[TUHS] Does anybody recall how the TU10 bootstrap code actually operates?
will.senn at gmail.com
Wed Dec 23 16:13:50 AEST 2015
I am in the process of gaining a deeper understanding of PDP-11 machine
instructions and how the bootstrap loader and its cousins function. As
part of that process, I am analyzing the code. I am concurrently working
through the DEC bootstrap loader and the bootstrap loader that is
described in the v6 documentation. The DEC bootstrap loader, while
fascinating and elegant, is relatively straightforward, given its
enormous range and the fact that it is self-modifying. I wrote up my
preliminary notes here:
The code that is in the v6 documentation on the other hand is not
yielding easily to reasonable interpretation and I was hoping y'all
might be able to shed some light on how it works.
The following is the TU10 (TM11) bootstrap code from "Setting Up Unix -
The author's notes around the code are:
The tape should move and the CPU loop. (The TU10 code is not the DEC
bulk ROM for tape; it reads block 0, not block 1.)
Halt and restart the CPU at 0. The tape should rewind. The console
should type ‘=’.
Of course, following the instructions results in a successful outcome,
but understanding what is happening is difficult given that this is a
virtual environment and no discernible tape movement can be detected.
My attempt at interpretation is along the following lines, I
manufactured the dissasembly based on my reading of the PDP-11/40
handbook and the machine codes:
012700 MOV #172526, R0 ; moves the TM11 Current Memory Address Register
(MTCMA) address into R0
172526 ; the immediate operand
010040 MOV R0,-(R0) ; moves the contents of R0, 172526, into
memory location 172524, the TM11 Byte Record Counter (MTBRC)
012740 MOV #60003,-(R0); moves 60003 into memory location 172522, the
TM11 Command Register (MTC)
060003 ; immediate data
This seems like gobbledegook to me. It moves the MTCMA (Magtape Current
Memory Address) into R0, then it moves the MTCMA into the MTBRC (Magtape
Byte Record Count), then it moves 60003 into the MTC (Magtape control
register), which causes a read operation with 800BPI 9 Channel density.
172526 is -5252 in 2's complement.
Am I misinterpreting the byte codes or is this some idiosyncratic way to
get the Magnetic tape to rewind or something (the TM11 has a control
function to rewind, so it seems unlikely that this is the case, but I'm
I single stepped through the code in the simulator, and the TM11
registers appear to be pretty unobservable (examining these three
registers always displays 0's, but if I change from referencing the TM11
registers to another area of memory, say 100500 I see the values I would
expect to see as they are being moved from the registers into memory).
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the TUHS