View source for 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]] by [[Stanly Frankel]], 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 [[bit]]s 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.) The [[logic]] was built out of [[vacuum tube]]s, and it contained only 113 tubes and 1,450 [[diode]]s; it could perform 500 additions per second.

[[Input/output]] used a [[Flexowriter]], which had a [[paper tape]] reader attachment (a higher speed reader/punch was available as an option).

It appears to have been an influence on the later [[Packard Bell PB 250]], no doubt via Stanly Frankel, who consulted on the latter machine.

==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 [[head]]s, so as to reduce their [[access time]].

The entire machine contained only fifteen [[flip-flop]]s!

{| class="wikitable"
! 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 [[interleaving|interleaved]] so that waiting was usually minimized.

==Further reading==

* Stanley P. Frankel, [https://ed-thelen.org/comp-hist/StanleyFrankelIRE-PaperOnLGP-30.pdf ''The Logical Design of a Simple General Purpose Computer''], I.R.E. Transactions on Electronic Computers, March 1957
* Harry D. Huskey, Granino A. Korn, (editors), ''Computer Handbook'', McGraw-Hill, New York, 1962 - the LGP-30 is covered on pp. 20-12&mdash;20-15

==External links==

* [http://www.bitsavers.org/pdf/royalPrecision/LGP-30/ LGP-30] - documentation at [[Bitsavers]]
** [http://www.bitsavers.org/pdf/royalPrecision/LGP-30/LGP-30_Operations_Manual.pdf LGP-30 Operations Manual]
* [https://www.computerhistory.org/brochures/q-s/royal-mcbee-corporation/ Royal McBee Corporation]
** [http://archive.computerhistory.org/resources/text/Royal_McBee/RPC.LGP-30.1956.102646223.pdf LGP-30] - fairly detailed brochure
* [https://ed-thelen.org/comp-hist/lgp-30.html LGP-30] - a vast amount of material
** [https://ed-thelen.org/comp-hist/lgp-30-man.html Royal Precision Electronic Computer LGP-30 Programming Manual]
* [https://www.masswerk.at/nowgobang/2019/lgp-30 LGP-30 — A Drum Computer of Significance]
* [http://computermuseum.informatik.uni-stuttgart.de/dev_en/lgp30/lgp30_1.html LGP-30] - at the Computermuseum der Stuttgarter Informatik, has many images
* [https://web.archive.org/web/20190704220349/https://webdocs.cs.ualberta.ca/~smillie/ComputerAndMe/Part19.html LGP-30] - memories of someone who was taught to program it, but never actually used one
* [https://web.archive.org/web/20230205001912/https://librascopememories.com/Welcome.html Librascope Memories]

[[Category: Early Computers]]