Making RAM For A TMS9900 Homebrew Computer

Over on YouTube [Usagi Electric] shows us how to make RAM for the TMS9900.

He starts by remarking that the TI-99/4A computer is an excellent place to start if you’re interested in getting into retro-computing. Particularly there are a lot of great resources online, including arcadeshopper.com and the AtariAge forums.

The CPU in the TI-99 is the TMS9900. As [Usagi Electric] explains in the video this CPU only has a few registers and most actual “registers” are actually locations in RAM. Because of this you can’t do much with a TMS9900 without RAM attached. So he sets about making some RAM for his homebrew TMS9900 board. He uses Mitsubishi M58725P 16 kilobit (2 kilobyte) static RAM integrated circuits; each has 11 address lines and 8 data lines, so by putting two side-by-side we get support for 16-bit words. Using six M58725Ps, in three pairs, we get 6 kilowords (12 kilobytes).

He builds out his RAM boards by adding 74LS00 Quad 2-input NAND gates and 74LS32 Quad 2-input OR gates. Anticipating the question as to why he uses NAND gates and OR gates he explains that he uses these because he has lots of them! Hard to fault that logic. (See what we did there?)

After a quick introduction to the various animals in his household [Usagi Electric] spends the rest of the video assembling and testing his RAM. For testing the RAM with full feature coverage a simple assembly program is written and then compiled with a custom tool chain built around a bunch of software available on the internet. Success is claimed when the expected trace is seen on the oscilloscope.

Of course we’ve seen plenty of TMS9900 builds before, such as with this TMS9900 Retro Build.