The PDP-12 – A Glimpse of a Computer From the Past
PDP or Programmed Data Processor refers to a series of mini-computers (a later coined term) that Digital Equipment Corporation created and sold between 1957 and 1990. They were called PDPs to avoid using the word “computer”. Computers at the time of release were huge and expensive. Ken Olsen, the creator of the PDP series, said computers users of the time”could not believe that in 1960 computers could do the job that could be built for less that $1 million” It was DEC's intention to market an inexpensive computer for smaller corporations and laboratories that would, in fact, do the job. The PDP series ranged from PDP-1 to PDP-16, with many technological advances along the way.
The PDP-12 itself, released in 1969, was a direct descendant of the Linc-8 (Laboratory Instrument Computer). Both mini-computers operated in two modes, one of those being LINC and the other PDP-8. These modes operated independently of each other on both mini-computers. Interestingly enough, the Linc-8 actually used two processors to accomplish the dual modes, while the PDP-12 was able to handle both modes on the same processor. This essentially made the Linc-8 obsolete and indeed, only a couple hundred were sold. Although not many of the PDP-12s were actually sold, they did manage to sell 755 units world-wide. These were fairly cheap for a computer during those days, selling for about $28,000. The PDP-12 had no direct successor and was the last mini-computer with the capability to operate in LINC mode. In fact, NEC skipped over PDP-13, possibly for superstitious reasons, but it is completely plausible that it was skipped to avoid confusion.
As stated earlier, the PDP-12 was much smaller than a traditional computer. It's cabinet was able to fit into a small laboratory. Within the cabinet, there was a 6.5 inch x 9 inch cathode ray tube (CRT) monitor, memory, the processor, an I/O bus, analog inputs, a relay buffer, tape drive, 8 inch floppy drives and a keyboard/printer. The ASR-33 keyboard/printer was connected to the I/O bus and could be accessed for both data input and output. It could accomplish input by means of a hole punch reader or by means of typing. It accomplished output by either printing characters on to paper or by punching holes into a card. Ten characters per second was the maximum transfer rate for input or output.
The memory capacity of the PDP-12 was initially 4 kilobytes using 12-bit words but could be upgraded in 4 kilobytes increments for a maximum total of 32 kilobytes of memory. In LINC mode the banks were divided into four 1024-word segments and only two of these segments were active at any given time. These segments included the Data Field, which was purely for indirectly accessed data, and the Information Field, which had all the directly accessible data and the executable program. Alternatively, the PDP-8 mode used 32 pages of 128 words each on 4 kilobytes of memory. If data was on a single page it could be accessed directly. If it was on different pages it could only be accessed indirectly. One of the advances that the PDP-12 made was that it was able to retain its memory contents and CPU registers when it was turned off.
The PDP-12 was aimed at scientists and engineers that had a lower budget and less space then some of the big laboratories and companies. The shortened words in the memory allowed for a smaller size, but also meant that programming these mini-computers was all done in machine language. At the time IBM was already coming up with easier ways to program, however their machines were much more expensive.
The main contribution of the PDP family in historical terms was the PDP-8 processor
This was small enough to fit into other devices. At the time of conception, IBM's mainframe computer was bulky and expensive and no one would consider using it for small applications such as controlling lights. The PDP family changed all this. It enabled many students, lab technicians, teachers, and even some of the general public to get their hands on a computer for the first time. Also, because of the inexpensive price, more and more labs and business were able to buy these machines to take on the task of some of their computing. This put IBM in the race to build a smaller, less expensive computer. Without the advent of the PDP family of mini-computers, personal computers as we know them today may not even exist.
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