Slide Rule, Minimum Latency Calculator for the UNIVAC Solid-State Computer


This circular device was an aid to programming the UNIVAC solid-state computer. It consists of a paper disc with equal divisions running from 1 to 200 near the edge, and a clear plastic rotating disc that are pivoted together at the center. The upper disc is marked in red with two perpendicular diameters. The solid state computer had a magnetic storage drum on which locations were specified numerically. The latency calculator allowed programmers to write code for the machine to make the most efficient possible use of the drum memory. The back of the instrument gives a list of instruction codes and corresponding word times. Recieved in bag. Reference: Sperry Rand Corporation, Programming: Simple Transition to Electronic Processing UNIVAC Solid-State 80, 18-26.

Compare 2005.0271.01. Date based on date of documents 2015.3097.03 and 2015.3097.04.

According to Kirk Lubbes, who programmed the Univac Solid State Computer:

"The SS90 had a drum memory, i.e. memory was not random accessible. One had slow memory and fast memory. The slow memory had only a single read/write head per track on the drum and fast memory had four read/write heads spaced at 90 degrees, so therefore the drum had to rotate a full revolution to access a memory word in slow memory and only a quarter turn to access fast memory.

The trick in programming the SS90 was to have the instruction and its operand accessible at an optimal time so that the instruction could access its operand without waiting for the drum very far. As one started a program, this was not much a problem. The programmer new how much time that a given instruction would take to execute and the speed of the drum. Therefore, he calculated the position of the next instruction, based these two parameters. The minimum latency calculator was a mechanical device to help in this calculation. The problem was that as the programmer progressed, collisions occurred, i.e. the optimal location of an instruction or an operand was already taken by a previous instruction or operand. Since the drum was arranged in bands and the read/write heads were at the same location on each band, if one had a collision, you could put the necessary instruction or operand in a parallel band at the same position. This worked the bands all filled up.

The basic approach was to get a program working using the best latency that you could. Then the programmer would go back and start rearranging instructions and operand locations to achieve minimum latency. In those early times, machine time was expensive and memory severely limited. So it was important that production programs were efficient."


Nonccession file 2015.3097.

Date Made: ca 1960

Maker: Remington Rand Univac

Location: Currently not on view

Place Made: United States: New York, New York

See more items in: Medicine and Science: Computers, Computers & Business Machines, Slide Rules


Exhibition Location:

Credit Line: Gift of R. Kirk Lubbes

Data Source: National Museum of American History

Id Number: 2015.3097.01Nonaccession Number: 2015.3097Catalog Number: 2015.3097.01

Object Name: slide rule

Physical Description: paper (base, bag material)plastic (covering disc material)metal (rivet material)Measurements: overall: .1 cm x 14.5 cm x 14.5 cm; 1/32 in x 5 23/32 in x 5 23/32 in


Record Id: nmah_1764336

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