mode could be arranged in such a way as to use less memory but still provide improved color or improved pattern layout, leading to the popularity of so-called "half-bitmap" modes. In fact these modes were not undocumented modes of the VDP (which fully documented this masking) but simply clever layout of Bitmap mode.
  • Multicolor mode: 64×48 pixels (each pixel may be any color, all 32 sprites are available)
  • All of the above comprise 36 "layers" starting with the video overlay input, then the background color, then two graphics mode layers, then a layer for each of the 32 sprites. A higher layer would obscure a lower layer in hardware, unless that higher layer was transparent.
• Sound: TI TMS9919, later SN94624, identical to the SN76489 used in many other systems.
  • 3 voices, 1 noise (white or periodic).
  • Voices generate square waves from 110 Hz to approximately 115 kHz.
  • Console ROM includes interrupt-driven music list playback.