Editing ZX81 (section)

== Features ==
[[File:Sinclair ZX81 PCB Revision 3 Keyboard.JPG|thumb|Sinclair ZX81 PCB Revision 3 keyboard]]

The ZX81 has a base configuration of 1&nbsp;KB of on-board memory that can officially be expanded externally to 16&nbsp;KB. Its single [[circuit board]] is housed inside a wedge-shaped plastic case measuring {{convert|167|mm|in}} wide by {{convert|40|mm|in}} high. The memory is provided by either a single 4118 (1024 bit × 8) or two 2114 (1024 bit × 4) RAM chips. There are only three other onboard chips: a 3.5&nbsp;MHz [[Z80A]] [[8-bit computing|8-bit]] [[microprocessor]] from [[NEC]], an [[Gate array|uncommitted logic array]] (ULA) chip from [[Ferranti]], and an 8&nbsp;KB ROM providing a simple [[Sinclair BASIC#8K BASIC|Sinclair BASIC interpreter]]. The entire machine weighs just {{convert|350|g|oz}}.<ref name="ZX81OS">[[#ZX81OS|''ZX81 Operating Supplement'' (1982)]]</ref> Early versions of the external RAM cartridge contain 15&nbsp;KB of memory using an assortment of memory chips, while later versions contain 16&nbsp;KB chips with the lowest addressed kilobyte disabled.

The front part of the case is occupied by an integrated 40-key [[membrane keyboard]]. The keyboard is mechanically very simple, consisting of 40&nbsp;pressure-pad switches and 8 diodes under a plastic overlay, connected in a matrix of 8 rows and 5 columns.{{sfn|Thomasson|1983|p=40}}

The ZX81 uses a standard [[QWERTY]] [[keyboard layout]] displaying 20&nbsp;graphic and 54&nbsp;inverse video characters.<ref name="ZX81OS" /> However ZX81 BASIC commands are not typed in letter by letter, instead each key has up to five key functions. This is how the user displays the ZX81's BASIC keywords, functions, mathematical operations and graphics.

The ZX81 key's function is determined by a combination of context in the command and mode selection e.g. <code>SHIFT</code> and <code>FUNCTION</code> keys <!--accessed depending on context?--> to select the under key keyboard functions. For example, the P key combines the letter <code>P</code>, the <code>"</code> character, and the [[BASIC]] commands <code>PRINT</code> and <code>TAB</code>.

Context mode feedback is displayed by the cursor displaying an inverted letter;
* <code>K</code> for Keywords (above key), 
* <code>L</code> for Letters, 
* <code>F</code> for Functions (under key) or 
* <code>G</code> for Graphics characters.
The fact that to effect a <code>RUBOUT</code> or backspace/delete operation took 2 key presses encouraged ZX81 programmers to be brief and type carefully when entering text or code.

The ZX81's primary input/output is delivered via four sockets on the left side of the case. The machine uses an ordinary UHF television set to deliver a [[Binary image|monochrome]] picture via a built-in RF modulator. It can display 24 lines of 32 characters each, and by using the selection of 2×2 [[text semigraphics|block character graphics]] from the [[ZX81 character set|machine's character set]] offers an effective 64 × 44 pixel graphics mode, also directly addressable via BASIC using the PLOT and UNPLOT commands, leaving 2 lines free at the bottom. Two 3.5&nbsp;mm jacks connect the ZX81 to the EAR (output) and MIC (input) sockets of an audio cassette recorder, enabling data to be saved or loaded. This stores each data bit as a number of pulses followed by an inter-bit silence of 1300 μs. Each pulse is a 150 μs "high" then a 150 μs "low". A "0" bit consists of four pulses while a '1' bit is nine pulses, so the baud rate varies between 400 bit/s for all "0"s and 250 bit/s for all "1"s. A file with equal amounts of '0's and "1"s would be stored at 307 bit/s (38 B/s).<ref name="MyPrius">{{Cite web |title=Sinclair ZX81 Data Tapes |url=http://www.myprius.co.za/tape_storage.htm}}</ref> This provides a somewhat temperamental storage medium for the machine, which has no built-in storage capabilities. The ZX81 requires 420 [[milliamp|mA]] of current at 7–11 [[Volt|V]] [[direct current|DC]], delivered via a custom 9 V Sinclair DC power supply.<ref name="ZX81OS" />

The ULA chip, described by the ZX81 manual as the "[[dogsbody]]" of the system, has a number of key functions that competing computers share between multiple chips and integrated circuits. These comprise the following:{{sfn|Thomasson|1983|p=14}}
* Synchronising the screen display;
* Generating a 6.5&nbsp;MHz clock, from which a 3.25&nbsp;MHz clock is derived for the processor;
* Outputting an audio signal to a cassette recorder in SAVE mode;
* Processing the incoming cassette audio signal in LOAD mode;
* Sensing keystrokes;
* Using memory addresses provided by the CPU to decide when ROM and RAM should be active;
* Controlling general system timing.

The ZX81's built-in [[RF modulator]] can output a video picture to a [[UHF]] [[625 lines|625-line]] television (used in the UK, Australia, and most western European countries). France and Luxembourg required a slightly modified version of the machine to match the positive video modulation of [[CCIR System L]] sets, while the United States and Canada required a resistor adding to link a ULA pin to ground and different  modulator to cope with their [[525 lines|525-line]] VHF ([[NTSC]]) television systems. Both the ZX81 and its predecessor, the ZX80, have a significant drawback in the way that they handle visual output. Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display. On the ZX80, this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place.<ref>[[#NS070280|''New Scientist'' (7 February 1980)]].</ref>

[[File:ZX81 Interface.jpg|thumb|Left side and rear views of the ZX81, showing its [[edge connector]], the three input/output sockets (TV, EAR, MIC) and the 9 V DC power socket|alt=Two views of the ZX81, one showing the left side with four sockets marked "TV", "EAR", "MIC" and "9V DC" respectively, and one showing the rear with the edge of the circuit board visible through a gap in the case.]]

The ZX81's designers adopted an improved approach, involving the use of two modes called SLOW and FAST respectively. In SLOW mode, also called "compute and display" mode, the ZX81 concentrates on driving the display. It runs the current program for only about a quarter of the time – in effect slowing the machine down fourfold, although in practice the speed difference between FAST and SLOW modes depends on what computation is being done.<ref>For comparisons of processing speed in the two modes, see "ZX81 v IBM PC", ''Which Micro?'', April 1983, p. 36.</ref> In FAST mode, processing occurs continuously, but the display is abandoned to its own devices – equivalent to the ZX80's standard operating mode.{{sfn|Thomasson|1983|p=13}}

Another hardware quirk produced one of the most distinctive aspects of the ZX81's screen display – during loading or saving, moving zigzag stripes appear across the screen. The same pin on the ULA is used to handle the video signal and the tape output, producing the stripes as an interference pattern of sorts. The ULA cannot maintain the display during SAVE and LOAD operations, as it has to operate continuously to maintain the correct baud rate for data transfers. The interference produces the zigzag stripes.{{sfn|Thomasson|1983|p=14}}

The unexpanded ZX81's tiny memory presents a major challenge to programmers. Simply displaying a full screen takes up to 793&nbsp;bytes, the system variables take up another 125&nbsp;bytes, and the program, input buffer and stacks need more memory on top of that.{{sfn|Thomasson|1983|p=26}} Nonetheless, ingenious programmers are able to achieve a surprising amount with just 1&nbsp;KB. One example is ''[[1K ZX Chess]]'' by David Horne, which includes most of the [[rules of chess]] in 672 bytes.<ref name="horne198302">{{Cite news |url=https://archive.org/stream/YourComputer_198302/YourComputer8302#page/n99/mode/2up |title=Full ZX-81 Chess in 1K |last=Horne |first=David |date=February 1983 |work=Your Computer |access-date=5 March 2018 |pages=100–102}}</ref> The ZX81 conserves its memory to a certain extent by representing entire BASIC commands as one-byte tokens, stored as individual "characters" in the upper reaches of the machine's unique (non-[[ASCII]]) [[ZX80 character set|character set]].{{sfn|Nash|1984|p=102}}

The edge connector or external interface at the rear of the ZX81 is an extension of the main printed circuit board. This provides a set of address, control, and data lines that can be used to communicate with external devices.{{sfn|Thomasson|1983|p=43}} Enthusiasts and a variety of third-party companies make use of this facility to create a wide range of add-ons for the ZX81.

=== Comparisons with other computing devices ===
The following table provides a comparison between the capabilities of the ZX81 and various other competing [[microcomputer]]s that were available in June 1981, about the time that the first ZX81 orders were delivered. The prices given are as of June 1981 from Your Computer UK.
{{sort-under}}
{| class="wikitable sortable sort-under" style="font-size:80%; margin:1em auto 1em auto;"
|-
! scope="col" | Device
! scope="col" | RAM<br/>stan&shy;dard
! scope="col" | Expand&shy;able to
! scope="col" | CPU
! scope="col" | Keyboard
! scope="col" | BASIC
! scope="col" | Sale price
! scope="col" | Number of<br />colours
! scope="col" | Maximum<br />resolution
! scope="col" | Sound
|-
| [[Apple II Plus]]
| 16&nbsp;KB
| 48&nbsp;KB
| [[MOS Technology 6502]] @ 1&nbsp;MHz (8-bit)
| Typewriter
| Integer only
| £549
| 16
| 280 × 192 pixels
| beeper
|-
| [[Atari 8-bit computers|Atari 400]]
| 8&nbsp;KB
| 48&nbsp;KB
| [[MOS Technology 6502]] @ 1.78&nbsp;MHz
| Membrane
| extra
| £289
| 256
| 320 × 192 pixels
| 4 voice
|-
| [[Commodore PET|PET]]
| 16&nbsp;KB
| 40&nbsp;KB
| [[MOS Technology 6502]] @ 1&nbsp;MHz
| Typewriter
| MS Level I
| £399
| Monochrome
| 320 × 200 text only
| beeper
|-
| [[VIC-20]]
| 5 KB
| 32 KB
| [[MOS Technology 6502]] @ 1.02&nbsp;MHz
| Typewriter
| MS Level I
| £189
| 8
| 176 × 184 pixels
| 4 voice
|-
| [[Video Genie]]
| 16&nbsp;KB
| 48&nbsp;KB
| [[Zilog Z80]] @ 1.76&nbsp;MHz
| Typewriter
| MS Level II
| £279
| Monochrome
| 128 × 48 block
| beeper
|-
| [[Compukit UK101|Compukit 101]]
| 1&nbsp;KB
| 48&nbsp;KB
| [[MOS Technology 6502]] @ 1&nbsp;MHz
| Typewriter
| MS Level II
| £149/£199 assembled
| Monochrome
| 128 × 48 block
| extra
|-
| [[TRS-80|TRS-80 Model I]]
| 4&nbsp;KB
| 48&nbsp;KB
| [[Zilog Z80]] @ 1.78&nbsp;MHz (8-bit)
| Typewriter
| Restricted
| £399
| Monochrome
| 128 × 48 block
| extra
|-
| [[Acorn Atom]]
| 2&nbsp;KB
| 12&nbsp;KB
| [[MOS Technology 6502]] @ 1&nbsp;MHz
| Typewriter
| TinyBASIC hybrid
| £140/£173 assembled
| 8
| 256 × 192 mono, 64 × 64 colour
| beeper
|-
| [[Tangerine Microtan 65|MicroTan]]
| 1&nbsp;KB
| 48&nbsp;KB
| [[MOS Technology 6502]] @ 0.75&nbsp;MHz
| extra
| extra
| £65/£99 assembled
| Monochrome
| 128 × 48 block
| extra
|-
| [[TI-99/4A]]
| 16&nbsp;KB
| 48&nbsp;KB
| [[Texas Instruments TMS9900|TI TMS9900]] @ 3.0&nbsp;MHz (16-bit)
| Typewriter
| Restricted
| £399
| 16
| 256 × 192 pixels
| 3 voices and white noise
|- style="color:green; font-weight:bold;"
| ZX81 / TS1000
| 1&nbsp;KB / 2&nbsp;KB
| 32&nbsp;KB
| Zilog or NEC Z80 @ 3.25&nbsp;MHz (8-bit)
| Membrane
| MS Level II equiv.
| £49/£69 assembled
| Monochrome
| 64 × 48 block
| No
|}

{{smalldiv|1=
[[Nota bene|N.B.]]: [[Commodore PET]] includes monitor and cassette recorder.  The Video Genie includes a built-in cassette recorder.  The Compukit 101 is an [[Ohio Scientific]] SuperBoard made under licence in Britain.  Compukit 101 and MicroTan are kit computers and are not supplied with a case.

The [[Atari 400]] and [[TRS-80]] model I were officially only expandable to 16&nbsp;KB, but expansion to 48&nbsp;KB was available from third-party suppliers.

The ZX81 and the other systems referred to as having "block" graphics have a resolution of 256&nbsp;× 192 pixels, there are 32 columns&nbsp;× 22 rows of text and graphics can be addressed in quarter character blocks.
}}