| T80 (Z80) CPU in Multicomp [message #8452] |
Sun, 11 April 2021 01:11 |
nealcrook
Messages: 127
Registered: October 2015
Location: UK
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Senior Member |
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TL;DR -> If you use T80 in an FPGA design, go get the latest version from https://github.com/EisernSchild/t80
Going back a few years, there was a lot of interest here in FPGA-based Z80 machines based on Grant Searle's "multicomp" design. The Z80 version used the synthesisable T80 core from opencores.org.
Recently I have been working on a reproduction of an old UK computer (the NASCOM 2) using that same core. When a "known good" CP/M boot loader failed to work I dived down a debug rabbit-hole which ended in the discovery that the T80 "ini" instruction did not work.
ini: read from port, store at (HL), increment HL, decrement B
On T80, HL does not increment. Finding this was "fun" especially as it was part of the floppy disk data load loop and so could not be single-stepped (with a WD2797 there is no data buffer and so data read/write is a real-time activity).
After a couple of hours looking at and simulating the T80 code, I did some internet searching and learned that this was a Known Bug (reported in the bugs list on opencores), and that T80 seems not be maintained on opencores any more. However, there is a maintained version/fork on github at https://github.com/EisernSchild/t80
I had to make one change to my code in order to switch from the opencores version to this version: the input clock port has been renamed from clk_n to clk (there is no change in function, this is just cosmetic).
So, if you are using this code, I suggest that you consider an upgrade.
Neal.
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