[TUHS] Does anybody recall how the TU10 bootstrap code actually operates?

[Will Senn]

All,

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:

http://decuser.blogspot.com/2015/12/analysis-of-pdp-11-bootloader-code.html

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 - 
Sixth Edition":
TU10
012700
172526
010040
012740
060003
000777

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
000777  HALT

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 
mystified)?

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).

Thanks,

Will
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