CSIRAC
Australia's first digital computer and oldest surviving first-generation electronic computer.
CSIRAC (Commonwealth Scientific and Industrial Research Automatic Computer), first called the CSIR Mk 1, was Australia’s first digital computer and ranks as the fifth stored-program computer ever built. It is the oldest surviving first-generation electronic computer—the Zuse Z4 at the Deutsches Museum is older but electro-mechanical, not electronic—and it was the first computer to produce digital music.
A team led by Trevor Pearcey and Maston Beard built CSIRAC, working largely without knowledge of similar projects in Europe and the United States. It ran its first test program, a multiplication of numbers, in November 1949. Restricted operation began in late 1950, and the machine was publicly demonstrated and fully operational in 1951.
The design was typical of early valve-driven computers. It used mercury acoustic delay lines for primary storage, holding 768 20-bit words, plus a parallel disk device with 4,096 words and an access time of 10 milliseconds. The memory clock ran at 1,000 Hz; the control unit initially took two clock cycles per instruction, later doubled to one cycle. The bus, called the “digit trunk,” was serial—it transferred one bit at a time. Most of its roughly 2,000 valves were 6SN7 triodes, 6V6 tetrodes, KT66 tetrodes, and EA50 diodes. George Semkiw later redesigned the drum-read electronics using germanium transistors.
Input came from punched 3-inch-wide, 12-track paper tape (punch cards were tried but found unsatisfactory). A console let operators step through programs one instruction at a time and featured CRT displays showing register contents. Output used a standard teleprinter or punched tape. The instruction set included basic arithmetic and logical operations, plus conditional and relative jumps, enabling subroutine libraries. Instructions had three parts: a 5-bit destination, a 5-bit source, and a 10-bit address. For main store access, bits P15–P20 selected one of 64 logical delay lines; bits P11–P14 specified the word within that delay line. There were 32 destination gates and 32 source gates; the 10 address bits identified a data word when store access was needed. Of the 1,024 possible source/destination combinations, only about 256 were commonly used. The machine had three 20-bit registers (A, B, C), two used in multiplication; one 10-bit register that could link to either half of a word; and 16 additional 20-bit registers
- type
- First-generation electronic computer
- country
- Australia
- year_first_test
- November 1949
- year_operational
- 1951
- primary_storage
- Mercury acoustic delay lines (768 20-bit words) and a parallel disk (4096 words)
- valves
- Approximately 2000 valves (6SN7, 6V6, KT66, EA50)
- known_for
- First computer to play digital music; oldest surviving first-generation electronic computer
Lore & Background
CSIRAC was constructed by a team led by Trevor Pearcey and Maston Beard, working largely independently of similar efforts overseas. It ran its first test program (multiplication of numbers) in November 1949, began restricted operation in late 1950, and was publicly demonstrated in 1951. The machine used mercury acoustic delay lines for primary storage and a serial bus called the 'digit trunk'. Input was via 12-track punched paper tape, and output through a teleprinter or punch tape. In 1950, CSIRAC was used to play music, the first known use of a digital computer for that purpose; the music was never recorded but has been accurately reconstructed.
Reader's Guide
CSIRAC is historically significant as the fifth stored-program computer in the world and the oldest surviving first-generation electronic computer. It represents early Australian innovation in computing, built independently of European and American efforts. Its use in 1950 to play digital music marks a milestone in computer music history. After its 1964 shutdown, it was preserved and eventually exhibited at Melbourne Museum and later at Scienceworks in 2018. It is listed on the Victorian Heritage Register and as a National Engineering Landmark by Engineers Australia. An emulator has been written for it, and many programs have been preserved. In 2025, Australian electronic DJ Ninajirachi released a song titled 'CSIRAC' on her debut album, paying tribute to the computer.
Did You Know?
- CSIRAC is the oldest surviving first-generation electronic computer.
- It was the first computer to play digital music, in 1950.
- Its bus, called the 'digit trunk', transferred one bit at a time (serial).
- After being retired, it was stored for decades before being exhibited at Melbourne Museum and later at Scienceworks in 2018.
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