LGP-30
The LGP-30 (the 'LGP' was the acronym of 'Librascope General Purpose', later 'Librascope General Precision') was an early low-cost (and thus low-performance) computer, produced by Librascope (later part of Royal McBee). It based on a design done at Caltech, and implemented there in a prototype called MINAC, in 1954. It was first delivered in September, 1956. Around 500 were sold.
It was a serial machine which used a drum for its main memory, each track (of 64 tracks) holding 64 words, each 32 bits long. (For arithmetical operations, there was a sign bit, only 30 bits in single-precision math, and a spacer bit, always zero in memory, though.) It could perform 500 additions per second, and contained only 113 vacuum tubes and 1,450 diodes. Input/output used a Flexowriter, which had a paper tape reader attachment (a higher speed reader/punch was available as an option).
Internals
The drum also held three 'circulating register' tracks, each used for an internal register (the Program Counter, the Instruction Register, and the accumulator); they were replicated around the track, with multiple read heads, so as to reduce their access time.
The entire machine contained only fifteen flip-flops!
| Flop | Function |
|---|---|
| F, G, H | Phase of instruction execution |
| K | Drum sector search during fetch |
| L | Carry |
| Q1-Q4 | Operation code of current instruction |
| P1-P6 | Track selection |
K, Q2, and P1-P6 were used for other purposes during times in which they were not needed for their primary purpose.
Instructions contained a 12-bit address (6 each track and sector), and 4-bit opcode (in octal; ones marked with '!' do not use the 'address' field for an address):
- 0! - Stop
- 1 - Load
- 2 - Store address only
- 3 - Save return address
- 4! - Input
- 5 - Divide
- 6 - Truncated multiply (low order part of result)
- 7 - Short multiply (high order part of result)
- 10! - Output
- 11 - And
- 12 - Jump
- 13 - Jump if negative
- 14 - Store
- 15 - Store and clear
- 16 - Add
- 17 - subtract
The high order half of the word was unused by instructions. Since there was no 'next instruction field, optimum programming was not possible; words on the drum were interleaved so that waiting was usually minimized.
Further reading
- Stanley P. Frankel, The Logical Design of a Simple General Purpose Computer, I.R.E. Transactions on Electronic Computers, March 1957
External links
- LGP-30 - documentation at Bitsavers
- Royal McBee Corporation
- LGP-30 - fairly detailed brochure
- LGP-30 - a vast amount of material
- LGP-30 — A Drum Computer of Significance
- LGP-30 - at the Computermuseum der Stuttgarter Informatik, has many images
- LGP-30 - memories of someone who was taught to program it, but never actually used one
- Librascope Memories
