RT-11 idle light pattern

Charles Anthony charles.unix.pro at gmail.com
Fri Dec 29 10:17:51 CST 2017

(Not RT-11, PDP-11, or even DEC...)

The Multics process scheduler quantum is 1/4 second; running processes are
interrupted and rescheduled every 1/4 second.

The idle process updates the light pattern and sleeps until the 1/4 timer
runs out or an interrupt occurs.

" Idle with a flashing pattern in the lights.
" This is a convenient place to recompute tc_data$max_timer_register,
" which is the product of the number of CPUs and the tuning parameter
" tc_data$pre_empt_sample time.  Either of these values may have
" changed due to reconfiguration or tuning parameter changes, and
" we don't have much else to do here, so ...

          ldaq      flash_pattern

          inhibit   off       <-><-><-><-><-><-><-><-><-><-><-><->
idle_dis: dis       0
          inhibit   on        <+><+><+><+><+><+><+><+><+><+><+><+>

" The following code checks to see if a connect has been delayed.
" This is done by seeing if the ring alarm register is nonzero.
" If it is, the connect must be reissued now.

          sznc      pds$alarm_ring  is ring alarm set?
          tze       *+2                 if not, skip connect reissue
          cioc      scs$cow_ptrs,1*

          staq      prds$idle_temp      save flash pattern
          ldq       tc_data$pre_empt_sample_time
          mpy       tc_data$ncpu
          tze       reload_flash        bogus for some reason
          staq      tc_data$max_timer_register

          ldaq      prds$idle_temp
          alr       35
          qlr       1

          eax7      0,au                flash pattern in X7
          lxl4      tc_data$n_eligible  # of eligible processes in X4
          ldx5      bp|apte.term_processid  idle type in X5
          lxl6      tc_data$stat+2      # of ready processes in X6

          tra       idle_dis

          inhibit   off       <-><-><-><-><-><-><-><-><-><-><-><->

At the top, the initial pattern is loaded into the A and Q registers; the
'dis' is 'delay until interrupt set': idle the CPU until the count timer
expires or interrupt. A few lines later, the A and Q registers are saved in

At reload_flash, the pattern is restored from prds$idle_temp into A and Q;
the 'alr 35' (A Left Rotate) does a circular rotate of the A register;
being 36 bits, this is equivalent to rotating it one bit to the right. The
'qlr 1' (Q Left Rotate) rotates the Q resister to the left one bit.

The 'eax7 0,au' (Effective Address to X7) effectively copies the high 18
bits of A into the X7 register; as far as I can tell, this and the next few
instructions are strictly diagnostic; the contents of those registers can
be displayed on the maintenance panel, but unless the machine was idle,
they would quickly be overwritten.

Then the code branches back up to the sleep instruction. When the system is
idle, the A and Q registers will hold each pattern for a 1/4 second.

The initial pattern is:

          zero      -1,0
          zero      0,-1

Which is 6180 assembler weirdness for putting 0777777000000 in A and
0000000777777 into Q.

The rotate instructions will make the 18 one bits in the A register
circulate to the right; to the left in the Q register. On the maintenance
panel, the registers are displayed stacked, A above Q; because of the
relative layout of the bits, this produces the illusion that the bits are
rotating out of the right end of A into the left end of Q, and similarly of
the other end, so the bits are traveling in a circle.

The pattern can be seen starting at 1:44 in

-- Charles

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