29324Re: IMSAI 8080
- Feb 22, 2013
> --- In email@example.com, "DanielB" <danlb_2000@> wrote:COOL! Good to hear!
> I finally got the IMSAI cleaned up and powered up successfully. Currently I have it running with the front panel, CPU and one memory board. I can successfully deposit and examine data in RAM using the front panel. One thing I have noticed is that the front panel does not behave exactly like the manual says. For example when pushing the Examine button up it should display memory and not increment the address but in my case both Examine and Examine Next increment the address. Could this be because it's a Z80 instead of an 8080?
The Examine button causes the CPU to execute a JUMP to the address selected by the 16 Address Switches. It releases the "Wait" state and feeds the CPU with 3 bytes. 0xC0 (Jump instruction), Address Low, Address High. It clocks these bytes on each pSync pulse.
The Examine Next causes the CPU to execute a NOP (0x00). Same as the jump, but with a single byte instruction.
For these instructions to execute correctly, the CPU must be in an M1 state. The front panel should stop the CPU on the M1 cycle, but that might not happen on Z80's. That is due to the M1 status not being placed on D05 during a pSync, a fundamental difference between Z80 and 8080. Systems with an 8080 will always stop on the M1 cycle.
The designers of these systems offer the single step work around.
Stop your CPU, single step until M1 LED is on, then try the Examine / Examine Next functions.
I hope that helps, if not, then there's an issue with your front panel.
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