Misuse of word "microcode" 106
 |
I haven't looked at the patents ... what I know about the 68K's internals is mostly based on the computer press articles published when it first appeared. I...
George Neuner
Legend has it that Apple negotiated with Motorola about putting special instructions in microcode for use by Quickdraw. Motorola was reluctant, and about the time that Motorola gave in and decided to let Apple do it, Apple decided not to.
I've never heard that Apple got any 68K chips with WCS, but then I'd never heard of any 68K chips with WCS until George's post. The structure of the 68K control store was not very amenable to WCS. They had microcode and "nanocode". However, contrary to the name, the nanocode was not an interpreter that executed microcode. Rather, it was an arrangement where there were certain groups of fields in the horizontal microcode that often shared common values, so they were collapsed into a smaller PLA called nanocode. There was still only one microcode address that went into both the microcode ROM and the nanocode PLA.
A 68K with WCS would presumably use a single RAM array for microcode and do away with the nanocode concept. Then when the microcode was finalized, a simple automatic tool could transform it into the separate microcode ROM and nanocode PLA.
The internal design of the MC68000 is described in published articles as well as several US Patents, most notably patent 4,325,121 "Two-level control store for microprogrammed data processor".
Eric