Preliminary Specifications: Programmed Data Processor Model Three (PDP-3) October, 1960, Digital Equipment Corporation [book club suggestions .txt] 📗
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The Core Memory may be supplemented by Magnetic Tape Storage. This is described under Input-Output.
STANDARD INPUT-OUTPUTThe PDP-3 is designed to accommodate a variety of input-output equipment. Standard input-output units include a Paper Tape Reader, Paper Tape Punch and an Electric Typewriter.
A single instruction, In-Out Transfer (see Central Processor), performs all in-out operations through the 36 bit In-Out Register. The address portion of this instruction specifies the in-out function. One bit of the instruction selects an in-out halt as required.
PAPER TAPE READERThe Paper Tape Reader of the PDP-3 is a photoelectric device capable of reading 300 lines per second. Six lines form the standard 36 bit word when reading binary punched eight hole tape. Five, six and seven hole tape may also be read.
The reader will operate in one of two basic modes or in a third special mode.
Alphanumeric Mode
rpa iot 1
In this mode, one line of tape is read for each In-Out Transfer. All eight holes of the line are read. The information is left in the right eight bits of the In-Out Register, the remainder of the register being left clear. The standard PDP alphanumeric paper tape code includes an odd parity bit which may be checked by the program. Tape of non-standard width would be read in this mode.
Binary Mode
rpb iot 2
For each In-Out Transfer instruction, six lines of paper tape are read and assembled in the In-Out Register to form a full computer word. For a line to be recognized in this mode, the eighth hole must be punched; i.e., lines with no eighth hole will be skipped over. The seventh hole is ignored. The pattern of holes in the binary tape is arranged so as to be easily interpreted visually in terms of machine instruction.
Read-In Mode
This is a special mode activated by the "Read-In" Switch on the Console. It provides a means of entering programs which neither rely on read-in programs in memory nor require a plug board. Pushing the "Read-In" Switch starts the reader in the binary mode. The first group of six lines and alternate succeeding groups of six lines are interpreted as "Read-In" mode instructions. Even-numbered groups of 6 lines are data. The "Read-In" mode instructions must be either "deposit in-out" (dio Y) or "jump" (jmp Y). If the instruction is dio Y, the next group of six binary lines will be stored in memory location Y and the reader continues moving. If the instruction is jmp Y, the "Read-In" mode is terminated and the computer will commence operation at the address of the jump instruction.
PAPER TAPE PUNCHThe standard PDP-3 Paper Tape Punch has a nominal speed of 20 lines per second. It can operate in either the alphanumeric mode or the binary mode.
Alphanumeric Mode
ppa iot 5
For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit 35 conditions hole #1. Bit 34 conditions hole #2, etc. Bit 28 conditions hole #8.
Binary Mode
ppb iot 6
For each In-Out Transfer instruction one line of tape is punched. In-Out Register bit five conditions hole #1. Bit four conditions hole #2, etc. Bit zero conditions hole #6. Hole #7 is left blank. The #8 hole is always punched in this mode.
TYPEWRITERThe Typewriter will operate in the input mode or the output mode.
Output Mode
tyo iot 3
For each In-Out Transfer instruction one character is typed. The character is specified by the right six bits of the In-Out Register.
Input Mode
tyi iot 4
This operation is completely asynchronous and is therefore handled differently than any of the preceding in-out operations.
When a Typewriter key is struck, Program Flag Number One is set. At the same time the code for the struck key is presented to gates connected to the right six bits of the In-Out Register. This information will remain at the gate for a relatively long time by virtue of the slow mechanical action. A program designed to accept typed-in data would periodically check the status of Program Flag One. If at any time Program Flag One is found to be set, an In-Out Transfer instruction with address four must be executed for information to be transferred. This In-Out Transfer normally should not use the optional in-out halt. The information contained in the Typewriter's coder is then read into the right six bits of the In-Out Register.
OPTIONAL INPUT-OUTPUTThe 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 SYSTEMAn 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 CHANNELThe 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 TAPEThe 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
IOT Code Abbreviation Function 73....nn 60 mrb Read a binary record. 73....nn 61 mra Read an alphanumeric (BCD) record. 73....nn 62 mbb Backspace a binary record. 73....nn 63 mba Backspace an alphanumeric record. 73....nn 64 mwb Write a binary record. 73....nn 65 mwa Write an alphanumeric record. 73....nn 66 mlp Move tape to lead point (rewind).Where the octal digits, nn, specify the unit number.
The motion commands have the deferred bit, thus, the program halts. If the TC is free, the command will be transferred to the tape control for action and the program restarts immediately. If the tape control is currently busy with an instruction, i.e., it hasn't finished a previous command, the motion command is held up until TC is free to execute the new command.
The transfer of information from the computer to the TC is accomplished with the pause and skip command, MPS or IOT 70. This command has the deferred bit and halts a program until the TC can handle the transfer. On completion, the transfer occurs and the program restarts. This is used exclusively to synchronize the flow of information between a tape unit and the computer. This command normally skips the following instruction. If a flag is set in the TC, indicating incorrect information flow, the skip does not take place.
The TC contains a 36 bit buffer which holds a complete word while information is read or written. When an MPS order is given and the unit is reading, the TC buffer is read into the IO. The MPS order given during writing causes the IO to be transferred to the TC buffer.
Various conditions occurring in the TC cause the no-skip condition, when an MPS is given. Tape control flags are examined by the command, examine and clear flags, MEC or IOT 71. When MEC is given, the flags are put into the IO for program interrogation, and the flags cleared. The flags are: parity, end of tape, an end of record flag, and reading-writing check.
The parity flag is set if the parity condition is not met while the tape is being read (during MWA, MWB, MRA, or MRB).
The end of tape flag is set when the tape comes to the end of tape, moving in either direction.
Three conditions set the read-write check flag: 1) If TC is inactive, i.e., no unit or function selected, and an MPS instruction is given. The MPS becomes a no-operation, no-halt instruction. 2) When reading information and not emptying the TC buffer, by giving an MPS before more information arrives from tape. 3) A unit becomes unavailable during a normal sequence.
The end of record flag is set during reading or backspacing when the tape comes to an end of record gap.
Writing a Record of Information
Information is written on the tape by giving a MWB or MWA command. This sets a write binary or a write alphanumeric into the TC and selects the unit. A motion select command is executed immediately if the TC is free, otherwise, the command waits until it can be executed. Normal programming can continue after the MWA or MWB is given for approximately 5 milliseconds. At this time, an MPS order is given and the program pauses until information can be written. When the MPS is restarted, information is transferred to the TC buffer from the IO. If no flags have been set, the following instruction is skipped.
Three-quarter inches of blank tape is written by giving either the MWA or MWB order. An end of file is written as follows: 1) Four MWA commands write three inches of blank tape. 2) Then end of file character is written by giving the MPS order.
Information is read and checked for correct parity while writing.
If too many program steps are given between the motion select command, MWA or MWB and the first MPS, the unit will
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