Over on YouTube [The 8-Bit Guy] shows us how the TI-99/4A home computer worked.

[The 8-Bit Guy] runs us through this odd 16-bit home computer from back in the 1980s, starting with a mention of the mysterious extra “space” key on its antiquated keyboard. The port on the side is for two joysticks which share a bus, but you can find boards for compatibility with “newer” hardware, particularly the Atari-style joysticks which are easier to find. The AV port on the back is an old 5-pin DIN such as was typical from Commodore and Atari at the time (also there is a headphone port on the front). The other DB9 port on the back of the device is the port for the cassette interface.

The main cartridge interface is on the front right of the machine, and there’s a smaller expansion socket on the right hand side. The front interface is for loading software (on cartridges) and the side interface is for peripherals. The system boots to a now famous “press any key” prompt. (We know what you’re thinking: “where’s the any key!?” Thanks Homer.)

One curiosity is that when the system is waiting for a command the screen background color is a light blue, and when it’s running a command the background color changes to a light green. [The 8-bit Guy] demos some equation calculator software which has support for variables and expressions. In addition to the equation calculator the same cartridge has a version of BASIC (called TI BASIC) and a version of Space Invaders (called TI INVADERS). (Yes, the interface is all uppercase.)

When they were designing the system the TI-99/4A engineers had been considering an 8-bit CPU but they settled on the 16-bit TMS9900 instead. However, much of the board had already been designed for an 8-bit CPU, which lead it to being a bit of a weird hybrid. The CPU only has 15 address lines but it makes up for it by addressing two bytes at a time, allowing it to read up to 64K.

[The 8-Bit Guy] goes on to discuss the computer architecture, the Graphic Programming Language (GPL), and its various BASIC implementations. Also the internals of the cartridges are explored along with the Video Display Processor (VDP) which supported rudimentary graphics mode (32×24 characters with 15 colors and 32 sprites) in addition to a text mode (40×24 characters). The 4-voice sound generator chip was the SN76489, this chip proved to be useful in many other products as well.

[The 8-Bit Guy] finishes his video with a look at the expansion capabilities, which basically just daisy chain off the right hand side. Each of the peripheral devices demands its own power supply too!

If you’re interested in the TI-99/4A check out Persistence Pays In TI-99/4A Cassette Tape Data Recovery and Don’t Mess With Texas – The TI-99/4A Megademo.