histcomp:pdp-10 [UWiki XXX]

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====== PDP-10 ======
Because of its prominent role in the early ARPANet and the influential operating systems developed for it, the PDP-10 remains an important fixture in hacker lore.

===== Instruction set =====
==== Conventions ====
The PDP-10 is a word-oriented machine. All instructions are 36 bits, and all memory addresses refer to whole words. By convention, all numbers are octal unless followed by ''.''. 

The PDP-10 instruction set is quite complex. Instructions are generally in one of these formats:

   000000000 0111 1 1111 112222222222333333
   012345678 9012 3 4567 890123456789012345
   ________________________________________
  |         |    | |    |                  |
  |   OP    | AC |I| X  |        Y         |  General instructions
  |_________|____|_|____|__________________|
  
   000 0000000 111 1 1111 112222222222333333
   012 3456789 012 3 4567 890123456789012345
   _________________________________________
  |   |       |   | |    |                  |
  |111|  DEV  |IOP|I| X  |        Y         |  I/O instructions
  |___|_______|___|_|____|__________________|
  
These should be kept in mind, as instruction mnemonics are laid out in almost the same way: ''OP AC,@Y(X)'', where any zeroed fields are omitted. 

As indicated in the above diagrams, bits are numbered left to right, or most significant to least significant. 

==== Effective addresses ====
All instructions use the X, Y, and I fields to calculate an //effective address//, which may or may not actually be an address. In the simplest case, X and I are zero, so the effective address is Y. If X is nonzero, then the referenced accumulator is used as an index register. 

More complex is the case when I is 1. The result of the above step is used to fetch a word from memory. The procedure is repeated on that word, including the indirect step if its I bit is set.