S100 Computers - Dual Systems 68000 CPU Board

The board ran with a 8MHz clock and could address up to 16MG of RAM address space (24 bits). It met all IEEE-696 S-100 bus specifications. No wait states were requires for 4Mhz type RAM boards.

They had a second board called the CPU-68000M which also ran at 8MHz but also included the Motorola MC68451 Memory management Unit LSI chip. This allowed sophisticated segmented memory management. There was also a CPU-68000M-L10 which was identical but ran at 10MHz.

There were onboard 3 LED's. RUN (green), HALT (red) and HOLD (yellow). As per the IEEE-696 specs the phantom lime could be put on the bus during power on.

Port IO addressing and the 68000.
The 68000 instruction set does not have an explicit Input/Output instruction. Motorola architects intended for all 68000 I/0 to be memory mapped. Memory mapped I/0 takes advantage of the many powerful addressing modes of the 68000 for fast, efficient I/0 routines.

To support S-100 I/0 mapped peripherals and to allow communication between the 68000 and the 68151, Dual dedicated a 64 kilobyte page of the address space for I/0 and the MMU registers. This page could be based in the physical memory space at one of two following addresses:

I/0-MMU page:-    1)    7F0000 to 7FFFFF (L0)
                           2)    FFOOOO to FFFFFF (HI)

The jumper which selected the location of the physical base address for the I/0-MMU page has two settings: HI and L0. The HI and L0 jumpers refed to the position of the I/0-MMU page at either a HI (FFOOOO) or L0 (7FO00O) base address. The CPO/68000/M came configured with the I/0-MMU page jumper set to the L0 position. Any access by the CPU/68O00/M to that I/0 page caused the appropriate I/0 status signals to be asserted on the S-100 bus.

The MMU registers resided in the top 64 bytes of the I/0-MMU page, from 7FFFC0 to 7FFFFF, or FFFFC0 to FFFFFF in physical memory, depending on where the I/0-MMU page was located. On any access to the MMU registers, all S-100 strobes are rescinded until the data transfer between the CPU and the MMU was complete. Transfers between the 68000 and the MMU occur at 8 MHz.

For Example:-
If the I/0 page jumper was set to the HI position, the I/0 page base address is FF0000H. Hex address FF0002
corresponds to I/0 port address 2.      So the 68000 instruction:-

    M0VE.B OFF0002H, DO
    is functionally equivalent to the 8080 instruction
    IN 02H

The manual for this board may be obtained here.

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