Chapter 11 The Pilot ACE 199
destination 23. If a transfer with an odd characteristic is made from any source to destination 23 then, from then on, DS14 behaves as though it were two single length accumulators in series. This means that carries are suppressed at the end of each of the single words. This condition persists until a transfer is made to destination 23 using an even characteristic, when DS14 behaves as an accumulator for double length numbers with their least significant parts in even minor cycles and more significant parts in odd minor cycles.
The operation TS20 is modified by transfers to destination 21. If a transfer with an odd characteristic is made to destination 21 then TS20 ceases to have an independent existence and from then on is fed continuously from DL10. Source 20 then gives the con tents of DL10 one minor cycle later than from source 10. TS20 reverts to its former condition when a transfer with an even characteristic is made to destination 21. The facility is used to move the 32 words in DL10 round one position so that the word in minor cycle n is available in minor cycle (n + 1).
Assessment of optimum coding
A detailed assessment of the value of optimum coding is by no means simple. Roughly speaking, subroutines are on an average about 4 or 5 times as fast as on an orthodox machine using the same pulse repetition rate. In main tables a somewhat lower factor is usually achieved. The factor of 4 or 5 would be exceeded if less of the advantage given by optimum coding were used to overcome disadvantages due to the rudimentary nature of the arithmetic facilities on Pilot ACE. Even so, the bald statement of the average ratio of speeds does not do full justice to the value of optimum coding on the Pilot ACE. Its value springs as much from the fact that it has made possible the programmes in which computing is done between the rows of cards and also the high output speed of decimal numbers. The binary decimal conversion routines for punching out several decimal numbers simultaneously on a card and also decimal-binary conversion routines for reading several numbers, achieve a ratio of something like 14 to 1, and on a machine which is being used extensively for scientific computation on a commercial basis this is of immense importance.
Engineered versions of the Pilot Model are now under construction by the English Electric Company. These machines will be similar to the Pilot Model but will have a little more high-speed store, an automatic divider, two quadruple length stores and a subtractive input on the double length accumulator besides several minor modifications including a rationalization of the numbering of the stores! In addition a magnetic drum intermediate store with the equivalent of 32DL's storage capacity will be added. A full scale machine will probably soon be under development employing a 4 address code. Typical instructions will be of the form
A + B C
and will select the next source of instruction. This code is more economical in instruction storage space and since all single word stores will then become complete accumulators with all facilities except multiplication on them, it will be possible to take much fuller advantage of optimum coding.