The PDP-3 is designed to accommodate a variety of input-output equipment. Of particular interest is the ease with which new, and perhaps unusual, external equipment can be added to the machine. Optional in-out devices include Cathode Ray Tube Display, Magnetic Tape, Real Time Clock, Line Printer and Analog to Digital Converters. The method of operation of PDP-3 with these optional devices is similar to the standard input-output equipment.
SEQUENCE BREAK SYSTEM
An optional in-out control is available for PDP-3. This control, termed the Sequence Break System, allows concurrent operation of several in-out devices and the main sequence. The system has, nominally, 16 automatic interrupt channels arranged in a priority chain.
A break to a particular sequence may be initiated by the completion of an in-out device, the program, or an external signal. If this sequence has priority, the C(AC), C(IO), C(PC), and C(IA) are stored in three fixed memory locations unique to that sequence. Since the C(PC) and C(IA) are eighteen bits each, these two registers are stored in one memory location. The next instruction is taken from a fourth location. This instruction is usually a jump to a suitable routine. The program is now operating in the new sequence. This new sequence may be broken by a higher priority sequence. A typical program loop for handling an in-out sequence would contain 3 to 5 instructions, including the appropriate iot. These are followed by load AD and load IO from the fixed locations and a special indirect jump through the location of the previous C(PC). This special jump also loads the IA. This last instruction terminates the sequence.
HIGH SPEED IN-OUT CHANNEL
The device connected to an in-out channel communicates directly with memory through the Memory Buffer Register. At the completion of each machine instruction, a check is made to see if the in-out channel has a word for, or needs a word from, the memory. When necessary, a memory cycle is taken to serve the channel. The operation is initiated by an in-out command. The in-out transfer command indicates the nature of the transfer. The left half of the In-Out Register must contain the starting address of the transfer, and the right half must contain the number of words to be transferred. If the Sequence Break System is connected, the completion of the transfer will signal the proper sequence. If no Sequence Break System is connected, the completion of the in-out channel transfer sets a program flag.
MAGNETIC TAPE
The system consists of tape units connected to the PDP-3 through a tape control (TC). This tape is read or written in IBM 729I format. Two hundred characters, each having 6 bits plus a parity bit, are written on each inch of tape and the tape moves at 75 inches/sec. The tape control has the job of connecting a specific unit to the PDP-3 and is a switch. It also has the function of controlling the format of information that is read or written on tape. In-out class commands instruct TC to the type of information transfer and select the tape unit. Another IOT command synchronizes the transfer of information through the TC to the computer.
The IOT order to select the unit and function is decoded as follows: 1) Three bits specify the function of TC. 2) The remaining 6 bits select the unit.
IOT Motion Commands for Magnetic Tape Units