Anoraks Corner
by Tony Sale

Welcome to Anoraks Corner, speeding up Colossus

1. The MK II Colossus incorporated a new feature, Multiple Testing, which gave it a five fold increase in speed for setting wheels and rectangling to break patterns.

This was achieved by "remembering" selected bit streams 1. 2. 3 and 4 bits back from the current bit. Then as each character was read from the paper tape, the Z stream, the selected algorithm could be evaluated five times, all in parallel and the results counted into five counters. The bit stream being "remembered" could then be stepped on five start positions and the results in the five counters printed.

This was the first known use of what we now call a shift register.

This feature has now been got to work in the real Colossus Rebuild of a MK II and is also now incorporated into Virtual Colossus, so you can try it.

1. Multiple Testing on X1 and X2.

A simple first example is to repeat the X1, X2 wheel setting using Multiple Testing (MULT) on X1. Start again with a new Virtual Colossus. Set cipher? to BR2cipher, set patterns? to BRpatterns. Scroll down the left hand panel to the red addition keys at the bottom of the panel. On each of the five rows put down key 2, put down the yellow staggered R bus key and up the yellow = dot key. Now scroll right to the red counter switches. Put down counter 5 key on row 1, counter 4 key on row 2, etc ending with counter 1 key on row 5.

Now scroll left the right hand panel and set the big ganged black Q bus keys to Delta Z and Delta K.

On the master switch panel set up the red X1 MULT key on the top row of switches. Set the lower row of black start increment keys to down X1 (fast) and up X2 (slow).

Now set down the "10" switch just to try a part run, then scroll down to set the Set Totals switches to 1928. Put the X2 start to 12 and X1 to 17 then press "M". Because the Set Total should be 2928, but is 1928, all results will print out. This allows you to see the complete layout.

You will see that the K1 start increments by 5 and on each row the 5 counter outputs are shown, preceeded by "a" for counter 1 up to "e" for counter 5. but since the first row of yellow R keys, R1, has been switched into counter 5, "e", the K1 start position refers to this undelayed output, ie the count in counter 5. So the starts read back from the far right position. On the seventh line down the maximum count of 3009 has letter c in front of it so its position is 6 - 2 = 4.

If the Set Total is now increased to 2928 and the run done again, only one result shows, 6 12 c3009, which is interpreted as K1 = 4, K2 = 12, count = 3009.

On a 600 Mhz lap top:

Time comparisons: .... Virtual Colossus .... Real Colossus Direct . 50 starts ...... 100 secs ............ 60 secs Mult ... 50 starts ....... 60 secs ............ 12 secs
So speed-up by using Mult is 1.5 for VC (software) and 5 for Real Colossus (hardware).

2. Multiple Testing on X4 and X5.

Another Mult run is for 4=5=/1=2 with Mult on X4.

To set this up its quickest to reload VC. Load BR2cipher and BRpattern. Now on the big switch panel set down keys 1, 2 and 5 and the yellow R1 key, also the yellow negate key. Go down to row 2 and put up keys 1, 2 and 5 and the yellow R1 key, put down the yellow negate key.

Now do the rows in pairs as per the top two rows.

Now scroll across to the counter keys. Put row 1 and row 2 into counter 5, down to row 9 and row 10 into counter 1.

On the bottom row of yellow counter negates set all keys down.

Now scroll the right hand panel to the big ganged keys and set down to Delta, Z and K. Scroll further across and set down key 4 on the lower set of black keys, and up key 5. Set down the red key to X4 Mult.

Set K1 start to 4 and K2 start to 12, K4 to 10 and K5 to 7.

Set down the "10" key and press down "M" to do the run. This shows a maximum count of 883 at K4 = 19, K5 = 8 on counter 5 ("e").

With Set Total = 809, 19 8 = 883 shows as the maximum in the 50 starts tested.

On a 600 Mhz lap top:

Time comparisons: .... Virtual Colossus .... Real Colossus Direct . 50 starts ...... 235 secs ............ 60 secs Mult ... 50 starts ....... 95 secs ............ 12 secs

So speed-up by using Mult is 2.5 for VC (software) and 5 for Real Colossus (hardware).

The /// run to set K3 is so short that it is hardly worth setting up Mult.

There are some unresolved problems in Virtual Colossus in using Mult on the M2 motor wheels and Psi wheels.

This page was originally created by the late Tony Sale, the original curator of the Bletchley Park Museum,

Anoraks Corner by Tony Sale

Welcome to Anoraks Corner, speeding up Colossus

Anoraks Corner
by Tony Sale