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The PDP-8 was a 12-bit computer with 4096 words of memory. At its inception, it had only eight instructions, one full register, the accumulator (AC), and a single-bit register, the link (L) bit. The machine operated at a clock rate of 1 MHz, and took 10 clocks for each instruction, so that it ran at 0.1 MIPS. Later machines added a second register (the "MQ" Multiplier/Quotient Register), actual multiply and divide instruction options, and much faster operation.
The PDP-8 was a historically important computer because of the advances in technology, I/O, software development, and operating system design that occurred during its reign.
The earliest PDP-8 models used discrete transistor technology and were approximately the size of a compact refrigerator. Intermediate systems (the PDP-8/I and /L, the PDP-8/E, /F, and /M, and the PDP-8/A) used TTL MSI logic. The last revisions of PDP-8 models used single custom CMOS microprocessors. There was never a historical " system on a chip". However, in recent years enthusiasts have created entire PDP-8 systems using single FPGAA field-programmable gate array or FPGA is a gate array that can be reprogrammed after it is manufactured, rather than having its programming fixed during the manufacturing — a programmable logic device. FPGAs are generally slower than their ASIC counterp devices.
The I/O systems underwent huge changes in the PDP-8 era. Early PDP-8 models used a front-panel interface, a paper-tapePunched tape is an old-fashioned form of data storage, consisting of a long strip of paper in which holes are punched to store data. The earliest forms of punched tape come from weaving looms and embroidery, where cards with simple instructions about a ma reader and a teletype printer with optional paper-tape punch. Over time I/O systems such as magnetic tapeDECtape was a magnetic tape storage medium used with early Digital Equipment Corporation computers, including the PDP-8, PDP-10, PDP-11, and the PDP-15. DECtapes were 3/4 inch wide and formatted into blocks of data that could be read or written individual, RS-232RS-232 (also referred to as EIA RS-232C or V. 24 is a standard for serial binary data interchange between a DTE ( Data terminal equipment) and a DCE ( Data communication equipment). It is commonly used in personal computer serial ports. History This stand and current loopA communication interface akin to RS-232 that uses current instead of voltage for signaling. 20mA current loop uses the absence of 20mA current for high, and the presence of current in the loop for low. Used for long-distance and optically isolated links. dumb terminalA computer terminal is an electronic or electromechanical hardware device. It is used for entering data into, and displaying data from, a computer or a computing system. Historical Early terminals were Teletypes (TTYs), later ones use a Visual Display Unis, punched card readers, and magnetic drum "disks" were added. Toward the end of the PDP-8 era, floppy disks and hard disk drives were popular I/O devices. Modern enthusiasts have created standard PC style IDE hard disk adapters for real and simulated PDP-8 computers.
I/O was supported through several different methods:
A rudimentary form of DMA was supported called "data break"; this required the assistance of the processor although later forms of data break required less help.
Software development systems for the PDP-8 series began with the most basic front panel entry of raw binary machine code. In the middle era, source code was often stored on paper tape, read into memory, and saved to paper tape, and later assembled from paper tape into memory. Toward the end of the PDP-8 era, operating systems such as OS/8 allowed a traditional line-mode editor and command-line compiler development system using languages such as FORTRAN and BASIC in addition to assembly.
Early PDP-8 systems did not have an operating system, just a front panel and run and halt switches. Various papertape "operating systems" were developed, as were single user disk operating systems. Toward the end of the PDP-8 era, fairly modern and advanced RTOS 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).
The instruction set:
A wide variety of operations are available through the OPR microcoded instructions. In general, the operations within each Group can be combined by or'ing the bit patterns for the desired operations into a single instruction. If none of the bits are set, the result is the NOP instruction.
Group 1 operations:
Group 2 operations:
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