Editing PDP-8 (section)

==Description==
{| class="infobox" style="font-size:88%;width:26em;"
|-
|+ PDP-8 registers
|-
|
{| style="font-size:88%;"
|-
| style="width:10px; text-align:center;"| <sup>1</sup><sub>1</sub>
| style="width:10px; text-align:center;"| <sup>1</sup><sub>0</sub>
| style="width:10px; text-align:center;"| <sub>9</sub>
| style="width:10px; text-align:center;"| <sub>8</sub>
| style="width:10px; text-align:center;"| <sub>7</sub>
| style="width:10px; text-align:center;"| <sub>6</sub>
| style="width:10px; text-align:center;"| <sub>5</sub>
| style="width:10px; text-align:center;"| <sub>4</sub>
| style="width:10px; text-align:center;"| <sub>3</sub>
| style="width:10px; text-align:center;"| <sub>2</sub>
| style="width:10px; text-align:center;"| <sub>1</sub>
| style="width:10px; text-align:center;"| <sub>0</sub>
| style="width:auto;" | ''(bit position)''
|-
|colspan="13" | '''Main registers'''
|- style="background:silver;color:black"
| style="text-align:center;" colspan="12"| AC
| style="width:auto; background:white; color:black;"| '''AC'''cumulator
|- style="background:silver;color:black"
| style="text-align:center;" colspan="12"| MQ
| style="width:auto; background:white; color:black;"| '''M'''ultiplier '''Q'''uotient
|-
|colspan="13" | '''Program counter'''
|- style="background:silver;color:black"
| style="text-align:center;" colspan="12"| PC
| style="background:white; color:black;"| '''P'''rogram '''C'''ounter
|-
|colspan="13" | '''Status flags'''
|- style="background:silver;color:black"
| style="text-align:center; background:white" colspan="11" | &nbsp;
| style="text-align:center;"| [[Carry flag|L]]
| style="background:white; color:black" | '''L'''ink register
|}
|}

The PDP-8's predecessor, the PDP-5, shared many architectural features with the 18-bit PDP-4.<ref>{{cite manual|author1=Gordon Bell|title=Programmed Data Procesor - 4 Technical Specification|publisher=Digital Equipment Corporation|year=1962|url=https://bitsavers.org/pdf/dec/pdp4/M-1142_PDP4spec_May62.pdf}}</ref> It also used ideas from several 12-bit predecessors such as the [[LINC]] designed by [[Wesley A. Clark|W.A. Clark]] and [[Charles Molnar|C.E. Molnar]], who were inspired by [[Seymour Cray]]'s [[CDC 160]] minicomputer.<ref name="pdp8-faq-pdp5"/><ref>{{cite book
|author1= C. Gordon Bell
|author2= J. C. Mudge
|author3= J. E. McNamara
|title= Computer Engineering: A DEC View of Hardware Systems Design
|pages= [http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf#page=198 175],[http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf#page=201 178]-[http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf#page=202 179]
|publisher= [[Digital Press]]
|year= 1978
|isbn= 0-932376-00-2
|url= http://bitsavers.org/pdf/dec/_Books/Bell-ComputerEngineering.pdf
|author-link1= Gordon Bell
}}</ref>
[[File:PDP 8-e.jpg|thumb|162x162px|PDP-8/e at the [[Living Computers: Museum + Labs|Living Computers Museum]].]]
The PDP-8 uses 12 bits for its [[word (computer architecture)|word]] size and arithmetic (on unsigned integers from 0 to 4095 or signed integers from −2048 to +2047). However, software can do [[Arbitrary-precision arithmetic|multiple-precision arithmetic]]. An interpreter was available for [[Floating-point arithmetic|floating point]] operations, for example, that uses a 36-bit floating point representation with a two-word (24-bit) [[significand]] (mantissa) and one-word exponent.<ref>{{cite manual|url=http://bitsavers.org/pdf/dec/pdp8/software/DEC-08-YQYB-D_PDP-8_Floating-Point_System_Programmers_Reference_Manual_Sep69.pdf|title=PDP-8 Floating-Point System Programmers Reference Manual|publisher=Digital Equipment Corporation|id=DEC-08-YQYB-D|date=September 1969}}</ref> Subject to speed and memory limitations, the PDP-8 can perform calculations similar to more expensive contemporary electronic computers, such as the [[IBM 1130]] and various models of the [[IBM System/360]], while being easier to interface with external devices.

The memory address space is also 12 bits, so the PDP-8's basic configuration has a [[main memory]] of 4,096 (2<sup>12</sup>) twelve-bit words, or 6&nbsp;[[KiB]] in modern terms. An optional memory-expansion unit can [[bank switching|switch banks of memories]] using an IOT instruction. The memory is [[magnetic-core memory]] with a [[clock rate|cycle time]] of 1.5 [[microsecond]]s (0.667 [[MHz]]), so that a typical two-cycle (Fetch, Execute) memory-reference instruction runs at a speed of 0.333 [[million instructions per second|MIPS]]. The 1974 Pocket Reference Card for the PDP-8/E gives a basic instruction time of 1.2 microseconds, or 2.6 microseconds for instructions that reference memory.

The PDP-8 was designed in part to handle contemporary telecommunications and text. [[Six-bit character code]]s were in widespread use at the time, and the PDP-8's twelve-bit words can efficiently store two such characters. In addition, a six-bit teleprinter code called the [[teleprinter#Teletypesetter|teletypesetting]] or TTS code was in widespread use by the news wire services, and an early application for the PDP-8 was typesetting using this code.<ref>{{cite web|url=http://www.pdp8online.com/miscdocs/PDP-8.pdf|title=PDP-8 Typesetting System|publisher=Digital Equipment Corporation|year=1965}}</ref>

PDP-8 instructions have a three-bit opcode, so there are only eight major instructions. The programmer can use many additional instruction mnemonics, which the assembler translates to specific OPR or IOT instructions. The PDP-8 has only three programmer-visible [[processor register|register]]s: A 12-bit [[accumulator (computing)|accumulator]] (AC), a [[program counter]] (PC), and a [[carry flag]] called the "link register" (L). Additional registers not visible to the programmer are a [[memory buffer register|memory-buffer register]] and a [[memory address register|memory-address register]]. To save money, these serve multiple purposes at different points in the operating cycle. For example, the memory buffer register provides arithmetic operands, is part of the instruction register, and stores data to rewrite the [[magnetic-core memory|core memory]], which is erased when read.

For input and output, the PDP-8 has a single interrupt shared by all devices, an I/O bus accessed by I/O instructions and a [[direct memory access]] (DMA) channel. The programmed I/O bus typically runs low to medium-speed peripherals, such as [[printer (computing)|printer]]s, [[teleprinter|teletypes]], [[paper tape]] punches and readers, while DMA is used for [[cathode-ray tube]] screens with a [[light pen]], [[analog-to-digital converter]]s, [[digital-to-analog converter]]s, [[tape drive]]s, and [[disk drive]]s.

To save money, the design uses inexpensive main memory for many purposes that are served by more expensive [[flip-flop (electronics)|flip-flop]] registers in other computers,<ref>C. Gordon Bell, et al.; Computer Structures, Principles and Organization, 1982, PDP-8 chapter</ref> such as auxiliary counters and subroutine linkage.

Basic models use software to do multiplication and division. For faster math, the Extended Arithmetic Element (EAE) provides multiply and divide instructions with an additional register, the Multiplier/Quotient (MQ) register. The EAE was an option on the original PDP-8,<ref>{{cite book|chapter=Chapter 8: Extended Arithmetic Element Type 182|chapter-url=https://archive.org/stream/bitsavers_decpdp8pdpokMay66_22437012/F-85_PDP-8_Users_Handbook_May66#page/n55/mode/1up|title=PDP-8 User's Handbook|publisher=Digital Equipment Corporation|year=1966|page=41}}</ref> the 8/I,<ref>{{cite book|chapter=Chapter 4: System Description and Operation|title=Introduction to Programming|publisher=Digital Equipment Corporation|year=1969|at=pp. 4–22|url=http://bitsavers.org/pdf/dec/pdp8/handbooks/IntroToProgramming1969.pdf}}</ref> and the 8/E, but it is an integral part of the [[Intersil 6100]] microprocessor.

The PDP-8 is optimized for [[reduced instruction set computer|simplicity of design]]. Compared to more complex machines, unnecessary features were removed and logic is shared when possible. Instructions use autoincrement, autoclear, and indirect access to increase the software's speed, reduce memory use, and substitute inexpensive memory for expensive registers.

Because of their simplicity, early PDP-8 models were less expensive than most other commercially available computers. However, they used costly production methods often used for prototypes. They used thousands of very small, standardized logic-modules, with gold connectors, integrated by a costly, complex [[wire wrap|wire-wrapped]] backplane in a large cabinet.

In the later 8/S model, introduced in August 1966,<ref name="ricomputermuseum" /> two different logic voltages increased the [[fan-out]] of the inexpensive [[diode–transistor logic]].<ref>{{cite book |title=PDP-8/S Maintenance Manual |publisher=Digital Equipment Corporation |year=1971}}</ref> The 8/S also reduced the number of logic gates by using a serial, single-bit-wide data path to do arithmetic. The CPU of the PDP-8/S has only about 519 [[logic gate]]s. In comparison, small [[microcontroller]]s (as of 2008) usually have 15,000 or more. The reductions in the electronics permitted a much smaller case, about the size of a bread-box. The 8/S was designed by Saul Dinman.<ref>{{Cite web |title=PDP -8/S |url=http://www.pdp8.com/8s.htm |url-status=dead |archive-url=https://web.archive.org/web/20001023035335/http://www.pdp8.com/8s.htm |archive-date=2000-10-23 |access-date=2021-05-13 |website=PDP-8 Technology Before The Millennium}}</ref>

The even later PDP-8/E is a larger, more capable computer, but further reengineered for better value. It employs faster [[transistor–transistor logic]], in integrated circuits. The core memory was redesigned. It allows expansion with less expense because it uses the OMNIBUS in place of the wire-wrapped backplane on earlier models. (A personal account of the development of the PDP-8/E can be read on the Engineering and Technology History Wiki.<ref>{{cite web|url=https://ethw.org/First-Hand:PDP-8/E_OMNIBUS_Ride|title=First-Hand:PDP-8/E OMNIBUS Ride|author=Remo J. Vogelsang|year=2013}}</ref>)