More PDP 11 hacking (help!)

From: Jerome H. Fine <>
Date: Tue Feb 8 10:30:07 2005

>Pete Turnbull wrote:

>Depends on what's at location zero. You could try entering a copy of
>the normal RX02 bootstrap code at location 1000 and stepping through it
>to see what happens and where it goes wrong. Did you know that you can
>type "P" instead of "G" to make the processor restart at the last-used
>address, without resetting the bus and devices on it, and that you can
>do this with the HALT switch on, so it single-steps? So if enter a
>manual bootstrap at 1000, set the HALT switch, type "1000G" and then
>keep typing "P" you'll single-step through the code starting at 1000.
Jerome Fine replies:

While there are published bootstraps which can
be entered starting at 1000, as far as I know,
the hardware boot code in and EPROM reads block zero
of any device into memory starting at address zero.
After that, the PDP-11 starts execution at location
zero. Thus any bootstrap entered using ODT starting
at location 1000 will be different from a bootstrap
read from the floppy.

If you want to confirm this and assuming you
have the same version of RT-11 available, you can
start the boot process using an emulator and request
that it be paused just before the first instruction
is executed (after the code in block zero is read
into memory starting at location zero). If the
code that is seen starting in location zero is the
same, then it is likely that it came from the floppy

Note that the bootstraps typed in using ODT are
almost always very small and probably use less than
half of the code that would be scattered about in the
256 word device block zero of an RX02 floppy.

As for RT-11, every bootstrap I have ever seen that
is in block zero of the device then reads into
memory starting at location 1000 up to 4 more blocks
starting at block two - i.e. blocks 2,3,4,5 as required.
Normally, V05.0x versions of RT-11 required all 4
additional blocks.

Sincerely yours,

Jerome Fine
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Received on Tue Feb 08 2005 - 10:30:07 GMT

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