Classic computing fanatic Jeff Truck has constructed a microcomputer across the RCA 1802 “COSMAC,” notable as the primary business microprocessor to make use of a complementary metal-oxide semiconductor (CMOS) course of — whereas Truck’s host for the machine extends issues by including a reminiscence administration unit (MMU) to develop its deal with house all the way in which to a hefty 256kB.
“I spent a few yr on this construct,” Truck says of his newest creation, “and I feel the time spent considering issues out and designing it out, the endurance with having to scrap present concepts and provide you with new ones, I feel lastly paid off for me.”
The system in query is constructed across the RCA 1802 COSMAC, an eight-bit microprocessor launched in 1974 in a 40-pin DIP package deal. Its largest declare to fame, other than having pushed the COMSAC ELF equipment laptop at a time when most rivals have been choosing the MOS 6502 or Zilog Z80, is being the primary business microprocessor to modify from the N-type metal-oxide semiconductor (NMOS) course of to CMOS — therefore its title, the Complementary Symmetry Monolithic Array Pc.
As an eight-bit processor with 16-bit addressing, the COSMAC helps 64kB of RAM — beneficiant for the period — however whereas Truck’s earlier builds had adhered to this restriction, his newest opted for extra by including one thing lacking from earlier designs: a reminiscence administration unit (MMU).
“You would possibly ask your self ‘how within the heck can Jeff entry 256k of RAM from a 16-bit deal with bus?’ [Well], here is what I spent nearly all of time on over the winter: it is what I name my reminiscence supervisor unit, or MMU. I am most happy with this machine right here. I’ve spent fairly a little bit of time writing out design after design after design, emulating exercise in my very own thoughts — as a result of, in fact, nothing prefer it existed on the time, and I am happy to say the primary era of this board labored on day one.”
The MMU board robotically handles paging banks of reminiscence out and in as required. (📷: Jeff Truck)
The presence of the MMU implies that banks of reminiscence could be paged out and in as required, extending the pc’s deal with house nicely past the 64kB for which it was designed. “I can inform it, for every register, what web page or financial institution of reminiscence that it wants to modify to when that register’s getting used,” Truck explains.
“Via a program I can say ‘hey, when Register 11 is accessing reminiscence I need you to make use of Financial institution 01, when Register 12 is utilizing reminiscence I need to deal with, say, Financial institution 02 — utterly independently of each other — and this factor will swap the banks, primarily by toggling [these] two extra deal with traces.”
A video detailing the construct is on the market on Truck’s YouTube channel, whereas schematics, Gerbers, and microcode, plus 3D print information for mounting {hardware} and the entrance panel, can be found on GitHub underneath an unspecified open supply license.
