 | | | Sinclair ZX81 | | Type | Home computer | | Released | 1981 | | Discontinued | 1983 | | Processor | Z80 @ 3.25 MHz (most machines used the NEC µPD780C-1 equivalent) | | Memory | 1 KB (64 KB max. 56 KB useable) | | OS | Sinclair BASIC | The Sinclair ZX81 home computer, released by Sinclair Research in 1981, was the follow up to the company's ZX80. The case was black, with a membrane keyboard; the machine's distinctive appearance was the work of industrial designer Rick Dickinson. Video output, as in the ZX80, was to a television set, and saving and loading programs was via an ordinary home audio tape recorder to audio cassette. It was the first home computer under $100 USD (in kit form), and therefore sold in great volume. Timex Corporation manufactured kits as well as assembled machines for Sinclair Research. In the United States a version with double the RAM and an NTSC television standard was marketed as the "Timex Sinclair 1000". Image File history File links Download high resolution version (862x768, 95 KB) Translation of the original German summary: The photograph was taken by me[, the uploader]. I, the creator of this work, hereby release it into the public domain. ...
Children playing on a Amstrad CPC 464 in the 1980s. ...
The Zilog Z80 is an 8-bit microprocessor designed and manufactured by Zilog from 1976 onwards. ...
NECs μPD780C Z80 clone on a ZX Spectrum board. ...
The abbreviation KB can refer to: Kilobyte (kB), equal to 1,000 bytes, or Kibibyte (KiB), equal to 1,024 bytes. ...
Sinclair BASIC (taking its name from innovator Sir Clive Sinclair) is a dialect of the BASIC programming language used in the home computers from Sinclair Research and Timex Sinclair. ...
Image File history File linksMetadata Size of this preview: 599 × 599 pixelsFull resolution (1703 × 1704 pixel, file size: 831 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...
Image File history File linksMetadata Size of this preview: 599 × 599 pixelsFull resolution (1703 × 1704 pixel, file size: 831 KB, MIME type: image/jpeg) File history Legend: (cur) = this is the current file, (del) = delete this old version, (rev) = revert to this old version. ...
Children playing on a Amstrad CPC 464 in the 1980s. ...
Sinclair Research Ltd was a home computer company founded by Clive Sinclair in Cambridge, England. ...
1981 (MCMLXXXI) was a common year starting on Thursday of the Gregorian calendar. ...
The Sinclair ZX80 was a home computer brought to market in 1980 by Sinclair Research of Cambridge, England. ...
A membrane keyboard is a computer keyboard whose keys are not separate, moving parts, as with the majority of other keyboards, but rather have only outlines and symbols printed on a flat, flexible surface. ...
Dickinson designed the ZX81 personal computer, and holds a patent for its design. ...
Sony reel-to-reel tape recorder. ...
Typical 60-minute Compact Cassette. ...
Timex Group B.V. is the best-known American watch company. ...
Look up RAM, Ram, ram in Wiktionary, the free dictionary. ...
The references in this article would be clearer with a different and/or consistent style of citation, footnoting or external linking. ...
The Timex Sinclair 1000 (TS1000) was the first computer produced by Timex Sinclair, a joint-venture between Timex Corporation and Sinclair Research. ...
General description As with the ZX80, the processor was a NEC Zilog Z80-compatible, running at a clock rate of 3.25 MHz, but the system ROM had grown to 8192 bytes in size, and the BASIC now supported floating point arithmetic[1]. It was an adaption of the ZX80 ROM by Steve Vickers on contract from Nine Tiles Ltd, the authors of Sinclair BASIC. The new ROM also worked in the ZX80 and Sinclair offered it as an upgrade for the older ZX80 for a while. NEC Corporation (Jp. ...
One of the first Z80 microprocessors manufactured; the date stamp says well before July 1976. ...
Read-only memory (usually known by its acronym, ROM) is a class of storage media used in computers and other electronic devices. ...
Sinclair BASIC (taking its name from innovator Sir Clive Sinclair) is a dialect of the BASIC programming language used in the home computers from Sinclair Research and Timex Sinclair. ...
A floating-point number is a digital representation for a number in a certain subset of the rational numbers, and is often used to approximate an arbitrary real number on a computer. ...
Image:Steve Vickers. ...
Sinclair BASIC (taking its name from innovator Sir Clive Sinclair) is a dialect of the BASIC programming language used in the home computers from Sinclair Research and Timex Sinclair. ...
As suggested, the computer was similar to the ZX80, but was built around a semi-custom Ferranti ULA (Uncommitted Logic Array) instead of TTL logic. The redesigned system board therefore had only four or five ICs: the microprocessor, the ULA, the 8192 bytes ROM, and either one 1024 bytes RAM chip, or two 1024x4 bit RAM chips. The word ula might have several meanings: Ula Thurman ULA, the Underground Literary Alliance Ula (tribe), a Manchu tribe Ula (dance), an ancient Tongan dance Ula, Turkey, a district in Turkey Ula, Norway, a village in Norway Ūla River, a river in Lithuania Ula class submarine, a type of Norwegian...
A Gate array or Uncommitted Logic Array (ULA) is an approach to the design and manufacture of application_specific integrated circuits (ASICS). ...
A Motorola 68000-based computer with various TTL chips. ...
 A 16-KB RAM pack that plugged into the rear of the ZX81 The base system as supplied (in the UK for approximately GB£70 fully built and GB£50 as a kit requiring soldering [1], or US$100 in the US) had 1 KB of RAM. This RAM was used to hold the computer's system variables, the screen image, and any programs and data. The screen was text only, 32 characters wide by 24 high. However blocky graphics with a resolution of 64 by 48 pixels were possible by the use of the PLOT command, which ingeniously selected among a set of 16 graphics characters. Image File history File links Download high resolution version (1145x1130, 202 KB)Optional 16K RAM pack for the ZX-81. ...
Image File history File links Download high resolution version (1145x1130, 202 KB)Optional 16K RAM pack for the ZX-81. ...
The abbreviation KB can refer to: Kilobyte (kB), equal to 1,000 bytes, or Kibibyte (KiB), equal to 1,024 bytes. ...
Random access memory (usually known by its acronym, RAM) is a type of data storage used in computers. ...
To conserve memory, the screen bytes were stored as minimal length strings: for example, if a screen line was only 12 characters long, it would be stored as only those 12 characters followed by the code for a new line, the rest of the line being automatically assumed to be spaces. Using this knowledge, it was common to write programs that kept to the top left of the screen to save memory. As another memory-saving feature, BASIC keywords were stored as 1-byte tokens. If memory grew short, the number of lines displayed on the TV screen would be reduced.
Even so, there were many games and applications that ran in the minimalistic 1 KB, including a basic game of Chess. It was not that hard to get to know, understand, and control the computer completely, something almost impossible with the computers of today.
Memory expansions Even with all these space saving measures the little machine's memory did not go very far, so the ZX 16K RAM (or Timex-Sinclair TS1016) expansion pack was available with 16 KB of RAM (GB£49.95 n the UK[1], US$100 in the US). By mid-1982, third-party 32 KB and 64 KB expansion packs were available. These plugged onto the main circuit board expansion bus edge connector (the 16 KB Memopak from Memotech could be "stacked" with a 16 KB or 32 KB one) and were notorious for their loose and wobbly connection to the main board. A swift nudge or jolt to a powered-on ZX81 with such an expansion pack usually resulted in a computer crash, known as a 'whiteout', and the loss of hours of programming. Image File history File links Download high-resolution version (2680x1588, 439 KB) Schnittstellen des ZX81. ...
Image File history File links Download high-resolution version (2680x1588, 439 KB) Schnittstellen des ZX81. ...
-1...
Memotech was a company based in Witney in Oxfordshire, England. ...
Printers and add-ons
 The optional ZX printer; a simple spark printer using aluminised paper.
Timex Sinclair 2040 printer, sold in the U.S. and in Portugal. The Sinclair ZX Printer was also marketed to accompany the ZX81; this was a spark printer (although it was sometimes misleadingly called a "thermal printer") in which a wire point sparked the dot pattern into 4-inch-wide silvery-grey aluminised paper, accompanied by a distinct odour of ozone. Although there were to be FCC compliance issues, the ZX Printer was marketed in the US for a limited time, and later the Timex-Sinclair 2040 thermal printer was produced (also available in the UK as the Alphacom 32). Image File history File links Download high resolution version (1494x1323, 204 KB)Optional thermal printer for the ZX-81. ...
Image File history File links Download high resolution version (1494x1323, 204 KB)Optional thermal printer for the ZX-81. ...
A spark printer uses a special paper coated with a layer of aluminium over a black backing, which is printed on by using a pulsing current onto the paper via two styli that move across on a moving belt at high speed. ...
Image File history File links Timex-sinclair-2040. ...
Image File history File links Timex-sinclair-2040. ...
A spark printer uses a special paper coated with a layer of aluminium over a black backing, which is printed on by using a pulsing current onto the paper via two styli that move across on a moving belt at high speed. ...
Direct thermal printers create an image by selectively heating coated paper when the paper passes over the thermal print head. ...
It has been suggested that Ozone generator be merged into this article or section. ...
The abbreviation FCC can refer to: Face-centered cubic (usually fcc), a crystallographic structure Federal Communications Commission, a US government organization Farm Credit Corporation/Farm Credit Canada, a Canadian government organization Families with Children from China, an adoption support organization Florida Christian College, a college in central Florida Fresno City...
There were also a third-party RS-232 serial interface (at ~US$140) and a Centronics parallel interface (at ~US$105) that would allow the ZX81 to communicate to a standard printer, as well as a full-sized external keyboard (at ~US$85). Memopak also produced a programmable RS-232 dongle, as well as 16, 32, & 64 KB RAM expansion dongle's. RS-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). ...
Centronics logo 23 June 1986 Centronics logo 1971 Centronics Data Computer Corporation was a pioneering American manufacturer of computer printers, now remembered only for the parallel interface that bears their name. ...
Chained parallel port copy prevention dongles. ...
A number of companies such as DK'tronics and Fuller (with the FD42) sold a case and keyboard that, with some skill, could be used to replace the membrane keyboard and black "doorstop" case. Fuller Micro Systems Ltd. ...
Fast and slow In the ZX80 and ZX81, the video output was generated by the Z80 chip. In the ZX80, when a program ran the screen blanked until the program paused again for input. An improvement of the ZX81 over the ZX80 was that the ZX81 had two modes of operation. The ZX81 could run in FAST mode like the ZX80, blanking while programs ran, or in SLOW mode (approximately a quarter as fast[1]) in which the video was maintained since programs only ran during the blank top and bottom border area of the screen. Since a FOR-NEXT loop from 1 to 1000 took 19 seconds, it was common to run the machine in FAST all the time, even when editing a program, causing the TV to flash every time a key was pressed into the editor.
Other peculiarities The ZX81 did not have the ability to make sound, but by switching between FAST and SLOW mode in various combinations under the control of a program, it was possible to modulate the interference that the processor caused on the TV and create a VERY simple musical keyboard. (Producer Aphex Twin claims he was able to play music on a ZX81 when he had one at the age of 11. [1]) Aphex Twin (born Richard David James on August 18, 1971 in Limerick, Ireland) is an electronic music artist, credited with pushing forward the genres of techno, ambient, acid, and drum and bass. ...
The ZX81 did not use ASCII but had its own character set. Character code 0 was space, codes 1–10 were used for blocky graphics, codes 11–63 corresponded to punctuation, numbers and upper case characters. Character codes 128–191 were reverse video versions of the first 64 characters. Other codes represented BASIC keywords and control codes such as NEWLINE. There were no lower case characters. Another trait of the ZX81 was that it echoed the signal from the tape recorder to the screen whilst loading and saving programs using cassettes, causing the TV to display zigzagging patterns. There are 95 printable ASCII characters, numbered 32 to 126. ...
A character encoding is a code that pairs a set of characters (such as an alphabet or syllabary) with a set of something else, such as numbers or electrical pulses. ...
Majuscules or capital letters (in the Roman alphabet: A, B, C, ...) are one type of case in a writing system. ...
Reverse video is a way of displaying, wherein the background and text or the colors are reversed so that the appearance is that of a negative of the original display. ...
Minuscule, or lower case, is the smaller form (case) of letters (in the Roman alphabet: a, b, c, ...). Originally alphabets were written entirely in majuscule (capital) letters which were spaced between well-defined upper and lower bounds. ...
Because the display was generated primarily by software in the ZX81 ROM, it was possible to override the interrupt service routine and generate the display oneself. Several "hi-res" (meaning, 256×192, rather than 64×48) games did this, notably from a company called Software Farm. An Interrupt Handler is the modern progression of an interrupt service routine, a routine whose execution is triggered by an interrupt. ...
There was a notorious bug causing some ZX81s to give the square root of 0.25 as 1.3591409 rather than 0.5[citation needed]. Sinclair's reputation for poor quality control was due less to the existence of the bug in some machines, and more to the time it took to react once the bug had been reported. Conversely, an article in BYTE of the time, comparing mathematical accuracy of several mainstream and much more expensive computers of the time, reflected positively on the ZX81. A computer bug is an error, flaw, mistake, failure, or fault in a computer program that prevents it from working as intended, or produces an incorrect result. ...
In mathematics, a square root of a number x is a number r such that , or in words, a number r whose square (the result of multiplying the number by itself) is x. ...
It has been suggested that this article or section be merged with quality assurance. ...
In computer science a byte is a unit of measurement of information storage, most often consisting of eight bits. ...
The BASIC interpreter was fully proprietary, unlike most microcomputers of this era (except the original Apple II) which used a series of similar but incompatible Microsoft BASIC variants. This meant that there was no need to comply with ASCII or any other existing standards. Screenshot of Atari BASIC, an early BASIC language for small computers. ...
An interpreter is a computer program that executes other programs. ...
Apple IIc Generally, a microcomputer is a computer with a microprocessor (µP) as its CPU. Another general characteristic of these computers is that they occupy physically small amounts of space. ...
The Apple II was one of the most popular personal computers of the 1980s. ...
Microsoft BASIC is the foundation product of the Microsoft company. ...
Success and successors The Sinclair ZX81 was sold in the U.S. by Sinclair itself (from its facility in Nashua, New Hampshire) and also by Timex as the Timex Sinclair 1000. The TS1000 shipped with twice as much RAM (2 KB) Nickname: Gate City Location in Hillsborough County, New Hampshire Coordinates: Country United States State New Hampshire County Hillsborough Incorporated 1746 Government - Mayor Bernard A. Streeter Area - City 31. ...
The Timex Sinclair 1000 (TS1000) was the first computer produced by Timex Sinclair, a joint-venture between Timex Corporation and Sinclair Research. ...
The ZX81 sold in large numbers, until it was replaced by its greatly upgraded successor, the ZX Spectrum. The ZX Spectrum is a home computer released in the United Kingdom in 1982 by Sinclair Research Ltd. ...
ZX81 internal workings
The technical means used to implement the display and other parts of ZX81 was quite original — at a time when the entire "home" class of computers was in its infancy. The system operated as follows: Electronics is the study of the flow of charge through various materials and devices such as, semiconductors, resistors, inductors, capacitors, nano-structures, and vacuum tubes. ...
It has been suggested that Assembler be merged into this article or section. ...
A typical vision of a computer architecture as a series of abstraction layers: hardware, firmware, assembler, kernel, operating system and applications (see also Tanenbaum 79). ...
Hardware overview
 Manually designed ZX81 printed circuit board (ISSUE ONE), a later version was made using CAD tools. Image File history File links Download high-resolution version (2212x1660, 636 KB) Leiterkarte des ZX81 File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sinclair ZX81 ...
Image File history File links Download high-resolution version (2212x1660, 636 KB) Leiterkarte des ZX81 File links The following pages on the English Wikipedia link to this file (pages on other projects are not listed): Sinclair ZX81 ...
The integrated circuits The ZX81 contains (depending on RAM type) four or five chips; ROM, CPU, SRAM, and a Ferranti Gate array (or ULA – Uncommitted Logic Array). The ROM occupies addresses 0–8191 (but also addresses 8192–16383, due to minimal decoding hardware). The 1 KB (or 2 KB for Timex) SRAM is placed at address 16384 (but repeats up to address 32767). A15 is used for display purposes (see below), and the upper 32 KB memory area is therefore unusable for code execution. It may still be used to store data, such as BASIC programs or large arrays, however. Unless more than 16 KB RAM is installed, this upper 32 KB area is mirroring the lower 32 KB (except for code execution). Die of an Intel 80486DX2 microprocessor (actual size: 12×6. ...
Static random access memory (SRAM) is a type of semiconductor memory. ...
Ferranti or Ferranti International plc by the time of its collapse, was a major UK electrical engineering and equipment firm, known primarily for defence electronics and power grid systems. ...
A Gate array is an approach to the design and manufacture of ASICs. ...
A Gate array or Uncommitted Logic Array (ULA) is an approach to the design and manufacture of application_specific integrated circuits (ASICS). ...
The abbreviation KB can refer to: Kilobyte (kB), equal to 1,000 bytes, or Kibibyte (KiB), equal to 1,024 bytes. ...
Character display The computer uses a resizable display-file (screen buffer) meaning that it can be expanded or shrunk depending on the amount of installed memory and the amount of free space at the moment.
There is also a completely non-standard (non-ascii) character set in which 0–63 are printable characters and 128–191 the same characters in reverse video. Bit 6 has special meaning here as, under normal circumstances, the only value with bit 6 set that should be written to the display file is 118, which is a NEWLINE (and also the opcode for HALT!). Placing any other byte with bit 6 set into the display-file would cause unexpected results and may cause the machine to crash.
The ZX81 has the bitmaps (patterns) of the character set stored in the uppermost 512 bytes of its 8 KB BASIC ROM. For the use of the term raster in radio regulation, see frequency raster. ...
Multiplexing Resistors function as cheap multiplexers and are placed in series with the data lines (CPU & ULA on one side, ROM & RAM on the other) allowing the ULA to override data when the CPU reads from memory (see below). An ideal resistor is a component with an electrical resistance that remains constant regardless of the applied voltage or current flowing through the device. ...
The term multiplexer has uses in several fields of application: Electronics In electronics, a multiplexer or mux is a device that combines several electrical signals into a single signal. ...
This article or section does not adequately cite its references or sources. ...
There are also resistors in series with address lines A0–A8, separating the ROM and ULA from the CPU (and from any add-on hardware) on those lines; this is used by the ULA to read pixel patterns out of the ROM by overriding address bits A0–A8, while allowing the CPU to control address bits A9–A12 (see below). Hi all! Goog directory <a href=http://tajga. ...
Hardware is the general term that is used to describe physical artifacts of a technology. ...
This example shows an image with a portion greatly enlarged, in which the individual pixels are rendered as little squares and can easily be seen. ...
The Z80 The conversion of the character codes into pixels on the TV screen employs both well known and other Z80-specific capabilities, most notably the use of the R and I registers:
The register R is intended as a dynamic RAM refresh counter; during the last part of each opcode fetch, the value of this counter is fed onto the lower portion of the address bus, and the RFSH control signal becomes active. However, also the interrupt vector register, I, is output* during the refresh cycles, but on the upper portion of the address bus.
The HALT instruction is also of central importance in the ZX81, it's necessary to know that a halted Z80 executes repeated NOPs until an interrupt occurs, and that these NOPs causes the refresh counter to tick, just as normal NOPs do. NOP or NOOP (short for No OPeration) is an assembly language instruction, sequence of programming language statements, or computer protocol command that does nothing at all (besides wasting clock cycles). ...
* Undocumented by Zilog and other manufacturers (such as NEC) at the time. Zilog, often seen as ZiLOG, is a manufacturer of 8-bit, 16-bit, 24-bit, and 32-bit CPUs, and is most famous for its Intel 8080-compatible Z80 series. ...
NEC Corporation (Jp. ...
 Screenshot of ZX81 displaying its own printable character set Image File history File links ZX81. ...
Image File history File links ZX81. ...
Keyboard scanning The membrane keyboard is scanned during (and closely coupled to) the vertical retrace interval. The scan patterns is laid out by the upper* eight bits of the unbuffered address bus and read back through five TTL inputs (8×5 = 40). It thus takes eight readings to determine which "keys" are being pressed. Decoding and debounce are done in software.
* This exploits the undocumented feature that (for instance) IN A, (C) actually puts the whole BC register pair on the address bus.
TV picture generation The I register is normally set to point to the base of the character set bitmap table in ROM. The refresh counter, R, is used to count the 32 character positions on the screen during each scan line. The program counter, PC (see below), counts actual character codes, which may be less than 32 due to the fact that the display-file is dynamically sized. The program counter (also called the instruction pointer in some computers) is a register in a computer processor which indicates where the computer is in its instruction sequence. ...
During each scan line, the CPU enters a HALT state as soon as it encounters the NEWLINE (HALT) that terminates each line of characters in the display-file, and when the R register has counted all 32 positions, a maskable interrupt (INT) is generated to bring the processor out of the HALT state just in time to prepare for another raster line. In computer science, an interrupt is an asynchronous signal from hardware or software indicating the need for attention. ...
SLOW mode During the upper and lower blank parts of the screen, the computer executes application code (i.e. BASIC or machine code), but a non-maskable interrupt (or NMI) briefly interrupts even this, once every HSYNC period; a counter is updated by the NMI-routine, so it can decide whether it's time to go back and produce character patterns again. A Non-Maskable Interrupt (NMI) is a special type of interrupt that can not be ignored by standard interrupt masking techniques. ...
Unfortunately, the use of the SLOW (smooth multitasking mode) slows all other processing by approximately 75% compared to the FAST (flickery mode).
Executing characters To actually produce a TV raster scan line of 256 pixels, the interrupt routine literally jumps to the start of the currently scanned line of characters in the display file, but with address line A15 set (i.e. 32768 added); the Z80 control line M1 is also active (indicating an opcode-fetch), and this combination is detected by the ULA:
The CPU fetches the character codes (as if it were opcodes), enabling the ULA to easily latch the values; by forcing a NOP (all zeros) onto the Z80 data bus after each retrieved byte, the ULA ensures that "nothing happens" except that the R register keeps track of the character positions on the line, and that the program counter functions as an auto incrementing pointer into the display file. The program counter (also called the instruction pointer in some computers) is a register in a computer processor which indicates where the computer is in its instruction sequence. ...
As long as the retrieved data has bit 6 reset (a character), the CPU will continue "executing" characters (as NOPs), helping the ULA reading character codes out of the display file. When the ULA detects the HALT (bit 6 set), it allows the Z80 to execute it normally; the processor stays halted and executes NOPs until the R-register wraps around to zero and thereby generates an INT — this works because INT is hardwired to A6.
This process is repeated eight times for each line of characters, and 192 times for a full TV frame.
The ZX81 makes extensive use of rather intricate "instruction timing", in the ROM program, as well as some small but delicate hardware fixes to fine-tune this system and avoid glitches and jitter in the generated video picture.
The interrupts The INT routines are not proper interrupt routines, in that they mostly do not return. Instead, the return address is constantly discarded so that each interrupt can, technically, interrupt the previous without causing stack overflow. Only once every eight scan lines (pixel lines) is the pushed address used, it then points to the next line of characters in the (possibly) irregular display file, directly after the HALT instruction. In both conventional and electronic messaging, a return address is an explicit inclusion of the address of the person sending the message. ...
The NMI interrupt, however, always makes a conventional return to the application code (i.e. the BASIC interpreter or another machine code program) during the greater part of each blank line in the upper and lower parts of the screen (the upper and lower borders). However, when it's time to initiate the character pattern display, it turns the NMI generator off, and transfers control to the INT routines (and vice versa).
More details and timing Due to the refresh-mechanism, the Z80 opcode-fetch consists of four clock cycles, which (during generation of eight pixels) are spent as follows: Microprocessors perform operations using binary bits (on/off/1or0). ...
Look up fetch, fetching in Wiktionary, the free dictionary. ...
During the first and second (the opcode fetch), the processor attempts to fetch a character code (as if it were an instruction), but the ULA latches the actual data, while forcing a NOP onto the Z80-bus, as described above.
During the third and fourth (the DRAM refresh) the ULA composes the address to the actual byte of pixel-data; bits 0–5 of the latched code is fed onto bits 3–8 of the ROM address (selecting one of the 64 different character patterns); the interrupt vector register supplies the base address (bit 9-12), while bits 0–3 comes from a modulo 8 counter (in the ULA) clocked by HSYNC and thereby selecting one of the eight pixel-rows for all characters on a line simultaneously. Interrupt vector Computing term. ...
In computer architecture, a processor register is a small amount of very fast computer memory used to speed the execution of computer programs by providing quick access to commonly used values—typically, the values being in the midst of a calculation at a given point in time. ...
The byte from the ROM is then fed into a shift-register, controlled by the same "crystal" as the CPU, and clocked out to the TV set at twice the CPU frequency (8 pixels during the 4 cycle NOP).
TV synchronization pulses (HSYNC and VSYNC) are output by the ULA and mixed with the video signal such that white=5V, black=2.5V, and SYNC=0V and fed to the HF-modulator. HSYNC is autonomously generated in hardware, while VSYNC is generated under CPU control in connection with keyboard scanning.
Bit 7 from the original character byte is read by the ULA and controls inverse-video on a per-character basis. The video synchronization pulses uses the same individual I/O bit as is used to generate the output for the 250 bit/s cassette recorder interface. This is the reason for the strange patterns displayed on the TV while saving or loading programs.
RAM pack & add-ons
I/O-addressing The lower eight I/O address bits were used as individual chip selects for individual I/O devices within the ULA. Every lower address bit except that selecting the desired device would therefore have to be one, theoretically allowing up to eight I/O devices. In the standard configuration, the only I/O present (unless the optional external ZX Printer was plugged into the 40-pin bus edge connector) was one bit for the cassette input, one bit for the cassette/video sync output, a five-bit word of input from the keyboard (which resembled a car bumper sticker more than it resembled a proper keypad) and whatever control registers were required to enable the ULA itself for video generation. This meant that not all eight bits were used, allowing some limited room for external expansion. Video (Latin for I see, first person singular present, indicative of videre, to see) is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion. ...
ZX 16K RAM pack The 16K RAM pack tied the RAM-CS line on the 40-pin edge connector to +5V to disable the internal RAM. It used eight 4116 16 K x 1 bit dynamic RAM chips contained in 16-pin dual inline packages (1 data pin and 7 multiplexed address pins with /RAS, /CAS, /WE, and power). These old chips required +12 V, +5 V and -5 V so the RAM pack contained an oscillator and some inductors to convert +5V into the other required voltages as well as circuitry to multiplex the address lines, adding significantly to its internal complexity. IC in a DIP14-Package In electronics, a Dual In-line Package (DIP), sometimes called a DIL package, is an electronic device package with a rectangular housing and a row of electrical connecting pins along each of two opposite sides, usually the longer sides of the rectangle. ...
The ZX81's internal voltage was regulated by a simple 7805 5V linear regulator attached to a small heatsink. This could became rather warm as the voltage into the 3.5 mm jack could vary within an approximate 9V-18V range depending on factors such as actual load (RAM pack, printer, etc) and line voltage variations.
Unfortunately the 40 pin bus edge connector itself was not gold-plated (the contacts were covered with plain solder) and was very prone to oxidation. In addition the design of the Sinclair RAM pack was mechanically insecure, which rendered the upgraded ZX81 system very crash-prone. This would become annoying as it would take eight minutes to reload the full 16 KB RAM from an (often-unreliable) cassette tape. Home-brew "kludge" solutions to this problem varied from physically bolting the computer and RAMpack to a solid substrate to placing the whole works in a larger case with a proper surplus keyboard in place of the original. GOLD refers to one of the following: GOLD (IEEE) is an IEEE program designed to garner more student members at the university level (Graduates of the Last Decade). ...
A solder is a fusible metal alloy, with a melting point or melting range of 180-190°C (360-370 °F), which is melted to join metallic surfaces, especially in the fields of electronics and plumbing, in a process called soldering. ...
The most fundamental reactions in chemistry are the redox processes. ...
User defined charset and hires Another less-common upgrade made by some end-users was to connect static RAM (as "pseudo-ROM") in place of the ROM mirrored at addresses 8192–16383. This RAM would need to be connected to the same side of the data bus resistors as the ROM itself so that it could be used to store a user-defined character set of up to 64 characters. One variant on this theme added a one-bit latch to latch the high data bit of the original character (when M1 and A15 were both active) in order to use it to drive one of the address bits, allowing all 128 character bitmaps to be redefined.
As the main RAM was on the wrong side of the data and address bus resistors for this to work, the extra static RAM was required for this approach. Loading the I register to point to the main RAM would not produce the desired result, instead displaying garbage as pixels.
Clones The ZX81 was cloned for sale outside Europe, with Timex Sinclair, a joint venture, producing the TS1000 for the US market. Timex Corporation, best known for the production of clocks, watches, and other timepieces, attempted to enter the fledgling home computer market of the late 1970s and early 1980s with the Sinclair line. ...
The Timex Sinclair 1000 (TS1000) was the result of a joint effort between British company Sinclair Research and Timex Corporation in an effort to gain an entry into the rapidly-growing early 1980s home computer market in the United States. ...
The ZX81 was also cloned in the Brazilian Market by many local companies, among them: Apply, Ritas, Microdigital and Prológica (these two being the main competitors for the market). Microdigital produced several ZX80 clones (TK 80 and TK 82/82C), a ZX81 clones (the TK 83), a TS1500 clone (TK 85), and two ZX Spectrum clones (TK 90X and TK 95)[2]. Prológica produced NE-Z80 (ZX80 clone), NEZ-8000, CP-200 and CP-200S (late cheaper version). The Sinclair ZX80 was a home computer brought to market in 1980 by Sinclair Research of Cambridge, England. ...
TK82C was the ZX-81 clone made by Microdigital Eletronica, a computer company located in Brazil. ...
It has been suggested that this article or section be merged into Sinclair ZX81. ...
The ZX Spectrum is a home computer released in the United Kingdom in 1982 by Sinclair Research Ltd. ...
TK 90X The TK90X was the first Brazilian ZX Spectrum clone made in the 1980s by Microdigital Eletronica, a company located at Sao Paulo, Brazil that manufactured some ZX81 clones before (TK82C, TK83 and TK85). ...
The TK95 was the evolution of TK 90X made in the 1980s by Microdigital Eletronica, a company located at Sao Paulo, Brazil that manufactured some ZX81 clones before (TK82C, TK83 and TK85). ...
External links Image File history File links Commons-logo. ...
The Wikimedia Commons (also called Wikicommons) is a repository of free content images, sound and other multimedia files. ...
References - ^ a b c d Tebbutt, David (June 1981). "Bench Test Sinclair ZX81". Personal Computer World: 67-70,154.
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