The Harwell Dekatron computer, aka the WITCH computer is at the UK National Computer Museum. More information about it can be found at http://www.computerconservationsociety.org/witch.htm.
In developing this simulator, I have been particularly influenced by the users manual, the table of order codes, the Electronic Engineering article describing the arithmetic units, and the example programs. All of these resources can be found liked from the CCS site linked above.
I also corresponded with Delwyn Holroyd, and would not have been able to figure out the details of some of the design without his helpful replies.
I started this as a Clojure programming exercise. I chose the WITCH machine specifically because my parents learned to program this machine while studying at Wolverhampton.
Download the Leiningen environment from http://leiningen.org and then run the code using the following command line from the root of the repository sandbox.
$ lein run <tape-file>
-t : Traces the execution of the system
$ lein run resources/demo-1
$ lein run resources/demo-2
$ lein run -t resources/exercise-1
The tape file format tries to be similar to the examples given on the WITCH computer website. Several tapes are described in each file. Blank lines are ignored, as are lines which start ';'. A line can contain one of:
| Syntax | Meaning | Example |
|---|---|---|
| ==tape | The beginning of the next tape | ==tape |
| #n | A block marker | #1 |
| NNNNN | A five digit positive number, typically used for orders | 21020 |
| +NNNNNNNN | An eight digit positive number. '+' is mandatory | +1234567 |
| -NNNNNNNN | An eight digit negative number. '-' is mandatory | -1234567 |
The first non-comment line must be a '==tape' marker. The contents of tape 1 follows. When another '==tape' marker is seen, then tape 1 is finished, and tape 2 follows the marker. As in the real machine, tape 1 should contain a block 1 marker, because the machine will search for it an execute from that point on booting.
For example:
; This is an example
==tape
#1
; Search for block 2 on tape 2 and jump to it
03202
02102
==tape
#2
; Stop the machine
00100
There is an implicit decimal point after the first digit of any number.
See the files in the 'resources' directory for some examples.
Unit tests can be run from Leiningen environment.
$ lein test
The code is broken into the following namespaces:
- core.clj - Implements the main function. It initialises the state, executes the fixed search and transfer instructions, and enters the main loop.
- machine.clj - Implements functions the simulate the hardware of the machine. Input, output, register access, transfer unit, and machine state are all in here.
- decode.clj - Implements instruction decode.
- nines.clj - Implements some nines complement utility routines.
The machine execution loop (decode/run) calls the step function until the machine state indicates that the machine has stopped. The step function (decode/step) fetches an instruction from the current PC and dispatches it to the decoding function. The decoding functions take the machine state and produce a new machine state with the result of the instruction.
Because a read can sometimes be a state-modifying operation (reading a tape is the most obvious one), the read functions return a modified machine state, with the value read in an input holding place (:sending-value). This partially mirrors the real machine. The transfer unit takes this value and transforms it according to the 'complement' and 'shift' states, as I believe the real machine does. Finally, the store/accumulator outputs will perform the summation as in the real machine.
I believe that in the real machine, clear is an operation whereby the complementary pulse chain is not fed back into the sending dekatrons as their value is read out. I do not simulate this. The sending dekatrons effectively keep their value unless they are explicitly cleared.
- Finish off the unit tests
- Write some other example tapes (calculate bubble sort, quick sort, ...)
- Introduce punched hole tape format
- Is the shift value (set by 08xxxx) independent of the transfer shift. An example is if we set the shift value, perform a multiply, then do an add. Does this add have the shift value applied as set explicitly, or does it revert to 'no shift' following a multiplication?
- Is there a separate transfer unit for the accumulator, or a single one for both the accumulator and the stores?
- If there is 1 transfer unit, how do digits 8-14 become set when there is a store as the sending value? For accuracy they should be filled with the sign digit.
- When the accumulator is the sending address, are its digits 8-14 applied to the transfer unit inputs? This is important when the shift value is set to +1, where you could get digit 8 affecting the destination store.
Copyright © 2016 David Norman
Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.