[Simh] PDP-6 Microtape bootstrap

[Lars Brinkhoff]

Hello,

Below is the PDP-6 Microtape (DECtape) bootstrap code used at the MIT AI
Lab.  There are some of us wondering why it was written this way?  Why
add one to the input word and check that it's negative, and that the
right half is not zero?

  0/   JFCL             ;Nop.
  1/   MOVSI (JFCL)     ;Set 0 to nop.
  2/   CONO 635550      ;Reset processor.
  3/   CONO PI,11577    ;Reset priority interrupts.
  4/   CONO UTC,223110  ;Reset Microtape controller.
  5/   CONO DC,4010     ;Start transfer.
  6/   CONSO DC,1000    ;Ready?
  7/   JRST 6           ;No, loop.
  10/  DATAI DC,13      ;Yes, store word at 13.
  11/  AOJGE 13,6       ;Add one to contents of 13, and loop if positive.
  12/  TRNE 13,-1       ;Right half zero?
  13/  .                ;No, execute.
  14/  JRST 6           ;Yes, loop.

This was also posted to alt.sys.pdp10 some time ago.  Sorry to those who
see a duplicate.