tapeth: TI-99/4a

tapeth represents the type of computer which was the first my family ever owned. The TI was the machine I first learned to program on, and I have a certain affection for it as a result.

I have a number of users' club magazines for the TI, including some hardware projects for expanding it. The console unit alone is very basic: it uses cassette tape for storage and a TV for a display. You could buy a large peripheral expansion unit for it which permitted the addition of floppy drives and more memory.

Curiously enough for its time, the 99/4a is actually a 16 bit machine, (to be precise, 16bit databus + 16bit address bus + 14bit VDP address bus + 16bit GROM address bus [see below]) using the TMS9900 CPU. Unfortunately, TI crippled the machine with a weird architecture. There are actually only 256 bytes of RAM connected directly to the CPU. Most of the memory of a TI is either VDP RAM or Graphics RAM (GRAM), accessible only one word at a time. VDP RAM can only be accessed via the Video Display Processor; it contains the on-screen image and is also used to store BASIC programs(!). GRAM/GROM is used to store Graphics Programming Language (GPL) programs. These are interpreted by a GPL interpreter residing in real CPU ROM. Believe it or not, the BASIC interpreter is a GPL program. This means that it must in turn be interpreted by the GPL interpreter, and the whole machine is correspondingly slow...
A console contains:

A cartridge can contain up to 8K of CPU ROM and 30K of GROM (although the Extended BASIC cartridge has 12K of CPU ROM accessed via bank switching).
The optional memory expansion provides 32K of CPU RAM.
The ExtBas cartridge may also have some CPU RAM for storing ExtBas programs.

Much of this information is from an article by R.A. Green in TI*MES issue 17.


tapeth was originally owned by a friend's family. Like the one I used to own (which is unfortunately no longer in existence) it was bought for a very low price in 1984, immediately after Texas announced that they were leaving the home computer market and would no longer market the 99/4a.


View of Main PCB

main PCB
This is a (rather poor) shot of tapeth's main PCB. The only really obvious feature is the TMS9900 CPU, which is the big IC in the middle. The 40 pin IC with the heatsink gunk is video-output related, and the one to its right is a TMS9901. The white bars are insulated metal ground busbars.

Exterior View

view of case
Note the cartridge slot to the right. The TI has a bigger, more solid feel than many of the 8bit machines available at the time (compare it with hallath and tegath, for instance).
This page written by Peter Maydell (pmaydell@chiark.greenend.org.uk).