PDP-8
[2] Similar machines from DEC are the PDP-12 which is a modernized version of the PDP-8 and LINC concepts, and the PDP-14 industrial controller system.It uses diode–transistor logic packaged on flip chip cards in a machine about the size of a small household refrigerator.Using a one-bit serial arithmetic logic unit (ALU) allowed the PDP-8/S to be smaller and less expensive, although slower than the original PDP-8.The chief engineer who designed the initial version of the PDP-8 was Edson de Castro, who later founded Data General.The greatest historical significance was that the PDP-8's low cost and high volume made a computer available to many new customers for many new uses.Some ambitious programming projects failed to fit in memory or developed design defects that could not be solved.As design advances reduced the costs of logic and memory, the programmer's time became relatively more important.[11] Subsequent computer designs emphasized ease of programming, typically using larger and more intuitive instruction sets.The programmed I/O bus typically runs low to medium-speed peripherals, such as printers, teletypes, paper tape punches and readers, while DMA is used for cathode-ray tube screens with a light pen, analog-to-digital converters, digital-to-analog converters, tape drives, and disk drives.To save money, the design uses inexpensive main memory for many purposes that are served by more expensive flip-flop registers in other computers,[17] such as auxiliary counters and subroutine linkage.They used thousands of very small, standardized logic-modules, with gold connectors, integrated by a costly, complex wire-wrapped backplane in a large cabinet.Over time, I/O systems such as magnetic tape, RS-232 and current loop dumb terminals, punched card readers, and fixed-head disks were added.Modern enthusiasts have created standard PC style IDE hard disk adapters for real and simulated PDP-8 computers."Data break" places the processor in charge of maintaining the DMA address and word count registers.However, the I/O devices need more electronic logic to manage their own word count and transfer address registers.Early PDP-8 systems were shipped with no pre-installed software; each time the PDP-8 was powered up, the user hand-entered instructions using a bank of 12 toggle switches.Paper-tape versions of a number of programming languages became available, including DEC's FOCAL interpreter[35] and a 4K FORTRAN compiler and runtime.Languages include a special version of BASIC, a FORTRAN subset similar to FORTRAN-1 (no user-written subroutines or functions), an ALGOL subset, FOCAL, and an assembler called PAL-D. A fair amount of user-donated software for the PDP-8 was available from DECUS, the Digital Equipment Corporation User Society, and often came with full source listings and documentation.The combination CMA IAC, which the assembler lets you abbreviate as CIA, produces the arithmetic inverse of AC: the twos-complement negation.Since there is no subtraction instruction, only the twos-complement add (TAD), computing the difference of two operands, requires first negating the subtrahend.By De Morgan's laws, this means that the skip is performed if all of the specified inverted conditions are true.Unused bit combinations of OPR are defined as a third Group of microprogrammed actions mostly affecting the MQ (Multiplier/Quotient) register.A set of I/O instructions in the range 6200 through 6277 is handled by the Memory Extension Controller and give access to the DF and IF registers.It was more complicated for multiple-field programs to deal with field boundaries and the DF and IF registers than it would have been if they could simply generate 15-bit addresses, but the design provided backward compatibility and is consistent with the 12-bit architecture used throughout the PDP-8.Compare the later Intel 8086, whose 16-bit memory addresses are expanded to 20 bits by combining them with the contents of a specified or implied segment register.Moreover, additional 4K fields could be allocated to separate users, turning 4K FOCAL into a multi-user timesharing system.Fortunately, as a jump usually is the next instruction after CIF, this emulation does not slow programs down much, but it is a large workaround to a seemingly small design deficiency.The following piece of code shows what is needed just to compare two numbers: As shown, much of the text of a typical PDP-8 program focuses not on the author's intended algorithm but on low-level mechanics.Programs intended to be placed into ROMs approach this problem in several ways: The use of the JMS instruction makes debugging difficult.Although it is not a production design, as it uses more modern SSI and MSI components and solid state rather than core memory, the exercise provides a detailed description of the computer's operation.
The National Museum of ComputingBletchleyDigital Equipment CorporationProgrammed Data ProcessorMinicomputerPlatformPDP-1212-bitminicomputersDigital Equipment Corporation (DEC)instruction setPDP-14industrial controllerDECtapeStuttgart Computer Museumdiode–transistor logicflip chiprefrigeratorone-bit serialarithmetic logic unittransistor–transistor logicIntersilIntersil 6100Edson de CastroData Generalcompilersreduced instruction set computerW.A. ClarkC.E. MolnarSeymour CrayCDC 160multiple-precision arithmeticfloating pointsignificandIBM 1130IBM System/360main memoryswitch banks of memoriesmagnetic-core memorycycle timemicrosecondsSix-bit character codesregistersaccumulatorprogram countercarry flagmemory-buffer registermemory-address registercore memorydirect memory accessprintersteletypespaper tapecathode-ray tubelight penanalog-to-digital convertersdigital-to-analog converterstape drivesdisk drivesflip-flopsimplicity of designwire-wrappedfan-outlogic gatesmicrocontrollershybrid ICsLINC-8PDP-8/EmicroprocessorHarrisDECmateRaspberry Piemulatedopen-source hardwarefront panelpaper-tapeteletypemagnetic tapeRS-232current loopdumb terminalspunched cardfixed-head disksfloppy diskscartridgebackplanesystem busbootstrap loaderFORTRANline modeeditorcompilerreal-time operating systempreemptive multitaskingtime-sharinginstructionpage 0Two's complementreturn addresslogically 'or'De Morgan's lawsIntel 808616-bitsegment registerassembly language"Hello, world!"contextsubroutineinterruptreentrancyrecursionCID-201Douglas W. JonesMcCusker, J. J.American Antiquarian SocietySmithsonian InstitutionC. Gordon BellDigital PressGordon BellAllen NewellSimon Fraser UniversitySmithsonianSteve Gibson's18-bitPDP-1536-bit