The CPU board of the ORDINATOR houses the 4 Mhz Z80 main CPU of the system,
together with 4KB of ROM memory (2732 type) containing DIAMOND and 1KB
of static 2114 RAM memory. The board also contains the MMU
(Memory Management Unit, consisting of 2 7489 RAM chips totaling 16 * 8 bits) that translates the 16-bit
Z80 logical address bus (capable of addressing 64KB) to the 19-bit physical
address bus (capable of addressing 512KB).
Given the right software, the CPU board is actually capable of
functioning in standalone mode, not needing any other circuit
board from the ORDINATOR bus. However, the only direct I/O capabilities
of the board are the three switches for RESET, NMI and INT and a
few status LEDs. This was used for testing the board originally,
requiring quite a bit of creativity, but in practice at least
a serial I/O board is needed to interface DIAMOND to the outside world.
RESET, NMI, INT and LEDs
The CPU board, being the first board we built, had several problems,
both logical and physical.
The logical problems were easily worked around but sometimes
irritating, being mostly related to the virtual address space switching
and its write protect implementation. The physical problems caused us more
headaches because they sometimes led to system malfunctions, being mostly
caused by the fragile type of wiring used on the board.
