23 Sep
2022
23 Sep
'22
11:16 a.m.
On Sep 23, 2022, at 1:38 PM, Christian Kennedy via
cctalk <cctalk(a)classiccmp.org> wrote:
On 23/09/22 10:22, Paul Koning via cctalk wrote:
Yes. I meant the panel is tied to channel 0, and deadstart runs an input from that
channel. To be fully precise:
The deadstart (master clear) signal forces a particular instruction state into each PP,
and it also sets the "deadstart synchronizer" -- the device that connects the
deadstart panel to channel zero -- into active state. The PPs then execute the newly set
instruction state. Since all other channels are inactive after master clear, PPn for n !=
0 would sit there waiting for channel active. PP 0 reads words from the deadstart
synchronizer: a zero word followed by the 12 words on the panel, and then the synchronizer
disconnects. That ends the IAM instruction and PP 0 resumes at the address pointed to by
location 0, which is 0, so the panel contents is executed (preceded by a 0 opcode which is
a NOP).
paul
I view the deadstart panel as a type of boot ROM, different from other boot ROMs only in
that it's easy to change. It was tied to an I/O channel; the deadstart operation
would run an I/O read operation on that channel to load the initial bits of code.
<pedantic>It wasn't tied to a channel, it forced the instructions into PP zero,
which in turn interacted with the channel.</pedantic>