The instructions for PDP-3 may be divided into three classes:
1. Indexable memory instructions
2. Non-indexable memory instructions
3. Non-memory instructions.
The layout of the instruction word is shown in Fig. 2.
The octal digits 0 and 1 define the instruction code, thus, there are 64
possible instruction codes, not all of which are used. The first bit of
octal digit 2 is the indirect address bit. If this bit is a ONE,
indirect addressing occurs.
The index address, X, is in octal digits 3, 4, and 5. These digits
address an index register for memory-type instructions. If these digits
are all ZERO, indexing will not take place. In main memory, 511 of the
registers can be used as automatic index registers.
The instruction base address, Y, is in octal digits 7 through 11. These
digits are sufficient to address 32,768 words of memory. Octal digit 6
is reserved for further memory expansion. Space is available in the
equipment frame for this expansion, should it prove desirable.
In those instructions which do not refer to memory, the memory address
digits, Y, and in some cases the index address digits also, are used to
specify the variations in any group of instructions. An example of this
is in the shift and rotate instructions in which the memory address
digits determine the number of shifts.