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Standardized control unit and microcode layout

A project log for Microcoding for FPGAs

A microcode compiler developed to fit into FPGA toolchain and validated to develop CPU and text-based video controller

zpekiczpekic 05/30/2020 at 18:011 Comment

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zpekic wrote 06/07/2020 at 18:11 point

Complex digital circuits can be described in different ways for the purpose of (re) creating them in FPGAs. One way that was curiously absent is the practice of microcoding. Looking at the history of computing in the last 70 years, this approach has been very popular for all sorts of devices from custom controllers to CPUs. This article describes the history of microcoding and its applications very well:

https://people.cs.clemson.edu/~mark/uprog.html

Coming to the era of particular interest to retrocomputing hobbyists (60, 70ies and 80ies), microcoding was extremely widespread technique. Most minis and mainframes of the era used it,for example PDP-11:

https://ia801908.us.archive.org/12/items/bitsavers_decpdp1111codeListingApr81_5149506/EY-C3012-RB-001_Microcode_Listing_Apr81.pdf

When the microprocessor revolution started, some of the early 8-bit CPUs were using "random logic" to implement their control unit (6502, Z80, 1802), but in order to build something more flexible and faster, microcoding was the only game in town. One could almost say that the microcoding was the standard "programmable logic" way of the day, just as today FPGAs are.

One company in particular made fame and fortune using microcoding: AMD. The Am29xx family of devices was the way to create custom CPUs and controllers, or re-create minis from previous era and shrink them from small cabinet to a single PCB. Alternatively, well-known CPUs could be recreated but much faster. For example:

https://en.wikipedia.org/wiki/Microcodehttps://en.wikichip.org/w/images/7/76/An_Emulation_of_the_Am9080A.pdf

(note: based on the well documented design above, I coded it in VHDL and got 8080 monitor to run, see link in main project page)

Once the complexity of single - chip CPUs rose, microcoding again gained prominence, and is present from the first iterations of 68k and 8086 processor families until now (for example, description of 68k microcode: https://sci-hub.st/https://doi.org/10.1145/1014198.804299 )

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