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[/] [zipcpu/] - Rev 25

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  • Rev 25 2015-08-22 00:49:58 GMT
  • Author: dgisselq
  • Log message:
    Lots of changes, hopefully all for the better. The result works in a
    simulator, although it has yet to be tested yet in an FPGA--so it may still
    have Xilinx build errors.

    1. The wires brought from the CPU to the Zip System for the debug command
    register were adjusted. They now include GIE and SLEEP, but no longer include
    the step or break enable bits as these were fairly useless anyway.

    2. The user and master A-Stall counters were re-labeled as instruction count
    counters (which is what they are now anyway). This is for performance reasons
    so that, after the fact, you can measure how many instructions per clock
    you were actually able to achieve.

    3. The CPU debug access port stall was adjusted so that the data port no longer
    stalls when the CPU isn't halted. This can be useful, for example, when trying
    to determine where th program counter is at without stalling the CPU. (You'll
    still need to read two registers, the supervisor and user program counters, and
    reading these registers still requires a write to the debug command port first,
    so this still requires 4 single operand wishbone bus cycles.)

    4. Signed and unsigned 16-bit multiply capabilities were added to the ALU
    (cpuops.v) and support added in the Zip CPU master file as well.

    5. The ZIP CPU now spports the TRAP bit in the CC register, so that after a user
    interrupt the supervisor can tell that it was a user interrupt versus a hardware
    interrupt. This bit is set any time the user disables the GIE bit, and cleared
    any time the supervisor sets the GIE bit.

    6. A reserved position was created in the CC register for a floating point
    enable flag. This flag is permanently false, however, on the current
    implementation as it doesn't implement floating point.

    7. Logic was added to handle the break instruction. This instruction has now
    been tested successfully in the simulator. If a break is issued, the CPU will
    either halt (if in supervisor mode, or if in user mode with the break enable
    bit set in the CC register), or the CPU will trip an interrupt for the
    supervisor to transfer execution to a user-level debugging task.

    8. After watching the CPU stall on a LDIHI followed by an LDILO, logic was
    adjusted to keep the pipeline from stalling in thesee conditions. This lew
    logic works for an 'A' operand, or equivalently for a 'B' operand with no
    immediate. In the cases of such logic, the operand is loaded directly from the
    output of the ALU into the input of the ALU skipping the operand read stage of
    the pipelinle. This logic has not been tested on an FPGA yet, so it isn't clear
    if it will break timing requirements or not. (Goal is 100 MHz clock.) As
    of this new change, the CPU can now execute 0.48 instructions per clock, versus
    the 0.44 it was getting before, across the test set.

    9. Sleep logic was adjusted to prevent the user from switching to supervisor
    mode and putting the processor to (infinite) sleep at the same time. The
    justification was the fact that a user should not be able to halt the CPU when
    other processes that might want it might still exist.

    Other changes were made as well, but to other portions of the project. Those
    will be checked in shortly.
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