In-Out Transfer instructions that do not include the optional wait function require 5 microseconds. If the in-out device requires a wait time for completion, the operating time depends upon the device being used.
If an instruction includes reference to an index register, an additional 5 microseconds is required. Each step of indirect addressing also requires an additional 5 microseconds.
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.
The PDP-3 is a "fixed" point machine using binary arithmetic. Negative numbers are represented as the 1's complement of the positive numbers. Bit 0 is the sign bit which is ZERO for positive numbers. Bits 1 to 35 are magnitude bits with bit 1 being the most significant and bit 35 being the least significant.
The actual position of the binary point may be arbitrarily assigned to best suit the problem in hand. Two common conventions in the placement of the binary point are:
1. The binary point is to the right of the least significant digit, thus, numbers represent integers.