The SKIM microprogrammer’s guide
W. Stoye · CL Technical Reports · 2021
This paper describes the design and implementation of the SKIM microprocessor. The processor has a 24 bit ALU with 16 general purpose registers. The main unique feature is a large microcode store of up to 64K 40 bit words, with the intention that the microcode could be used like the machine code on a conventional processor, with operating system primitives being programmed in microcode. The processor has been constructed from TTL logic, with a microcode assembler running on Phoenix. A debugger for both the hardware and microcode programs runs on the host machine, currently a BBC Microcomputer. The processor architecture is discussed, with examples of microcode programming. Comparisons with other processors are made, and some of the limitations of the present design are noted.