TMS 9918A Video Upgrade ROM

Developer(s): Fred Nachbaur
Date: 1986
Type: ROM
Platform(s): TS 1000
Tags: ROM

The two EPROMs from the John L. Oliger TMS 9918A Video Upgrade for the ZX81/TS1000, which adds the TMS 9918A video chip used in the TI-99/4A, ColecoVision and MSX: colour, graphics up to 256×192, sprites and lower-case characters. Both were 2764 8K EPROMs on a JLO 8329 board (U1 74LS00, U2 74LS27, U3 74LS00, U4 74LS32) with no apparent changes to the board. One is the replacement system ROM (labelled “9918A VDP LATEST”) and the other is labelled “Printer, ETC @16383”.

The upgrade was published as a construction project: see TMS9918A Video Upgrade For The TS1000 and the two-part TMS9918A Video Upgrade series.

What’s in the download

  • 9918A_VDP.bin: the system EPROM, 8,192 bytes, from socket Ea.
  • Printer.bin: the second EPROM, 8,192 bytes, from socket Eb.
  • Printer_9918A.txt: notes on the board, chips and labels.
  • 9918A.txt: a z80dasm disassembly of 9918A_VDP.bin.
  • Four photos of the JLO 8322/8329 boards, front and back.

Program Analysis

9918A_VDP.bin

This is the Sinclair ZX81/TS1000 ROM with the VDP support added. It differs from the stock ROM in about 620 bytes: short patches throughout the lower 7.5K, which redirect the display and NMI handling to the new code, and a 512-byte block of new code and lower-case character patterns at 1E00–1FFFh, where the ZX81 keeps its character set. It talks to the VDP through ports 3Fh and 7Fh.

Printer.bin

Only the first 516 bytes are used; the rest is blank. It starts with a ZX81-to-ASCII translation table and printer control codes (ESC, line feed, carriage return), followed by a short routine and an upper- and lower-case ASCII table. It appears to be a driver for a full-size printer.

Potential Issues

  • 9918A.txt was disassembled with origin 0100h, but the EPROM replaces the ROM at 0000h, so every address in that listing is 256 too high.

Related Products

Two board, extensive upgrade that integrates the TMS 9918A video display processor into the ZX81/TS1000.

Related Articles

As can be expected with any major hardware upgrade, there are a few conflicts when the Oliger video system is used in conjunction with other equipment. Summarizes the fixes we and others have come up with to date.
The TMS9918A video project consists of two pc boards, a small modification/addition to the main computer board, and a +5, +12, -5 VDC power supply (an expansion board of some type is necessary).
This second installment involves some explanation of the changes to the operating system and how they have been implemented.

Image Gallery

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.

Tags

Downloadable John Oliger Co. TS 1000 ROM