4 May
2015
4 May
'15
10:54 a.m.
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.
Noel
4 May
4 May
11:34 a.m.
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
6:41 p.m.
On 05/04/2015 12:54 PM, Noel Chiappa wrote:
Yes, some
models of the PDP-11 do not handle DMA requests
when halted. I'm pretty sure the 11/45 does.
And, I'm pretty sure the 11/05 does NOT, as I recall not
being able to perform disk requests directly from the console,
but had to write a short program that went into an infinite
loop after commanding the controller card to perform a transfer.
Jon
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
3519
days inactive
3520
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2 comments
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participants (3)
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elson＠pico-systems.com -
jnc＠mercury.lcs.mit.edu -
wilson＠dbit.com