Possibly the best-known 12-bit CPU is the PDP-8 and its relatives, produced in various incarnations from August 1963 to mid-1990. Many ADCs have a 12-bit resolution. Some PIC microcontrollers use a 12-bit word size....
PDP-8 was the first successful commercial minicomputer
Minicomputer
A minicomputer is a class of multi-user computers that lies in the middle range of the computing spectrum, in between the largest multi-user systems and the smallest single-user systems...
Digital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
in the 1960s. DEC introduced it on 22 March 1965, and sold more than 50,000 systems, the most of any computer up to that date. It was the first widely sold computer in the DEC PDP
Programmed Data Processor
Programmed Data Processor was the name of a series of minicomputers made by Digital Equipment Corporation. The name 'PDP' intentionally avoided the use of the term 'computer' because, at the time of the first PDPs, computers had a reputation of being large, complicated, and expensive machines, and...
series of computers (the PDP-5 was not originally intended to be a general-purpose computer). The chief engineer who designed the initial version of the PDP-8 was Edson de Castro
Edson de Castro
Edson de Castro is a computer engineer perhaps best known for designing the Data General Nova series of computers.De Castro was founder and CEO of Data General Corporation throughout the 1970s, the 1980s and into the 1990s when he was replaced by Ronald L Skates, a former Price Waterhouse Coopers...
Data General was one of the first minicomputer firms from the late 1960s. Three of the four founders were former employees of Digital Equipment Corporation. Their first product, the Data General Nova, was a 16-bit minicomputer...
.
The earliest PDP-8 model (informally known as a "Straight-8") used diode-transistor logic
Diode-transistor logic
Diode–transistor logic is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function is performed by a diode network and the amplifying function is performed by a transistor .- Implementations :The DTL circuit shown in...
Flip-Chip modules were used in the DEC PDP-7 , PDP-8, PDP-9 and PDP-10, beginning on August 24, 1964. The trademark "Flip-Chip"' was filed on August 27, 1964...
cards, and was about the size of a minibar-fridge.
This was followed by the PDP-8/S, available in desktop and rack-mount models. By using a one-bit serial ALU
Arithmetic logic unit
In computing, an arithmetic logic unit is a digital circuit that performs arithmetic and logical operations.The ALU is a fundamental building block of the central processing unit of a computer, and even the simplest microprocessors contain one for purposes such as maintaining timers...
implementation, the PDP-8/S was smaller, less expensive, but vastly slower than the original PDP-8. The only mass storage peripheral available for the PDP-8/S was the DF32 disk.
Later systems (the PDP-8/I and /L, the PDP-8/E, /F, and /M, and the PDP-8/A) returned to a faster, fully parallel implementation but used much less-expensive TTL
Transistor-transistor logic
Transistor–transistor logic is a class of digital circuits built from bipolar junction transistors and resistors. It is called transistor–transistor logic because both the logic gating function and the amplifying function are performed by transistors .TTL is notable for being a widespread...
MSI logic. Most surviving PDP-8s are from this era. The PDP-8/E is common, and well-regarded because so many types of I/O devices were available for it. It was often configured as a general-purpose computer.
In 1975, early personal computers based on inexpensive microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
TRS-80 was Tandy Corporation's desktop microcomputer model line, sold through Tandy's Radio Shack stores in the late 1970s and early 1980s. The first units, ordered unseen, were delivered in November 1977, and rolled out to the stores the third week of December. The line won popularity with...
The Apple II is an 8-bit home computer, one of the first highly successful mass-produced microcomputer products, designed primarily by Steve Wozniak, manufactured by Apple Computer and introduced in 1977...
and others began to dominate the market for small general purpose computers.
The last commercial PDP-8 models in 1979 were called "CMOS-8s" and used custom CMOS microprocessors. They were not priced competitively, and the offering failed. The IBM PC
IBM PC
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...
in 1981 cemented the doom of the CMOS-8s by making a legitimate, well-supported small microprocessor computer.
Intersil Corporation is an American company that specializes in the design, development and manufacturing of high-performance analog semiconductors for four high-growth markets — Communications, Computing, High End Consumer and Industrial.-Company history:...
sold the integrated circuits commercially through to 1982 as the Intersil 6100 family. By virtue of their CMOS technology they had low power requirements and were used in some embedded military systems.
Architectural significance
The PDP-8 combined low cost, simplicity, expandability and careful engineering for value. The greatest historical significance was that the PDP-8's low cost and high volume made a computer available to many new uses and people. Its continuing significance is as a historical example of value-engineered computer design.
The low complexity brought other costs. It made programming cumbersome, as is seen in the examples in this article and from the discussion of "pages" and "fields." 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 more important. Subsequent computer designs emphasized ease of programming, typically using a larger and more intuitive instruction set.
Eventually, most machine-language programming came to be generated by compilers and report generators. The reduced instruction set computer
Reduced instruction set computer
Reduced instruction set computing, or RISC , is a CPU design strategy based on the insight that simplified instructions can provide higher performance if this simplicity enables much faster execution of each instruction. A computer based on this strategy is a reduced instruction set computer...
returned full-circle to the PDP-8's emphasis on a simple instruction set and achieving multiple actions in a single instruction cycle, in order to maximize execution speed, though the newer computers had much longer instruction words.
Description
The PDP-8 used ideas from several 12-bit predecessors, most notably the LINC
LINC
The LINC was a 12-bit, 2048-word computer. The LINC can be considered the first minicomputer and a forerunner to the personal computer....
Wesley Allison Clark is a computer scientist and one of the main participants, along with Charles Molnar, in the creation of the LINC laboratory computer, which was the first mini-computer and shares with a number of other computers the claim to be the inspiration for the personal computer.Clark...
Charles Edwin Molnar was a co-developer of one of the first minicomputers, the LINC , while a graduate student at the Massachusetts Institute of Technology in 1962. His collaborator was Wesley A. Clark....
Seymour Roger Cray was an American electrical engineer and supercomputer architect who designed a series of computers that were the fastest in the world for decades, and founded Cray Research which would build many of these machines. Called "the father of supercomputing," Cray has been credited...
The CDC 160 and CDC 160-A were 12-bit minicomputers built by Control Data Corporation from 1960 to 1965. The 160 was designed by Seymour Cray - reportedly over a long three-day weekend...
minicomputer.
The architecture had a simple programmed I/O bus, plus a DMA
Direct memory access
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
channel. The programmed I/O bus would typically run low to medium-speed peripherals, such as printers, teletypes, paper tape punches and readers, while DMA was used for cathode ray tube
Cathode ray tube
The cathode ray tube is a vacuum tube containing an electron gun and a fluorescent screen used to view images. It has a means to accelerate and deflect the electron beam onto the fluorescent screen to create the images. The image may represent electrical waveforms , pictures , radar targets and...
A light pen is a computer input device in the form of a light-sensitive wand used in conjunction with a computer's CRT TV set or monitor. It allows the user to point to displayed objects, or draw on the screen, in a similar way to a touch screen but with greater positional accuracy...
An analog-to-digital converter is a device that converts a continuous quantity to a discrete time digital representation. An ADC may also provide an isolated measurement...
In electronics, a digital-to-analog converter is a device that converts a digital code to an analog signal . An analog-to-digital converter performs the reverse operation...
A tape drive is a data storage device that reads and performs digital recording, writes data on a magnetic tape. Magnetic tape data storage is typically used for offline, archival data storage. Tape media generally has a favorable unit cost and long archival stability.A tape drive provides...
s, disk drives.
The word size, 12 bits, is large enough to handle unsigned integers from 0 to 4095 – wide enough for controlling simple machinery. This is also enough to handle signed numbers from -2048 to +2047. This is higher precision than a slide rule
Slide rule
The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...
An analog computer is a form of computer that uses the continuously-changeable aspects of physical phenomena such as electrical, mechanical, or hydraulic quantities to model the problem being solved...
s. Twelve bits could also store two six-bit ASCII subset characters.
To save money, the design uses inexpensive main memory for many purposes that are served by more expensive flip-flop
Flip-flop
Flip-flops, thongs, Japanese sandals, or jandals are an open type of outdoor footwear, consisting of a flat sole held loosely on the foot by a Y-shaped strap, like a thin thong, that passes between the first and second toes and around either side of the foot...
registers in other computers.
The PDP-8's basic configuration had a main memory of 4,096 twelve-bit words. An optional memory-expansion unit could switch banks of such memories using the IOT instruction.
At its inception, the programmer's view of the PDP-8 had only eight instruction
Instruction set
An instruction set, or instruction set architecture , is the part of the computer architecture related to programming, including the native data types, instructions, registers, addressing modes, memory architecture, interrupt and exception handling, and external I/O...
In computer architecture, a processor register is a small amount of storage available as part of a CPU or other digital processor. Such registers are addressed by mechanisms other than main memory and can be accessed more quickly...
In a computer's central processing unit , an accumulator is a register in which intermediate arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation to main memory, perhaps only to be read right back again for...
, AC, and a carry bit called the "link register", L). The machine used magnetic core memory
Magnetic core memory
Magnetic-core memory was the predominant form of random-access computer memory for 20 years . It uses tiny magnetic toroids , the cores, through which wires are threaded to write and read information. Each core represents one bit of information...
A microsecond is an SI unit of time equal to one millionth of a second. Its symbol is µs.A microsecond is equal to 1000 nanoseconds or 1/1000 millisecond...
s, so that a typical two-cycle (Fetch, Execute) memory-reference instruction ran at a speed of 0.333 MIPS. The 1974 Pocket Reference Card gave a basic instruction time of 1.2 microseconds, or 2.6 microseconds for instructions that referenced memory. Later machines added a second register (the Multiplier/Quotient Register, MQ). An extended arithmetic element option added actual multiply and divide instructions.
A one instruction set computer , sometimes called an ultimate reduced instruction set computer , is an abstract machine that uses only one instruction – obviating the need for a machine language opcode...
. The CPU of the serial model, the PDP-8/S, had only about 519 logic gates, while small microcontrollers (as of 2008) usually have 15,000 or more. Compared to more complex machines, unnecessary features were removed, and logic was shared when possible. Instructions used autoincrement, autoclear and indirect access to increase the software's speed, reduce memory use and substitute inexpensive memory for expensive registers. A basic PDP-8 CPU has only four 12-bit registers, the accumulator, program counter, memory-buffer register
Memory data register
The Memory Data Register is the register of a computer's control unit that contains the data to be stored in the computer storage , or the data after a fetch from the computer storage...
The Memory Address Register is a CPU register that either stores the memory address from which data will be fetched to the CPU or the address to which data will be sent and stored....
. To save money, these were all used for multiple purposes at different points in the operating cycle. For example, the memory buffer register provided arithmetic operands, was part of the instruction register, and stored data to rewrite the core memory. (This restores the core data destroyed by the read.) In spite of its basic simplicity, the use of very small modules made it an expensive machine, with a large part of the cost sunk into the small modules, the thousands of gold-plated connectors, and the complex wire-wrapped backplane. In the 8/S two different logic voltages were used, an inexpensive way to increase the fan-out of the inexpensive diode-transistor logic
Diode-transistor logic
Diode–transistor logic is a class of digital circuits that is the direct ancestor of transistor–transistor logic. It is called so because the logic gating function is performed by a diode network and the amplifying function is performed by a transistor .- Implementations :The DTL circuit shown in...
.
Versions of the PDP-8
The total sales figure for the PDP-8 family has been estimated at over 300,000 machines. The following models were manufactured:
LINC-8 was the name of a minicomputer manufactured by Digital Equipment Corporation between 1966 and 1969. It combined a LINC computer with a PDP-8 in one cabinet, thus being able to run programs written for either of the two architectures.-Architecture:...
Intersil Corporation is an American company that specializes in the design, development and manufacturing of high-performance analog semiconductors for four high-growth markets — Communications, Computing, High End Consumer and Industrial.-Company history:...
The Intersil 6100 family consisted of a 12-bit microprocessor and a range of peripheral support and memory ICs developed in the mid-1970s.The microprocessor recognised the PDP-8 instruction set....
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
Harris Corporation is a Florida-based international communications equipment company that produces wireless equipment, electronic systems, and both terrestrial and spaceborne antennas for use in the government, defense, and commercial sectors. It is also the largest private-sector employer in...
The Intersil 6100 family consisted of a 12-bit microprocessor and a range of peripheral support and memory ICs developed in the mid-1970s.The microprocessor recognised the PDP-8 instruction set....
CMOS single-chip PDP-8-compatible microprocessor (used in the DECmate
DECmate
DECmate was the name of a series of PDP-8-compatible computers produced by the Digital Equipment Corporation in the late 1970s and early 1980s. All of the models used an Intersil 6100 or Harris 6120 microprocessor which emulated the 12-bit DEC PDP-8 CPU...
word processors)
Latter-day implementations
Enthusiasts have created entire PDP-8 systems using single FPGA
Field-programmable gate array
A field-programmable gate array is an integrated circuit designed to be configured by the customer or designer after manufacturing—hence "field-programmable"...
devices. (This is possible because an entire PDP-8, its main memory system, and its I/O equipment is collectively much less complex than even the cache memories used in most modern microprocessors.)
Several software simulations of a PDP-8 are available on the Internet. The best of these correctly execute DEC's diagnostic software, and run historic operating systems and software. The software simulations often simulate late "large" PDP-8 systems with all possible peripherals. Even these use only a tiny fraction of the capacity of a modern personal computer.
Input/Output
The I/O systems underwent huge changes during the PDP-8 era. Early PDP-8 models used a front panel
Front panel
A front panel was used on early electronic computers to display and allow the alteration of the state of the machine's internal registers and memory. The front panel usually consisted of arrays of indicator lamps, toggle switches, and push buttons mounted on a sheet metal face plate...
Punched tape or paper tape is an obsolete form of data storage, consisting of a long strip of paper in which holes are punched to store data...
reader and a teletype printer with an optional paper-tape punch. Over time I/O systems such as magnetic tape
DECtape
DECtape, originally called "Microtape", was a magnetic tape data storage medium used with many Digital Equipment Corporation computers, including the PDP-6, PDP-8, LINC-8, PDP-10, PDP-11, PDP-12, and the PDP-15. On DEC's 32-bit systems, VAX/VMS support for it was implemented but did not become an...
In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
A current loop describes two different electrical signalling schemes.- Digital :For digital serial communications, a current loop is a communication interface that uses current instead of voltage for signaling...
A computer terminal is an electronic or electromechanical hardware device that is used for entering data into, and displaying data from, a computer or a computing system...
A punched card, punch card, IBM card, or Hollerith card is a piece of stiff paper that contains digital information represented by the presence or absence of holes in predefined positions...
readers, and fixed-head disks were added. Toward the end of the PDP-8 era, floppy disk
Floppy disk
A floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
drives were popular I/O devices. Modern enthusiasts have created standard PC
IBM PC compatible
IBM PC compatible computers are those generally similar to the original IBM PC, XT, and AT. Such computers used to be referred to as PC clones, or IBM clones since they almost exactly duplicated all the significant features of the PC architecture, facilitated by various manufacturers' ability to...
style IDE hard disk adapters for real and simulated PDP-8 computers.
I/O was supported through several different methods:
A backplane is a group of connectors connected in parallel with each other, so that each pin of each connector is linked to the same relative pin of all the other connectors forming a computer bus. It is used as a backbone to connect several printed circuit boards together to make up a complete...
dedicated slots for I/O controllers
A "Negative" I/O bus (using negative voltage signalling)
A "Positive" I/O bus (the same architecture using TTL signalling)
The Omnibus (a backplane of undedicated system bus
System bus
A system bus is a single computer bus that connects the major components of a computer system. The technique was developed to reduce costs and improve modularity....
Direct memory access is a feature of modern computers that allows certain hardware subsystems within the computer to access system memory independently of the central processing unit ....
called "three-cycle data break" was supported; this required the assistance of the processor. The "data break" method moved some of common logic needed to implement DMA I/O from each I/O device into one common copy of the logic within the processor. "Data break" placed the processor in charge of maintaining the DMA address and word count registers. In three successive memory cycles, the processor would update the word count, update the transfer address, and store or retrieve the actual I/O data word.
One cycle data break effectively tripled the DMA transfer rate because only the target data needed to be transferred to and from the core memory. However, the I/O devices needed more electronic logic to manage their own word count and transfer address registers. By the time the PDP-8/E was introduced, electronic logic had become less expensive and "one-cycle data break" became more popular.
Programming facilities
Early PDP-8 systems did not have an operating system, just a front panel
Front panel
A front panel was used on early electronic computers to display and allow the alteration of the state of the machine's internal registers and memory. The front panel usually consisted of arrays of indicator lamps, toggle switches, and push buttons mounted on a sheet metal face plate...
and run and halt switches. Software development systems for the PDP-8 series began with the most basic front panel entry of raw binary
Binary numeral system
The binary numeral system, or base-2 number system, represents numeric values using two symbols, 0 and 1. More specifically, the usual base-2 system is a positional notation with a radix of 2...
Machine code or machine language is a system of impartible instructions executed directly by a computer's central processing unit. Each instruction performs a very specific task, typically either an operation on a unit of data Machine code or machine language is a system of impartible instructions...
In computing, booting is a process that begins when a user turns on a computer system and prepares the computer to perform its normal operations. On modern computers, this typically involves loading and starting an operating system. The boot sequence is the initial set of operations that the...
entry).
In the middle era, various paper tape "operating systems" were developed. Many utility programs became available on paper tape. PAL-8 assembly language
Assembly language
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
In computer science, source code is text written using the format and syntax of the programming language that it is being written in. Such a language is specially designed to facilitate the work of computer programmers, who specify the actions to be performed by a computer mostly by writing source...
was often stored on paper tape, read into memory, and saved to paper tape. PAL assembled from paper tape into memory. Paper tape versions of a number of programming languages were available, including DEC's FOCAL interpreter and a 4K FORTRAN
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
compiler and runtime.
Toward the end of the PDP-8 era, operating systems such as OS/8
OS/8
OS/8 was the primary operating system used on the PDP-8 minicomputer developed by Digital Equipment Corporation of Maynard, Massachusetts. OS/8 was originally called MS/8 and, for a brief time, PS/8 before Digital settled on the name OS/8 in 1971.A virtually identical version of OS/8, called...
and COS-310 allowed a traditional line mode editor
Text editor
A text editor is a type of program used for editing plain text files.Text editors are often provided with operating systems or software development packages, and can be used to change configuration files and programming language source code....
A compiler is a computer program that transforms source code written in a programming language into another computer language...
development system using languages such as PAL-III assembly language, FORTRAN, BASIC, and DIBOL
DIBOL
DiBOL or Digital's Business Oriented Language is a general-purpose, procedural, imperative programming language, which is well-suited for Management Information Systems software development. It has a syntax similar to FORTRAN and BASIC, along with BCD arithmetic...
A real-time operating system is an operating system intended to serve real-time application requests.A key characteristic of a RTOS is the level of its consistency concerning the amount of time it takes to accept and complete an application's task; the variability is jitter...
and preemptive multitasking multi-user systems were available: a real-time system (RTS-8) was available as were multiuser commercial systems (COS-300 and COS-310) and a dedicated single-user word-processing system (WPS-8).
Time-sharing is the sharing of a computing resource among many users by means of multiprogramming and multi-tasking. Its introduction in the 1960s, and emergence as the prominent model of computing in the 1970s, represents a major technological shift in the history of computing.By allowing a large...
TSS-8 was a little time-sharing operating system co-written by Don Witcraft and John Everett at Digital Equipment Corporation in 1967. The operating system ran on the 12-bit PDP-8 computer and was released in 1968....
, was also available. TSS-8 allowed multiple users to log into the system via 110-baud terminals, and edit, compile and debug programs. Languages included a special version of BASIC, a FORTRAN
Fortran
Fortran is a general-purpose, procedural, imperative programming language that is especially suited to numeric computation and scientific computing...
subset similar to FORTRAN-1 (no user-written subroutines or functions), an ALGOL
ALGOL
ALGOL is a family of imperative computer programming languages originally developed in the mid 1950s which greatly influenced many other languages and became the de facto way algorithms were described in textbooks and academic works for almost the next 30 years...
subset, FOCAL, and an assembler called PAL-D.
A fair amount of user-donated software for the PDP-8 was available from DECUS
DECUS
The Digital Equipment Computer Users' Society was an independent computer user group related to Digital Equipment Corporation.The Connect User Group Community, formed from the consolidation in May, 2008 of DECUS, Encompass, HP-Interex, and ITUG is Hewlett-Packard’s largest user community...
, the Digital Equipment Corporation User Society, and often came with full source listings and documentation.
A bit field is a common idiom used in computer programming to compactly store multiple logical values as a short series of bits where each of the single bits can be addressed separately. A bit field is most commonly used to represent integral types of known, fixed bit-width. A well-known usage of...
of the 12-bit instruction word (labelled bits 0 through 2) are the operation code. For the six operations that refer to memory, bits 5 through 11 provide a 7-bit address. Bit 4, if set, says to complete the address using the 5 high-order bits of the PC
Program counter
The program counter , commonly called the instruction pointer in Intel x86 microprocessors, and sometimes called the instruction address register, or just part of the instruction sequencer in some computers, is a processor register that indicates where the computer is in its instruction sequence...
; if clear, zeroes are used. Bit 3 specifies indirection; if set, the address obtained as described so far points to a 12-bit value in memory that gives the actual effective address for the instruction. (The JMP instruction does not operate on a memory word, except if indirection is specified, but has the same bit fields.)
0
2
3
4
5
11
Operation
I
Z
Offset
Memory pages
This use of the instruction word divides the 4,096-word memory into 128-word pages; bit 4 of the instruction selects either the current page or page 0 (addresses 0000–0177 in octal
Octal
The octal numeral system, or oct for short, is the base-8 number system, and uses the digits 0 to 7. Numerals can be made from binary numerals by grouping consecutive binary digits into groups of three...
). Memory in page 0 is at a premium, since variables placed here can be addressed directly from any page. (Moreover, address 0000 is where any interrupt service routine must start, and addresses 0010–0017 have the special property of auto-incrementing preceding any indirect reference through them.)
The standard assembler places constant values for arithmetic in the current page. Likewise, cross-page jumps and subroutine calls use an indirect address in the current page.
It was important to write routines to fit within 128-word pages, or to arrange routines to minimize page transitions, as references and jumps outside the current page required an extra word. Consequently, much time was spent cleverly conserving one or several words. Programmers deliberately placed code at the end of a page to achieve a free transition to the next page as PC was incremented.
A bitwise operation operates on one or more bit patterns or binary numerals at the level of their individual bits. This is used directly at the digital hardware level as well as in microcode, machine code and certain kinds of high level languages...
Addition is a mathematical operation that represents combining collections of objects together into a larger collection. It is signified by the plus sign . For example, in the picture on the right, there are 3 + 2 apples—meaning three apples and two other apples—which is the same as five apples....
the memory operand to (a 13 bit signed value).
010 – ISZ – Increment the memory operand and Skip next instruction if result is Zero.
011 – DCA – Deposit AC into the memory operand and Clear AC.
In postal mail, a return address is an explicit inclusion of the address of the person sending the message. It provides the recipient with a means to determine how to respond to the sender of the message if needed....
in first word of subroutine!).
101 – JMP – JuMP.
110 – IOT – Input/Output Transfer (see below).
111 – OPR – microcoded OPeRations (see below).
IOT (Input-Output Transfer) instructions
The PDP-8 processor defined few of the IOT instructions, but simply provided a framework. Most IOT instructions were defined by the individual I/O devices.
0
2
3
8
9
11
6=IOT
Device
Function
Device
Bits 3 through 8 of an IOT instruction selected an I/O device. Some of these device addresses were standardized by convention:
00 was handled by the processor and not sent to any I/O device (see below)
01 was usually the high-speed paper tape reader
02 was the high-speed paper tape punch
03 was the console keyboard (and any associated low-speed paper tape reader)
04 was the console printer (and any associated low-speed paper tape punch)
Instructions for device 0 affected the processor as a whole. For example, ION (6001) enabled interrupt processing, and IOFF (6002) disabled it.
Function
Bits 9 through 11 of an IOT instruction selected the function(s) the device would perform. Simple devices (such as the paper tape reader and punch and the console keyboard and printer) would use the bits in standard ways:
Bit 11 caused the processor to skip the next instruction if the I/O device was ready
Bit 10 cleared AC
Bit 9 moved a word between AC and the device, initiated another I/O transfer, and cleared the device's "ready" flag
These operations took place in a well-defined order that gave useful results if more than one bit was set.
More complicated devices, such as disk drives, used these 3 bits in device-specific fashions. Typically, a device decoded the 3 bits to give 8 possible function codes.
OPR (OPeRate)
Many operations were achieved using OPR, including most of the conditionals. OPR does not address a memory location; conditional execution is achieved by conditionally skipping one instruction, which was typically a JMP.
The OPR instruction was said to be "microcoded." This did not mean what the word means today (that a lower-level program fetched and interpreted the OPR instruction), but meant that each bit of the instruction word specified a certain action, and the programmer could achieve several actions in a single instruction cycle by setting multiple bits. In use, a programmer would write several instruction mnemonics alongside one another, and the assembler would combine them with OR
Logical disjunction
In logic and mathematics, a two-place logical connective or, is a logical disjunction, also known as inclusive disjunction or alternation, that results in true whenever one or more of its operands are true. E.g. in this context, "A or B" is true if A is true, or if B is true, or if both A and B are...
to devise the actual instruction word. Many I/O devices supported "microcoded" IOT instructions.
Microcoded actions took place in a well-defined sequence designed to maximize the utility of many combinations.
The OPR instructions came in Groups. Bits 3, 8 and 11 identify the Group of an OPR instruction, so it was impossible to combine the microcoded actions from different groups.
In most cases, the operations are sequenced so that they can be combined in the most useful ways. For example, combining CLA (CLear Accumulator), CLL (CLear Link), and IAC (Increment ACcumulator) first clears the AC and Link, then increments the accumulator, leaving it set to 1. Adding RAL to the mix (so CLA CLL IAC RAL) causes the accumulator to be cleared, incremented, then rotated left, leaving it set to 2. In this way, small integer constants were placed in the accumulator with a single instruction.
The combination CMA IAC, which the assembler let you abbreviate as CIA, produced the arithmetic inverse of AC: the twos-complement negation. Since there was no subtraction instruction, only the twos-complement add (TAD), computing the difference of two operands required first negating the subtrahend.
A Group 1 OPR instruction that has none of the microprogrammed bits set performs no action. The programmer can write NOP
NOP
In computer science, NOP or NOOP is an assembly language instruction, sequence of programming language statements, or computer protocol command that effectively does nothing at all....
A bitwise operation operates on one or more bit patterns or binary numerals at the level of their individual bits. This is used directly at the digital hardware level as well as in microcode, machine code and certain kinds of high level languages...
In the x86 computer architecture, HLT is an assembly language instruction which halts the CPU until the next external interrupt is fired. Such interrupts are used by devices in order to signal to the CPU that an event occurred which the CPU shall react on...
– Halt
When bit 8 is clear, a skip is performed if any of the specified conditions are true. For example "SMA SZA", opcode 7540, skips if AC ≤ 0.
A Group 2 OPR instruction that has none of the microprogrammed bits set is another No-Op instruction.
When bit 8 is set, the Group 1 skip condition is inverted: the skip is not performed if any of the group 1 conditions are true, meaning that all of the specified skip conditions must be true. For example, "SPA SNA", opcode 7550, skips if AC > 0. If none of bits 5–7 are set, then the skip is unconditional.
Group 3
Unused bit combinations of OPR were defined as a third Group of microprogrammed actions mostly affecting the MQ (Multiplier/Quotient) register.
7501 – MQA – Multiplier Quotient with AC (logical or MQ into AC)
7441 – SCA – Step counter load into AC
7421 – MQL – Multiplier Quotient Load (Transfer AC to MQ, clear AC)
7621 – CAM – CLA + MQL clears both AC and MQ.
Typically CLA and MQA were combined to transfer MQ into AC. Another useful combination is MQA and MQL, to exchange the two registers.
Three bits specified a multiply/divide instruction to perform:
7401 – No operation
7403 – SCL – Step Counter Load (immediate word follows, PDP-8/I and up)
7405 – MUY – Multiply
7407 – DVI – Divide
7411 – NMI – Normalize
7413 – SHL – Shift left (immediate word follows)
7415 – ASR – Arithmetic shift right
7417 – LSR – Logical shift right
Memory control
A 12-bit word can have 4,096 different values, and this was the maximum number of words the original PDP-8 could address indirectly through a word pointer. As programs became more complex and the price of memory fell, it became desirable to expand this limit. To maintain compatibility with pre-existing programs, the designers chose the approach of using hardware outside the original design to add high-order bits to the effective addresses generated by the program.
Consequently, adding a "Memory Extension Controller" expanded the addressable memory by a factor of 8, to a total of 32,768 words. This expansion was thought sufficient because, with core memory then costing about 50 cents a word, a full 32K of memory would equal the cost of the CPU.
Each 4K of memory was called a "field." The Memory Extension Controller contained two three-bit registers: the DF (Data Field) and the IF (Instruction Field). These registers specified a field for each memory reference of the CPU, allowing a total of 15 bits of address. The IF register specified the field for instruction fetches and direct memory references; the DF register specified the field for indirect data accesses. A program running in one field could reference data in the same field by direct addressing, and reference data in another field by indirect addressing.
A set of IO instructions in the range 6200 through 6277 was handled by the Memory Extension Controller and gave access to the DF and IF registers. The 62X1 instruction (CDF, Change Data Field) set the data field to X. Similarly 62X2 (CIF) set the instruction field, and 62X3 set both. Pre-existing programs would never execute CIF or CDF; the DF and IF registers would both point to the same field, a single field to which these programs were limited.
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 was consistent with the 12-bit architecture used throughout the PDP-8. Compare the later Intel 8086
Intel 8086
The 8086 is a 16-bit microprocessor chip designed by Intel between early 1976 and mid-1978, when it was released. The 8086 gave rise to the x86 architecture of Intel's future processors...
, whose 16-bit memory addresses are expanded to 20 bits by combining them with the contents of a specified or implied "segment" register.
The effect of the CIF instruction was deferred to coincide with the next JMP or JMS instruction, so that executing CIF would not cause a jump. Additional IO instructions to the Memory Extension Controller retrieved the current value of these registers, letting software save and restore the machine state across an interrupt.
The extended memory scheme let existing programs handle increased memory with minimal changes. For example, 4K FOCAL normally had about 3K of code with only 1K left over for user program and data. With a few patches, FOCAL could use a second 4K field for user program and data. Moreover, additional 4K fields could be allocated to separate users, turning 4K FOCAL into a multi-user timesharing system.
Virtualization
On the PDP-8/E and later models, the Extended Memory Controller was enhanced to enable machine virtualization. A program written to use a PDP-8's entire resources could coexist with other such programs on the same PDP-8 under the control of a virtual machine manager. The manager would order all I/O instructions (including those that operated on the Extended Memory Controller) to cause a trap (an interrupt handled by the manager). In this way, the manager could map memory references, map data or instruction fields, and redirect I/O to different devices. Each original program had complete access to a "virtual machine" provided by the manager.
There was one problem with virtualization: Whether the CPU was in the process of deferring the effect of a CIF instruction (whether it had executed a CIF and not yet executed the matching jump instruction) was something that a program could not sense or modify. So the manager could not completely restore the machine state. The manager had to include a complete PDP-8 emulator (not difficult for an 8-instruction machine). Whenever a CIF instruction trapped to the manager, it had to emulate the instructions up to the next jump. Fortunately, as a jump usually was the next instruction after CIF, this emulation did not slow programs down much, but it is a large workaround to a seemingly small design deficiency.
By the time of the PDP-8/A, memory prices had fallen enough that 32K was not a huge budgetary item and more memory was desirable. The 8/A added a new set of instructions for handling more than eight fields of memory. The field number could now be placed in the AC, rather than having to be hard-coded into the instruction. However, by this time, the PDP-8 was in decline, so very little standard software was modified to use these new features.
An assembly language is a low-level programming language for computers, microprocessors, microcontrollers, and other programmable devices. It implements a symbolic representation of the machine codes and other constants needed to program a given CPU architecture...
as one might write for the PAL-III assembler.
Comparing two numbers
The following piece of code shows what is needed just to compare two numbers:
/Compare numbers in memory at OPD1 and OPD2
CLA CLL /Must start with 0 in AC and link
TAD OPD1 /Load first operand into AC (by adding it to 0); link is still clear
CIA /Complement, then increment AC, negating it
TAD OPD2 /AC now has OPD2-OPD1; if OPD2≥OPD1, sum overflows and link is set
SZL /Skip if link is clear
JMP OP2GT /Jump somewhere in the case that OPD2≥OPD1;
/Otherwise, fall through to code below.
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. An additional readability problem is that in conditional jumps such as the one shown above, the conditional instruction (which skips around the JMP) highlights the opposite of the condition of interest.
String output
This complete PDP-8 assembly language program outputs "Hello, world!
Hello world program
A "Hello world" program is a computer program that outputs "Hello world" on a display device. Because it is typically one of the simplest programs possible in most programming languages, it is by tradition often used to illustrate to beginners the most basic syntax of a programming language, or to...
" to the teleprinter.
*10 / Set current assembly origin to address 10,
STPTR, STRNG-1 / An auto-increment register (one of eight at 10-17)
*200 / Set current assembly origin to program text area
HELLO, CLA CLL / Clear AC and Link again (needed when we loop back from tls)
TAD I Z STPTR / Get next character, indirect via PRE-auto-increment address from the zero page
SNA / Skip if non-zero (not end of string)
HLT / Else halt on zero (end of string)
TLS / Output the character in the AC to the teleprinter
TSF / Skip if teleprinter ready for character
JMP .-1 / Else jump back and try again
JMP HELLO / Jump back for the next character
STRNG, 310 / H
345 / e
354 / l
354 / l
357 / o
254 /,
240 / (space)
367 / w
357 / o
362 / r
354 / l
344 / d
241 / !
0 / End of string
$HELLO /DEFAULT TERMINATOR
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
upon which to store registers or other context when a subroutine
Subroutine
In computer science, a subroutine is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code....
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
occurred. (A stack could be implemented in software, as demonstrated in the next section.) Instead, the JMS instruction simply stored the updated PC (pointing past JMS, to the return address) at the effective address and jumped to the effective address plus one. The subroutine returned to its caller using an indirect JMP instruction that addressed the subroutine's first word.
For example, here is "Hello, World!" re-written to use a subroutine. When the JMS instruction jumps to the subroutine, it modifies the 0 coded at location OUT1:
*10 / Set current assembly origin to address 10,
STPTR, STRNG-1 / An auto-increment register (one of eight at 10-17)
*200 / Set assembly origin (load address)
LOOP, TAD I STPTR / Pre-increment mem location 10, fetch indirect to get the next character of our message
SNA / Skip on non-zero AC
HLT / Else halt at end of message
JMS OUT1 / Write out one character
JMP LOOP / And loop back for more
OUT1, 0 / Will be replaced by caller's updated PC
TSF / Skip if printer ready
JMP .-1 / Wait for flag
TLS / Send the character in the AC
CLA CLL / Clear AC and Link for next pass
JMP I OUT1 / Return to caller
STRNG, "H / A well-known message
"e /
"l / NOTE:
"l /
"o / Strings in PAL-8 and PAL-III were "sixbit"
", / To use ASCII, we'll have to spell that out, character by character
" /
"w /
"o /
"r /
"l /
"d /
"! /
015 /
012 /
0 / Mark the end of our .ASCIZ string ('cause .ASCII hadn't been invented yet!)
The fact that the JMS instruction used the word just before the code of the subroutine to deposit the return address
Return address
In postal mail, a return address is an explicit inclusion of the address of the person sending the message. It provides the recipient with a means to determine how to respond to the sender of the message if needed....
Recursion is the process of repeating items in a self-similar way. For instance, when the surfaces of two mirrors are exactly parallel with each other the nested images that occur are a form of infinite recursion. The term has a variety of meanings specific to a variety of disciplines ranging from...
without additional work by the programmer. It also made it difficult to use ROM
Read-only memory
Read-only memory is a class of storage medium used in computers and other electronic devices. Data stored in ROM cannot be modified, or can be modified only slowly or with difficulty, so it is mainly used to distribute firmware .In its strictest sense, ROM refers only...
with the PDP-8 because read-write return-address storage was commingled with read-only code storage in the address space. Programs intended to be placed into ROMs approached this problem in several ways:
They copied themselves to read-write memory before execution, or
They were placed into special ROM cards that provided a few words of read/write memory, accessed indirectly through the use of a thirteenth flag bit in each ROM word.
They avoided the use of subroutines; or used code such as the following, instead of the JMS instruction, to put the return address in read-write memory:
JUMPL, DCA TEMP / Deposit the accumulator in some temporary location
TAD JUMPL+3 / Load the return address into the accumulator: hard coded
JMP SUBRO / Go to the subroutine, and have it handle jumping back
JUMPL+4 / Return address
The use of the JMS instruction made debugging difficult. If a programmer made the mistake of having a subroutine call itself, directly or by an intermediate subroutine, then the return address for the outer call would be destroyed by the return address of the subsequent call, leading to an infinite loop. If one module was coded with an incorrect or obsolete address for a subroutine, it would not just fail to execute the entire code sequence of the subroutine, it might modify a word of the subroutine's code, depositing a return address that the processor might interpret as an instruction during a subsequent correct call to the subroutine. Both types of error might become evident during the execution of code that was written correctly.
In computer science, a stack is a last in, first out abstract data type and linear data structure. A stack can have any abstract data type as an element, but is characterized by only three fundamental operations: push, pop and stack top. The push operation adds a new item to the top of the stack,...
, it could be implemented in software. Here are example PUSH and POP subroutines, simplified to omit issues such as testing for stack overflow and underflow:
PUSH, 0
DCA DATA
CLA CMA / -1
TAD SP
DCA SP
TAD DATA
DCA I SP
JMP I PUSH /Return
POP, 0
CLA CLL
TAD I SP
ISZ SP
JMP I POP
DATA, 0
SP, 0
And here is "Hello World" with this "stack" implemented, and "OUT" subroutine:
*200
MAIN, CLA CLL /Set the message pointer
TAD (MESSG /To the beginning of the message (literal)
DCA SP
LOOP, JMS POP
SNA /Stop execution if zero
HLT
JMS OUT /Otherwise, output a character
JMP LOOP
MESSG, "H
"e
"l
"l
"o
",
"
"w
"o
"r
"l
"d
"!
015
012
0
OUT, 0 / Will be replaced by caller's updated PC
TSF / Skip if printer ready
JMP .-1 / Wait for flag
TLS / Send the character in the AC
CLA CLL / Clear AC and Link for next pass
JMP I OUT / Return to caller
Linked list
Another possible subroutine for the PDP-8 was a linked list.
GETN, 0 /Gets the number pointed to and moves the pointer
CLA CLL /Clear accumulator
TAD I PTR /Gets the number pointed to
DCA TEMP /Save current value
ISZ PTR /Increment pointer
TAD I PTR /Get next address
DCA PTR /Put in pointer
JMP I GETN /return
PTR, 0
TEMP, 0
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
line on the PDP-8 I/O bus and interrupts were processed identically to having called a subroutine at location 0000 except that the interrupt system was also automatically disabled. Just as it was difficult to reentrantly call subroutines, it was difficult to nest interrupts and this was usually not done; each interrupt ran to completion and re-enabled the interrupt system just before executing the JMP I 0 instruction which acted as the exit from the interrupt.
Because there was only a single interrupt line on the I/O bus, the occurrence of an interrupt conveyed no information to the processor about the source of the interrupt. Instead, the interrupt service routine had to serially poll each active I/O device to see if it was the source of the interrupt; the code that did this was usually referred to as a skip chain because it consisted of a lot of PDP-8 "test and skip if flag set" I/O instructions. (It was also not unheard-of for a skip chain to reach its end and not have found any device in need of service.) The relative interrupt priority of the I/O devices was determined by their position in the skip chain with devices nearer the front of the skip chain having higher priority for service.
Books
An engineering textbook popular in the 1980s, The Art of Digital Design by David Winkel and Franklin Prosser, describes the process of designing a computer that is compatible with the PDP-8/I as an exercise. The function of every component is explained. Although it is not a production design, the exercise provides a detailed description of the computer's operation.
Simon Fraser University is a Canadian public research university in British Columbia with its main campus on Burnaby Mountain in Burnaby, and satellite campuses in Vancouver and Surrey. The main campus in Burnaby, located from downtown Vancouver, was established in 1965 and has more than 34,000...
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
Deutsche Bank AG is a global financial service company with its headquarters in Frankfurt, Germany. It employs more than 100,000 people in over 70 countries, and has a large presence in Europe, the Americas, Asia Pacific and the emerging markets...
centers and other financial institutes: Olympia Multiplex 80 (Olympia Business Systems)
http://www.pdp8.co.uk/ Blogs the restoration of PDP-8 computers
http://www.grc.com/pdp-8/pdp-8.htm Steve Gibson's explanation on how the PDP-8 works and how to program it.
The total sales figure for the PDP-8 family has been estimated at over 300,000 machines. The following models were manufactured: