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A project log for RC2016/99: TI-99/4A clone using TMS99105 CPU

Retrochallenge 2016/10 entry and winner: try to build a TI-99/4A clone using a TMS99105 CPU and an FPGA. In a month...

Erik PiehlErik Piehl 11/25/2016 at 09:370 Comments

I posted to youtube a new video showing the system running RXB 2015. I haven't had much time work on the design, but the little time I had I've been working on the disk support subsystem. Most of that code runs on the PC, and communicates with the system over USB. I'm starting to understand how the TI-99/4A disk system works.

In addition I remapped the memory of the FPGA system, so that the paged memory extension on the TI-99/4A clone now has access to 512K of RAM, up from the previous 256K.

I guess I need to try the game more on the classic99 emulator as I don't know if the game runs properly... At the end of video I got stuck... Sorry the video is pretty boring as I did not have time to work on it either. During loading you can see the sprite bugs in the top left hand corner - my VDP implementation still doesn't properly process the "no more sprites" code (vertical coordinate set to >D0), so it will spit out extra sprites. Sprite size doubling is not supported by the hardware, as can be seen from the size of the loading text.

From a technical point of view to get this far the system requires support for SAMS memory and the following disk subsystem opcodes: load, open, read, close. While they seem trivial, they are not exactly that. I went ahead and blatantly copied a bit of code from classic99 (the file buffering code)...

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