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Computermuseum der Fakultät Informatik				german


pdp8/l

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Type:          PDP8/L                             <<<    ***    >>>    ^^^
Manufacturer:  Digital Equipment Corp.
Technology:    TTL-IC
	       Magnetic Core Memory 4096W * 12Bit
Year:          1969
Price:         about DM 50000.-



The PDP-8 computers were the most widely spread minicomputers of the
world. In 1978, DEC was advertising them by having sold 50.000 units 
worldwide. The first version, the primary PDP8 was still assembled with
discrete transistors and had the same instruction set like the very 
older PDP5. Both have 12Bit words. This comes from the original main
application of the PDP5 as a small process computer, which should 
process the data from an analog/digital converter, that was 
integrated in the accumulator. All PDP8 commands consist of only one
word. The minimal word length is 12 Bits, which still allows an usable
machine instruction set. 
In the course of time, the PDP8 was reimplemented more than 10 times
with the always corresponding semiconductor technology, but the 
instruction set stayed effectively unmodified. On this computer-family,
you can clearly see which influence the semiconductor technology has
for the computers with otherwise unmodified conditions. The PDP8/L was
the third reimplementation after the PDP8/S and the PDP8/I.
It was the very smallest computer that an engineer or a scientist could
put on his desktop for personal use. Only a Teletype-machine was 
required as a peripheral device, which is described on an other 
position here. Because of the core memory of the PDP8/L, programs
have only be loaded once and then stay at the memory after switch-off.
Thus working with the quite slow Teletype was practicable.
The labeling PDP is by the way a mimikry: At the beginning, computers
were considered as extremely expensive devices, which needed much 
space and electricity and had to be cooled down complexly. For 
operating engineers, programmers and operators were necessary. To 
improve the opportunity of sale, DEC didn't name it 'computer' but 
'Programmable Digital Processor'(PDP).
Thus the instruction set of the single PDP8-generations always stayed
compatible, by the time real big software pool was accumulated. 
This software was traded and distributed by DECUS, the 
'DEC User Society'. On the PDP8/L exposed at the museum, in the 
4K main memory run for example Focal-69 (an interpreter language), 
LISP 1.5, FORTRAN, SPASTC (an emulation of an scientific calculator), 
CHECKMO-II (a chess program) as well as various floating point 
interpreters. Additional reasons for the popularity of ths computers 
was the simple, but though very powerful instruction set as well as 
the fact that the hardware was very well-documented by the 
manufacturer. In the famous 'Small Computer Handbooks', which came
with every delivered computer, there was a exact manual, how they could
assemble extenstions by themselves, for example to automate theis 
measurement setups or their controls in the laboratory or at the 
producation.  
On the left of the picture, the opened PDP8/L can be viewed: On the 
top the Wire-Wrap-Field, where the single modules are wired with, 
right beside a stack with puncehd tapes, containing the provided 
system software, right in the front the 'Small Computer Handbook',
with that you can really understand computers and in front of the 
frontpanel two single punched tapes.


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