TD Systems TDL-11 QBUS (SCSI?) card

John Wilson wilson at
Mon May 4 13:34:57 CDT 2015

On Mon, May 04, 2015 at 01:54:10PM -0400, Noel Chiappa wrote:
>Or maybe actually DMA indeed _doesn't_ work with the CPU stopped, and what
>happened was that the DMA request was waiting in the card, and as soon as the
>processor started, it did the DMA? But how did the first instruction fetch
>produce a valid instruction? Unless the first DMA cycle (to 01000) happened
>before the processor fetched the first instruction?

Ow my head!  I'm probably insane, but I have a hazy feeling that
arbitrating DMA while halted might be CPU-model-dependent thing?  Also,
different models can have different definitions of "halted".  The Q-bus
CPUs have uODT so they're mostly alive even when they aren't.  The older
models might be different -- not that it would matter if you're using one
with a ROM console emulator (so it's not halted at all when you're typing
"L 172150" etc.), but I wouldn't know what to expect with a lights 'n
switches console with the "HALT" switch flipped.  Then again, I also
don't know whether the DMA arbitration is usually done in microcode
or by external hardware.

John Wilson
D Bit

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