Thoughts on Future Expansion/Modifications

External Instructions

The TMS 9900 has an interesting feature where a handful of instructions can be decoded externally and used to trigger various hardware functions. These instructions are: | Mnemonic | A0 | A1 | A2 | A3-A14 | | —- | - | - | - | ———- | | RSET | 0 | 1 | 1 | Don’t care | | IDLE | 0 | 1 | 0 | Don’t care | | CKON | 1 | 0 | 1 | Don’t care | | CKOF | 1 | 1 | 0 | Don’t care | | LREX | 1 | 1 | 1 | Don’t care | | CRU operations | 0 | 0 | 0 | From R12 |

These five instructions implemented various hardware extensions in the TI 990 series of minicomputers.

For the TI CUBE, RSET could be decoded and used to trigger an external hardware reset. The IDLE instruction is typically used by the operating system to pause procesing of instructions while waiting for an external interrupt (I/O or timer). On TI 990 minicomputers this instruction was decoded to drive the front panel “IDLE” LED.

The CKOF/CKOF instructions are used for various implementation-specific functions (some microcomputers such as the Cortex used these for enabling/disabling the memory mapper function). The LREX (Load and Restart Execution) instruction was used in some hardware to implement a single-step function.

All of the above could be added to the TI CUBE by decoding the appropriate address lines (A0, A1 and A2) along with the CRUCLK signal using the existing 16V8 PLDs (or additional devices).

RAM Memory Expansion

The TI CUBE memory board decodes the entire address space of the TMS 9900 but this is a maximum of 65536 bytes (64 kiB). A number of other computers based on the TMS 9900 (e.g., Cortex and the TI 990/10 and 990/12) used memory mappers to expand the memory space up to 1 MiB or more of RAM via paging. The primary advantage of this expansion is that would allow running either MDEX or Unix, both courtesy of Stuart Conner who ported them for his Mini-Cortext project. I’d really like to try Unix on the TI CUBE as it might allow a rudimentary mulituser system to be constructed (when equipped with multiple additional serial cards).

Floppy and/or Hard Drive Controller

The next step after the memory expansion described above would be to add non-volatile storage using using a floppy or hard-drive controller. This would also be necessary to support booting the MDEX/Unix systems described above.

Can it run Doom?

The short answer is “no, not even close.” I’m working on an idea for a longer answer that starts with “yes, but…”.