TD Systems TDL-11 QBUS (SCSI?) card

Noel Chiappa jnc at
Mon May 4 12:54:10 CDT 2015

    > From: Seth Morabito

    > Well now I have egg on my face...

Don't worry, you're not the only one... :-)

    > No, actually it DOES work. 

Wow. Live and learn.

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?

I'll have to try throwing a 'scope on a QBUS and manually (via ODT) starting
a DMA transfer, with the CPU halted... see what (if anything) happens.


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