Origin of the term minicomputer (was Re: PDP-1 as minicomputer

Eric Smith eric at brouhaha.com
Fri Aug 20 23:13:57 CDT 2010

Allison wrote:
 > After the 8080 microprocessor tended to mean a cpu on one chip
 > or very few for example CDP1800/1801 a two chip predecessor
 > of the COSMAC CDP1802.

The CDP1801 was the two-chip processor.  There wasn't a CDP1800, though 
that designation was used to refer to the entire series of chips.

 > The PDP8 was a instruction and mostly compatible but improved
 > follow on to the all transistor PDP-5 (1963) but much larger than
 > the first version of the PDP-8 by a factor of three racks.  Note the
 > PDP-5 was almost as large as the PDP1 but was aimed at lower
 > cost to buy and use.

I don't think one can reasonably claim that the PDP-5 was "almost as 
large as the PDP-1", nor that it was "larger than the first version of 
the PDP-8 by a factor of three racks".

The PDP-1 CPU, 4K of core memory, high-speed reader and punch, occupied 
four racks.  Reference: DEC Technical Bulletin F-17, "Programmed Data 
Processor-1 Maintenance Manual", 1962, page 2-8:
"Dimensions, Standard PDP-1, Heighth 69 1/2 inches, Length 99 inches, 
including 17-inc console desk, width 27 inches".  Although the manual 
also describes one of the four racks as containing "all controls and 
standard input-output equipment" (p 2-2), that included the console and 
logic necessary to the operation of the CPU, and was not separable.

The PDP-5 CPU and 1K or 4K of core memory occupied a single rack. 
  Reference: DEC document number F-57, "Programmed Data Processor-5 
Maintenance Manual", October 1964, page 1-11: "The standard PDP-5 is 
contained in a single DEC computer cabinet 60-1/8 inches high, 47 inches 
wide, and 27-1/16 inches deep. A table 30 inches wide and 18 inches deep 
is attached to the front of the cabinet."  The 47 inch dimension is 
actually the dimension perpendicular to the front panel.  See figure 1-4 
"component locations" and the section "Specifications", both on page 1-12.

The PDP-8 CPU and 4K of core memory occupied a single rack.  Reference: 
DEC document number F-87, "Programmed Data Processor-8 Maintenance 
Manual", February 1966, "Physical Description" (page 1-11): "The 
standard PDP-8 is designed for either table-top or cabinet mounting, as 
specified by the customer. In the table-top configuration the computer 
is a single unit 34-1/16 inches high, 21-1/2 inches wide, and 21-3/4 
inches deep."

An expanded PDP-5 or PDP-8 might have two or more cabinets, but then an 
expanded PDP-1 will have more than four cabinets.

In conclusion, the PDP-8 was about half the size of the PDP-5, and the 
PDP-5 was about a quarter of the size of the PDP-1.


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