Playing with the PDP-11/40

Parent Category: Projects Category: BlinkenBone panel simulations Written by Administrator

First steps:

After starting the script "pdp1140.bat|sh", two application windows appear on your desktop:

  1. The SimH command window, which is also the serial console of the simulated PDP-11.
    After start, it shows the boot prompt of a RT-11 operating system.
  2. The graphical panelsim window.
    Initially it shows heavy blinking, as the simulated machine is executing a code loop while it waits for your input.

blinkenbone pdp1140 panel+simh

Arrange these two windows, so you can see both. (And you don't need to know much about PDP-11's, the RT-11 operating system or Blinkenlight panels.)


You now surely want to "Fingergrabbing and pressing the cnoeppkes from the computer" (see the jargon file). Well, try this:

Hint: Watch the SimH command window: it will reflect all operations made on the panel !

"Running light"

After destructing RT-11, it's time for some more constructive work.
Let's enter and run the "Hello world" program for Blinkenlight front panels: a running console light!

The corresponding PDP-11 assembler program was compiled with the MACRO-11 cross assembler.

See here spurce code, binary code, and how to enter it over the Blinkenlight panel:

PDP-11 assembler program:
adress  data      opcodes

operation on switches

001000  005000    clr     r0
001002  005200    inc     r0
001004  006100    rol     r0
001006  000005    reset
001010  000775    br      001004

activate HALT
data switches=001000, LOADADRS
data switches=005000, DEP
data switches=005200, DEP
data switches=006100, DEP
data switches=000005, DEP
data switches=000775, DEP

Start sequence:

de-activate HALT
data switches=001000, LOADADRS


Check this program again with EXAM operations! While converting octal digits to switch positions, you will make much more errors than you expect (we'll, at least I did ...).

After cross-checking, start the program. You should see a single light running in DATA from right to left.

The trick is using opcode "reset": "Reset" initializes all devices, it needs 70 milliseconds = 1/14 second to do this. In this time, the content of cpu register R0 is visible on the DATA LEDs.

To get a binary counter instead of a running light, replace this line
  001004  006100    rol     r0         ; data switches=006100, DEP
by this   
  001004  005200    inc     r0         ; data switches=005200, DEP

"Little Gauss"

Here comes another demo you can play: a little MACRO-11 assembler program, which adds the first <n> natural numbers:

sum = 1 + 2 + 3 + ... + n.

See the attachement for a complete listings and instructions how to toggle it in and let it run. Follow these basic steps:

  1. Toggle in the program starting at address 1000 (octal). It is just 11 opcodes long.
  2. verify it by EXAMing all opcodes.
  3. Enter a value for <n> over the SR data switches. Try at first n=100, which is octal 144.
  4. Start the program from address 1000. It will calculate the sum and HALT then. The resulting sum is show on the DATA LEDs. For n=100, the sum 1 + 2 + 3 .. 100 = 5050, (octal 11672)
  5. Enter another value for <n> as in step 3, then CONTinue program execution.

(You can tell your audience the story behind: The sum 1 + 2 + 3 + .. + n can quickly be calculated as sum = n*(n+1)/2. This formula was discovered by the 8-year old Carl Friedrich Gauss. He was forced in school to add the numbers 1+2+3+..+100, because a lazy teacher wanted to keep the kids occupied for an hour. Gauss quickly reordered the numbers to
    (1+100) + (2+99) + (3+98) ... (50+51) = 101 * 50 = 5050.
and had the result after a few seconds. Hence this formula is named "Little Gauss".

Download this file (sum_1140.blinkenlight.txt)sum_1140.blinkenlight.txt["Little Gauss": MACRO-11 program to calculate the sum of natural numbers (with toggle-in instructions)]10 kB