[TUHS] v6 RK05 bootloader question

Norman Wilson norman at oclsc.org
Mon Dec 28 06:32:12 AEST 2015

Something of a tangent:

In my early days with UNIX, one of the systems I helped look
after was an 11/45.  Normally we booted it from an SMD disk
with a third-party RP-compatible contorller, for which we
had a boot ROM.  Occasionally, however, we wanted to boot it
from RK05, usually to run diagnostics, occasionally for some
emergency reason (like the root file system being utterly
scrambled, or the time we ran that system, with UNIX, on a
single RK05 pack, for several days so our secretaries could
keep doing their troff work while the people who had broken
our air-conditioning system got it fixed--all other systems
in our small machine room had to stay shut down).

There was no boot ROM for the RK05, but it didn't matter:
one just did the following from the front-panel switches:

1.  Halt/Enable to Halt
2.  System reset (also sends a reset to the UNIBUS)
3.  Load address 777404
4.  Deposit 5.
(watch lights blink for a second or so)
5.  Load address 0
6.  Halt/Enable to Enable
7.  Continue

777404 is the RK11's command register.  5 is a read command.
Resetting the system reset the RK11, which cleared all the
registers; in particular the word count, bus address, and
disk address registers.  So when the 5 was deposited (including
the bit 01, the GO bit), the RK11 would read from address 0 on
the disk to address 0 in physical memory, then increment the
word-count register, and keep doing so until the word count
was zero after the increment.  Or, in higher-level terms, read
the first 65536 words of the disk into the first 65536 words
of memory.

Then starting at address 0 would start executing whatever code
was at the beginning of memory (read from the beginning of the

Only the first 256 words (512 bytes) of the disk was really
needed, of course, but it was harmless, faster, and easier to
remember if one just left the word-count at its initial zero,
so that is what we did.

The boot ROM for the SMD disk had a certain charm as well.
It was a quad-high UNIBUS card with a 16x16 array of diodes,
representing 16 words of memory.  I forget whether one inserted
or removed a diode to make a bit one rather than zero.

It's too bad people don't get to do this sort of low-level stuff
these days; it gives one rather a good feel for what a bootstrap
does when one issues the command(s) oneself, or physically
programs the boot ROM.

Norman Wilson
Toronto ON

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