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The other day I came across one of Grant Searle's projects that I had not noticed before: his "multicomp" project. It is a simple, low cost FPGA project, where he has more or less created a "software breadboard" for prototyping various retro computers. Googling for it shows various successful builds, including those with ready made PCB's, such as: https://www.retrobrewcomputers.org/forum/index.php?t=msg&th=111&start=0& (but there are several more such efforts). Maybe it is a nice idea to extend this idea to the 99xx world and create a few components to make it easy to prototype various small 99xx systems. The main thing missing in Grant's setup is the CRU bus and VHDL for CRU based chips. Probably the 9900 CPU core that 'speccery' did for last year's retro challenge can be reworked into a component for such a project without too much effort. Other chips to be added would be the 9902 and the 9901. As I have a need for a 9902 in VHDL for another project, I thought I might as well attempt to write one. Looking at he datasheet (figure 2 on page 3) it is not all that complex. There seem to be: - 6 plain registers - 2 shift registers - 3 counters - 3 controllers / FSM's The logic for the controllers is documented in flow charts on page 16, 18 and 19. I'm guessing it will be about 1,000 lines of VHDL? I'll post my public domain source as I go along. Peer review certainly welcome. PS : one guy did an interesting VHDL project around recreating the 9902.