Data General Corporation Nova 840 System

    The Data General Nova 840 is a "big cousin" to the 1200 series machines. It features the same architecture common to all the Nova processors in that it's a 16-bit minicomputer with a 15-bit address space.

    Unlike the DEC pdp-11 line, the Nova architecture makes use of separate memory and I/O busses. Memory is addressed as words; bytes are accessed using byte-swapping logic in the ALU. Like RISC machines of today, Novas operate on a load/store basis.

    The Nova sports three instruction formats: memory access, arithmetic/logic, and I/O. The memory access class of instructions included load/store of accumulator values and program flow control (jump, jsr, &c), arithmetic/logic instructions perform computational functions, and I/O instructions manipulate various peripherals and CPU states. If one "listens" very carefully, faint "echoes" of DEC's PDP-8 instruction set architecture may be "heard". I've made available here a document detailing the Nova basic instruction set .

    Construction of the Nova 840 is standard Data General practise for the era: a printed circuit backplane with wire-wrap pins for connection to the optional peripheral devices; some very common devices (paper tape reader and teletype) have built-in traces on the backplane.

    This machine was sometimes termed an 800 "Jumbo" in that it utilises a 14-slot backpanel. This particular example has an expansion backplane slung beneath the main system unit (visible in the photo just beneath the rails); the expansion is currently inoperative, but does not hinder use of the system.

    The 840 CPU is implemented on two 15 inch square PC boards utilising TTL logic and residing in slots 1 and 2 of the chassis (1 being the bottom slot). One board consists of the data paths and associated computational logic, the other implements the timing and control functions. Unlike the Nova 1200 class (which used a 4 bit "nibble" data path), the 840 has a full width 16 bit ALU. Hardware multiply/divide was an option on this machine, which this specimen posesses.

    The front panel on this specimen is, unfortunately, not the original, the original one having been swapped for the present Nova 1220 panel due to burnt out indicators. Its previous owners thought it easier to swap the panel than to replace the (soldered in) indicator lamps individually. This, of course, hints strongly at the compatibility of certain components across the Nova line, the fact of which is a mixed blessing to those interested in preserving these machines.

    The basic maximum memory size for the Nova line is 32 kw, in the case of the 840, of core memory. Mainstore capacities above 32 kw were made possible on the 800 series via the use of the MAP (Memory Allocation and Protection) option, again which this individual has. The mainstore size on this particular machine is 48 kw contained on six 8 kw boards. The memory modules, while similar to those used on the 1200 series, are not compatible with the 1200 machines.

    This individual is a fully operable system; the attached peripherals are as follows:

• Paper Tape Reader
• Teletype ASR-33
• Two Diablo Model 31 removeable disk drives
• Wangco Mod-10 9 track Tape Drive

    Other options implemented on this system are Line Printer, 2nd TTY, Card Reader, and Real Time Clock.

    The 840 in the collection runs Data General's RDOS operating system at Rev 5.0, FORTRAN and ALGOL compilers, DG Extended BASIC, and of course, assembler. Since this is a mapped machine, it is possible to have two discrete sessions running concurrently (one in the foreground and one in the background) each with its own controlling terminal. The ASR-33 is used as the foreground TTY.

    I have enough spares for this machine to keep it running (hopefully) well into the next century.

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