TI-99/4A FAQ: Technical Specifications

Technical specifications

from: https://en.wikipedia.org/wiki/Texas_Instruments_TI-99/4A#Technical_specifications


Texas Instruments TMS9900 @ 3 MHz, 16-bit, 64-pin DIP


  • 256 bytes scratchpad RAM for the CPU
  • 16 kB Video Display Processor RAM
  • 32k expansion ram on expansion bus (see memory expansion for more info)
  • 8k expansion ram on cartridge port (see supercart and mini memory for more info)


TMS9918A VDP, 40 pin DIP. The earlier 99/4 uses the TMS9918. PAL systems use the “9929” versions of each.

  • 32 single-color sprites in defined layers allowing higher-numbered sprites to transparently flow over lower-numbered sprites. Sprites are 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 is available in hardware for collision detection, and the console supports automatic movement via an interrupt routine in the ROM. There can only be 4 visible sprites per scan line.
  • 16 fixed colors (15 visible, one color reserved for “transparent” which shows the background color). Transparent is intended for the 9918’s genlock which is disabled in the system.
  • 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 access only 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).
  • 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 obscures a lower layer in hardware, unless that higher layer is transparent.


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 playback

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