Digital Equipment Corporation

“ 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. ”

“ The contents of octal digits 7 through 11 of the selected index register are added to the unmodified address (octal digits 7 through 11 of the instruction) .
This sum is used to locate the operand. The addition is done in the Index Adder which is a 15 bit 1's complement adder. The contents of the Accumulator and the In-Out Register are unaffected by the indexing process. An instruction which has used indexing is retained in memory with its original address unmodified. Memory registers 1-777 (octal) are available for use as normal memory registers if they are not being used as index registers. ”

“ An assembler or compiler prepares a machine language tape suitable for direct interpretation by the computer from a program tape in operator language. Generally speaking, one statement accepted by FRAP produces one instruction for the machine. A single statement written for the PDP-3 compiler, DECAL-3, may cause several instructions to be written. ”

“ 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. ”

“ PDP-3 is a stored program, general purpose digital computer. It is a single address, single instruction machine operating in parallel on 36 bit numbers. It features multiple step indirect addressing and indexing of addresses. The main memory makes 511 registers available as index registers.
The main storage is coincident current magnetic core modules of 4096 words each. The computer has a built-in facility to address 8 modules and can be expanded to drive 64 modules. The memory has a cycle time of five microseconds. ”

“ All computer flip-flops have indicator lights on the Console. These indicators are primarily for use when the machine has stopped or when the machine is being operated one step at a time. While the machine is running, the brightness of an indicator bears some relationship to the relative duty factor of that particular flip-flop. ”

“ 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 Central Processor requires a length of five and one half feet. The power cabinet is twenty-two inches long. A memory cabinet is thirty-two inches long and will hold three memory modules (12,288 words per cabinet) . Memory cabinets may be added at any time.
Magnetic tape units require twenty-two inches per transport. A tape unit cabinet may be connected as an extension of the Equipment Frame or may be a free-standing frame.
ENVIRONMENTAL REQUIREMENTS
The PDP-3 requires no special room preparation. The computer will operate properly over the normal range of room temperature. ”

“ For example, the operating time for a shift or rotate instruction is 5 +0.2N microseconds, where N is the number of shifts performed. The operating times for multiply and divide are functions of the number of ones in the multiplier and in the quotient, respectively. ”

“ Maintenance Routines are used exclusively to check the operation of the machine. These routines are operated while varying the bias supply voltages, and thus a check is made on possible degradation of all components which would affect the operation of the machine. ”

“ In addition to allowing program tapes to be prepared with off line equipment, an assembly program has other functions. Normally, the machine would require 36 bits or 12 octal digits to be written for each instruction used in the machine. ”