OPTIONAL INPUT-OUTPUT

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

IOT CodeAbbreviationFunction

73....nn 60mrbRead a binary record.

73....nn 61mraRead an alphanumeric (BCD) record.

73....nn 62mbbBackspace a binary record.

73....nn 63mbaBackspace an alphanumeric record.

73....nn 64mwbWrite a binary record.

73....nn 65mwaWrite an alphanumeric record.

73....nn 66mlpMove 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 deselect (or disconnect). The MPS is then a no-operation command.

Writing Program

As an example, a program to write k words in binary format from storage beginning in register A, using tape unit number 04, is shown. The following program is written in standard FRAP language. The program begins in register enterwrite.

enterwritemec,clear flags initially

mwb 400,73000000464

lir x/-k+1,initialize index register x

blio x/a+k-1,begin loop

mps,wait for TC then write C(Z)

jmp c,error

spx x/1,add 1 to index register x

jmp b,return of loop

jmp done,record written

cmec,tape error

ril 1

spi

jmp rwcstop,read-write error or tape fault

ril 1

spi

jmp b+3,tape end

hlt,tape parity

done,resume programming

Reading Information

Information is read by giving the MRA or MRB order. Almost 10 ms. is available after a read order is given before information actually enters the TC buffer.

To read a record of unknown length, the read order is first given. The MPS order halts the program until six characters are assembled in the TC information buffer. The next instruction after the MPS, a jump instruction, transfers control from the loop when any flag is set. The next instruction deposits the IO. The record length is determined by not skipping after the MPS order on the setting of the end of record flag. The read-write check flag or the end of record flag is then interrogated to see that the tape is actually at the end of record. If a tape is not at the end of record, then the tape is either at the end of the reel, or a parity check has occurred.

Reading Program

Program to read j binary words into storage beginning in register d, using tape unit 10, j is unknown. The program begins in register enteread.

entereadmec,clear flags initially

mrb 1000,730000001060

dzm x,put zero in memory location x

emps

jmp outcheck

dio x/d,store in location modified by x

snx x/+1,add 1 to C(x)

jmp e

outcheckmec,examine flags

spi,end of record?

jmp recordend,yes

hlt,error

recordendsnx x/+1,to find value of j

",resume programming C(IA) = j

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Forward Spacing

Forward spacing is done by giving an MRB or MRA order. This moves the tape forward with the read-write head positioned at the end of the following record. If n read orders are given, the tape is spaced forward n records. By giving the MEC order, parity flags are examined to see that information on tape has been read correctly.

Backspacing

By giving an MBA or MBB order the tape is moved backwards a record with the read-write heads positioned in the previous end of record gap. The end of record flag is set when the tape has moved backwards a record.

Rewinding

Rewinding is accomplished by giving the rewind order, move tape to load point, MLP. The rewind order starts a unit rewinding and does not tie up the TC. If a motion command is given which calls for a unit that is rewinding, the command is executed, but the action will not take place until the unit is available.

Unit Availability

A unit is unavailable to the program under the following conditions:

Unit is rewinding.

Tape is improperly loaded.

Cover door open.

Unit overloaded.

Unit under manual control.

Power off.

A selected but unavailable unit holds up the TC if a motion order is given for the unit. The TC will be held up until the unit is ready.

Flag Positions

IO BitFlag

0EOR — End of record

1RWF — Read-Write

2EOT — End of Tape

3Parity

Connection with High Speed Channel

The high speed channel directs the tape control, and word transfer, just as a program would. A unit is first started reading or writing. The high speed channel is given the memory location of the information, and the number of registers the words read or written will occupy. The channel effects the information transfer. Thus, a high speed channel connected to a tape control handles the programming for the unit word transfers.

Completion of the block transfer is signified by either setting a program flag, or entering the sequence break.

Connection with Sequence Break System

When the TC is connected to the Sequence Break System, the program is automatically interrupted each time an MPS command needs to be given.

Programming is unaffected during reading and a record may be read with no flags set. The TC initiates breaks so that an MPS may be given in time.

Similarly, the break is initiated during writing each time an MPS needs to be given.

Motion Command Summary

Time before first MPS Time between MPS's Time after End of Record to deselect Flags that may be set

MWA MWB 3 ms. 400 us. (longer time causes deselection) 10 ms. RWF (if unit is deselected and MPS given, or unit becomes unavailable), Parity, EOT.

MRA MRB 7 ms. 400 us. (longer time misses information, and rwc set) 5 ms. RWF, (if information is missed, or unit becomes unavailable), EOT, EOR, Parity.

MBA MBB — — 10 ms. RWF (if unit becomes unavailable), EOR, EOT.

CATHODE-RAY-TUBE DISPLAY

The PDP-3 Cathode Ray Tube Display is useful for presentation of graphical or tabular information to the operator. It uses a 16 inch round tube with magnetic deflection. For each In-Out transfer order, one point is displayed at the position indicated by the In-Out Register. Bits 0-9 of the IO indicate the X coordinate of the position, and bits 18-27 indicate the Y coordinate. The display takes 60 microseconds.

An additional display option is a Light Pen. By use of this device the computer is signaled that the operator is interested in the last point displayed. Thus the program can take appropriate action such as changing the display or shifting operation to another program.

A smaller display is available. This display uses a five inch, high resolution cathode ray tube. The tube is equipped with a mounting bezel to accept a camera or photomultiplier device. The operation of this display is similar to that of the 16 inch, except that 12 bits are decoded for each axis.

REAL TIME CLOCK

A special input register may be connected to operate as a Real Time Clock. This is a counting register operated by a crystal controlled oscillator. The clock can be reset to zero by manual operation. A toggle switch interlock prevents an accidental reset. The state of this counter may be read at any time by the appropriate In-Out Transfer instruction.

LINE PRINTER

A 72 column Anelex printer and control are available as an option for PDP-3. The control contains a one line buffer. This buffer is cleared by the completion of an order to space the paper one position (psp). The buffer is filled from the In-Out Register by a succession of 12 load buffer orders (plb). The first plb will put the six characters represented by C(IO) in the leading (left-hand) column positions of the buffer. After the buffer is loaded, the order, print (pnt), is given.