1956 Binary Desk Computer LIBRASCOPE LGP-30

[KC9UDX]

There was a Selectron for sale on an online auction site just about ten years ago. I don't think it sold.

 

[RandomVariations]

Each instruction in the LGP-30 had the address of the next instruction as part of the instruction.

The LGP-30 has a counter register which acts as a traditional program counter. It had an unconditional transfer and test (conditional transfer) instructions which contained the track and sector of the next instruction. The rest of the instructions either used the fields to specify the memory location involved in the instruction, a constant (mask,) a test value (stop,) output value (print,) or nothing at all (input.)

There are three tracks which hold the counter, accumulator, and instruction register. These have recirculating circuits that read and then write back the values to the track at the appropriate length.

Three additional tracks are hard coded at the factory with the correct clocks which mark the sector numbers, and masks to select the correct values from the serial streams.

These sector numbers are interleaved on the drum, which made it possible to use some interesting optimization tricks.

During the development of my simulator and FPGA designs, I started with a non-interleaved drum. Switching to an interleaved drum makes a significant change in the amount of time it takes for programs to run especially if a program is optimized for the interleave.

 

[Tor]

"one plus one" addressing - New term for me, but I like it.
Another machine using that scheme was the Norwegian "Nusse" from 1953, it was a computer (with a drum) based on A.D. Booth's All Purpose Electronic Computer. Nusse was a heavily modified model, as the "kit" they bought from England wasn't exactly complete. Nusse's instruction format appears to have been

• 7 bits for counter
• 10 bits for data address (x-addr)
• 5 bits for operation (op)
• 10 bits for address of next instruction (y-addr)

(the little that is known about it is on the computer history wiki on gunkies.org)

 

[Al Kossow]

There was a Selectron for sale on an online auction site just about ten years ago. I don't think it sold.

It sold. It wasn't obscenely expensive for what it was ( few thousand dollars)

The SECOND one which sold there within the past few years went for a lot of money. There was some
speculation at the time it was a flip of the original.

Details on the device can be found here:

http://www.computerhistory.org/collections/search/?s=selectron

JOHNNIAC replaced the Selectrons with core memory because of availability/reliability problems.