21 Mar
2021
21 Mar
'21
4:01 p.m.
On Sun, Mar 21, 2021, 18:07 Rick Murphy via cctalk <cctalk at classiccmp.org wrote:
The instruction that's waiting on the done bit to
be set is overwritten
with a NOP while the bootstrap is being read. This is common for
bootstraps: minimal user-entered code that gets overwritten by the boot
when it's being loaded into memory.
You have to read the code to understand this. :)
The setting of the WC to zero happens in the code I posted.
Rick, I think we're talking about two different bits of code.
The code I am referring to is here:
http://svn.so-much-stuff.com/svn/trunk/Eagle/projects/DEC/Mxxx/M847/m847/m8…
It does not get rewritten, at least not right away. It's at 07554, and the
first load happens at 07600, to get the rest of the bootloader code you're
referring to. There's another almost identical loader I've seen used by the
PDP-8/E Simulator for macOS, which is loaded at 0200 instead.
The handler makes some comments:
/BOOTSTRAP FOR DECTAPE MONITOR IS THE SAME AS FOR THE
/DEC LIBRARY SYSTEM, RL MONITOR AND POLY BASIC - OR JUST
/READ RECORD 0 INTO 7600 AND TRANSFER TO 7600 A LA DISK
/MONITOR SYSTEM ON DECTAPE
The code you're referring to has to get into memory somehow, and my
question involves this process, not afterward.
So, I think you're a step ahead here! :) I don't disagree with anything
you've said regarding the rest of the boot process, though.
Thanks,
Kyle