[Simh] Writeup of PDP-11 bootstrap loader analysis

[Will Senn]

All,

I have finished an analysis of the PDP-11 bootstrap loader code that may 
be of interest to folks working with SimH. As many of y'all know, I am 
pursuing a deeper understanding of Unix V6, which is pretty intertwined 
with PDP-11 architecture, and in this day of virtual machines, the SimH 
simulator (I don't have a PDP-11 laying around). Recently, my pursuit 
has gone down the road of assembly language and machine code. In getting 
a grip on these, I did a line by line analysis of how the bootstrap 
loader loads the absolute loader (which loads other absolute format 
paper tapes). Given that the bootstrap loader is 14 words long, you 
would think it would be really straightforward. Maybe I'm slow, but this 
turned out to be much more intricate a program than I thought it would 
be. It's self-modifying and the addressing modes of the PDP-11 are a bit 
difficult to work with (that and octal addition is strange to me).

I will follow up on this entry with a more user friendly writeup that 
explains how loading DEC-BASIC is accomplished using the bootstrap and 
absolute loaders.

Here is the link, I appreciate any feedback be it comments, suggestions, 
or criticisms:

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

Regards,

Will