CSIRAC: Our first computer

CSIRAC was, of course, a vacuum-tube machine; most of its 2000 tubes were 6SN7, 6V6, EA50 and KT66. Eventually button-based tubes were used in the delay line store electronics, germanium diodes and, much later, George Semkiw re-designed the disk read electronics using germanium transistors.

The logical design of CSIRAC was directed toward achieving engineering and programming simplicity, even at the expense of speed of execution.

From the engineering point of view, simplicity was achieved by the use of strictly serial processing and adopting a relatively small word length of 20 bits (today’s computers use mostly 16, 32 or even 64 bit word lengths). Storage delay lines each held 16 such words serially, and at first no attempt was made to minimise the time lost waiting for a word coincidence to occur, as was done, for instance, in the EDSAC . Consequently, with 1-microsecond digits at 3-microsecond digit periods and a delay line of 960-microsecond “length", the initial basic execution rate was about 500 instructions per second, and all instructions, except multiplication, took the same amount of time. Later the instruction execution rate was raised to about 1000 instructions per second (today’s computers have an execution rate of upwards of 100,000,000 instructions per second), and the store capacity was doubled using a method, developed by Reginald Ryan, of interleaving two trains of digits in each physical delay line so that each line held 32 words of 20 bits.

The instruction set

The instruction partitioning, although somewhat more complex than that of a simple one-address system like the EDSAC or even TREAC , was certainly less complex than that of the ACE or EDVAC . These features of simplicity of instruction format and economy in program length, resulting from flexibility and economy of instructions needed to perform a complex function, made CSIRAC notable.

Instructions consisted of three components, a 5-bit “destination” P₁-P₅, a 5-bit “source" P₆-P₁₀, and a 10-bit “address" P₁₁-P₂₀. For instructions that used the main store, the 6 bits P₁₅-P₂₀ selected one of the 64 logical delay lines. Bits P₁₁-P₁₄ determined the time at which 20 bits of data were written to or extracted from the delay line, and thus represented address of a word within the selected delay line. There were 32 destination gates and 32 source gates; the 10 address bits identified a data word within the store if either the source or destination required access to the store. The advantage of having the store address in the P₁₁-P₂₀ group of a word was that of simplifying address variation under program control. The total number of source and destination combinations, or different instruction functions, was 1024, although only about 256 of these were used often. This functional flexibility and small number of instruction segments undoubtedly made the machine attractive to users.

The 20 bit word was adopted deliberately! Although this short length limited the precision of arithmetic, it was adequate for most of the engineering-type calculations that occupied much of CSIRAC’s time. It also allowed further engineering simplicity. This short length was an advantage to the programming objective: using the machine as a vehicle to develop programming techniques. For instance, it enforced development of double multiple-word-length and floating-point arithmetic routines, and of interpretive processes. These routines, however, had the disadvantages of slowing some computations and occupying valuable extra storage.

CSIRAC had two main storage systems. These could be described as the RAM and the Hard Disk in today’s terms.

The disk/drum

Brian Cooper, a third senior member of the design team, designed and built the magnetic “drum” storage system. The design work on it started in 1952, and a number of trials of different fast and slow designs were performed, including a drum-based device of 1024-word capacity (disk drives of the 1990’s with a capacity of 1000000 bytes are used in entry-level personal computers) and a mean access time of only 5-milliseconds (disk drives today can only just match that speed!).

By 1956, a horizontal-axis disk-type device was permanently installed with one segment of 1024 words in use. This store was a four-segment, 20-bit parallel system (it had no seek time, as it read all 20 bits in parallel; the access time was due to the rotational latency of the spinning disk), each segment being addressed by the 10-bit address of an instruction (there was no Disk Operating System, as the data was directly addressable). Each segment was given its appropriate source or destination code, and a 10-bit coincidence between this address and the disk clock track count was detected as the drum rotated and serial-parallel transformations and time inter-clocking performed.

Mercury delay lines

CSIRAC’s main memory design allowed for thirty-two acoustic mercury delay lines, each with the capacity to store 16 words (of twenty bits); this was later upgraded to 32 words when a method of interleaving was devised by Reginald Ryan. A total of 1024 words of storage was therefore possible but only a maximum of 768 were ever available!

Mercury delay line technology was developed for radar systems during World War II. These worked by storing data as a series of acoustic pulses in a tube of mercury.

Some of the CSIRAC mercury tubes were about 10mm in diameter and 150cm long. A modulated pulse was generated by a transducer at one end of the tube, and 960 microseconds later it arrived at the other end of the tube where it was received by another transducer. The modulated pulse was detected, amplified, re-shaped and re-generated, being fed back into the beginning of the tube.

The tubes were housed in a box affectionately referred to as the coffin. This was a wooden box about the size of a real coffin and contained a heater which kept the tubes at a constant temperature of about 40 degrees Celsius (about the hottest Melbourne summer’s day!).

All of the incidental registers were also mercury delay lines, however they were only about 6 inches long and were housed in another box above one of the control racks.

The CRT displays allowed the user to monitor the state of the machine. The A, B, C and H registers were available and could be constantly monitored during debugging . Any bank of sixteen words of the main store could be viewed in binary as well as the index registers. As a result, users had a primitive (16 x 20) bit mapped display on which cartoon characters were often displayed!

The console also had a modified ex-PMG Teletype for printing, and an old Rola speaker for reproduction of “music". The speaker’s main purpose was for debugging. It was connected to the machine as an I/O device and instructions would be placed in the main program to produce “clicks” from the speaker. The operator would then know if the program had reached that part of the code successfully.

Programmers soon realised that CSIRAC could be instructed to play music, and experiments were soon implemented. These program tapes still survive and show that CSIRAC was probably the first computer to “play" music in the early 1950s.