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44 votes
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Why does the ZX81 not have user definable characters without extra hardware i.e. why could the character ROM not be in RAM?

The ZX80 and ZX81 use the Z80’s opcode fetch bus cycle for graphics collection — the opcode fetch is appropriated to obtain a character index and the refresh cycle is used to obtain a row of pixels. ...
Tommy's user avatar
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36 votes
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How did the ZX80 store both a useful program and screen memory?

Both the ZX80 and the ZX81 had a variable-size display file (DFILE). They didn't store the complete screen contents, but rather only the characters per line up to a terminating newline. This ...
tofro's user avatar
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21 votes
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Why does Sinclair BASIC have two formats for storing numbers in the same structure?

The ZX Spectrum has two formats for storing numbers, both 40 bits, or five bytes. Jup, like many other machines - one for float and one for integer. The second is some kind of 16-bit integer ...
Raffzahn's user avatar
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21 votes

Is there any significant difference in Sinclair BASIC of ZX80, ZX81 and ZX Spectrum?

Spectrum BASIC is a superset of the earlier BASICs (all the colour commands, etc), so pedantically the answer to the question is the title of the post is "yes, there are significant differences&...
Philip Kendall's user avatar
20 votes

Why does Sinclair BASIC have all of the characters twice in the token table?

You're right; the ZX81 does not use ASCII. That's because it uses the CPU to generate the displayed picture, from a compressed format in memory. The way this works is an absolute freakshow*, but from ...
Omar and Lorraine's user avatar
19 votes
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How do high-resolution graphics work on the ZX81?

The normal process is: CPU attempts to fetch an op-code instruction byte from the video file. ULA steals that byte as a character code and feeds the CPU a NOP op-code byte instead. Immediately ...
Tommy's user avatar
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16 votes
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Is it possible to PRINT AT 23,0 in ZX Spectrum BASIC?

POKE 16418,0 is for the ZX81, not the Spectrum - the equivalent system variable on the Spectrum is at 23659. You need to take care when poking this address, as it's liable to cause a crash if the ...
gasman's user avatar
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15 votes
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How can a peripheral work on both the ZX81 and on the ZX Spectrum?

The ZX printer is, like just about anything that carries the Sinclair label, a very minimalistic device - It only needs very few lines from the computer to actually work: Address bus: The printer ...
tofro's user avatar
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15 votes
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ZX81 screen corrupted

Thank you @Raffzahn for the comment that inspired me to think of something blocking the video signal. The initial problem described in the previous question was that the ZX81 was showing a white ...
Paul Ghobril's user avatar
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15 votes
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On the video signal generated by the ULA of the ZX81

That portion of the video signal is generated directly by the ULA; early ZX81s just don't generate a back porch, which was seemingly good enough for the majority of televisions but problematic enough ...
Tommy's user avatar
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14 votes

What does Spectrum BASIC have that ZX81 lacks?

From 'Reusing Code, Reverse Engineering And Collaboration': Grant and Vickers would’ve liked to start over, but there wasn’t time. So the ZX81 BASIC was used as the starting point, with the addition ...
Mark Williams's user avatar
13 votes
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Why did the ZX81 16K RAM pack use two PCBs?

It was neither due history or electronic but for design reason: The 16 KiB RAM pack was intended to be sold to existing (and new *1) ZX80 users as well. While both had their expansion port on back ...
Raffzahn's user avatar
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13 votes
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How did the ZX80 RAM pack use DRAM?

But as I understand it, the then current 16kbit RAM chips actually required three different voltages: -5, +5, +12. Right, and the 'missing' voltages (-5V,+12V) get generated from the +9V source via a ...
Raffzahn's user avatar
  • 224k
11 votes

Largest memory peripheral for Sinclair ZX81?

You might be willing to check this hardware list from World of Spectrum. It contains useful information and magazine advertisements of each hardware piece. Specifically, DK’Tronics memory expansion ...
DroidW's user avatar
  • 509
11 votes
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How did ZX81 assembly debuggers implement breakpoints?

MONS 3 in the ZX Spectrum, for example, changes the instruction at the break address by a CALL to an entry point in MONS. As MONS is executed, it replaces back the changed instruction with the ...
mcleod_ideafix's user avatar
10 votes
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Can you switch the ZX81 ROM to RAM w/o write protecting it and still use ZX81 BASIC?

Directly to answer your question: yes, if the RAM copy is substituted after the machine has started up; otherwise, not reliably, as it's likely the first five bytes have been altered. With reference ...
Tommy's user avatar
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10 votes
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What does Spectrum BASIC have that ZX81 lacks?

Hopefully, this is a pretty comprehensive list of the differences between the ZX81 and Spectrum (and, to a lesser extent, the ZX80) BASICs: String handling No FAST and SLOW UNPLOT/PLOT OVER ...
Greenonline's user avatar
  • 4,356
9 votes
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Did the ZX80 RAM pack pass the expansion bus through?

Did one or the other have a pass-through for the expansion bus? There was no other peripheral (by Sinclair) for the ZX80 than the 1..3 KiB static RAM or the later 16 KiB DRAM version (*1). Most (*2) ...
Raffzahn's user avatar
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8 votes
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(How) can I boost my ZX81 video output with an external device?

I would skip the existing output entirely. Buy a Chroma 81, which plugs into the expansion slot — no internal modifications necessary — and then monitors the internal bus to reproduce the video ...
Tommy's user avatar
  • 37k
8 votes

Membrane keyboard robustness

It depends on a number of factors, a major one being the material that the membrane is made from. Various types of plastics can be used, with different properties. Some will age and become brittle, or ...
user's user avatar
  • 15.2k
8 votes

Largest memory peripheral for Sinclair ZX81?

There were various companies producing these of which DK'Tronics was one. For example Ceedata, Bridge Software, Indescomp and Memotech,
Alan B's user avatar
  • 4,555
7 votes

Why does Sinclair BASIC have all of the characters twice in the token table?

TL;DR: That encoding is not just the token table but as well the BASIC charset and the screen charset which offers each of the 64 printable characters as Black-on-White (00h..3Fh) or White on Black (...
Raffzahn's user avatar
  • 224k
6 votes

Largest memory peripheral for Sinclair ZX81?

You may be thinking of the BASICare Micro System, an advert for which can be seen here: http://www.zx81stuff.org.uk/zx81/publisher/BASICare/mag/SinclairUser/Issue008/Pages/SinclairUser00800004.jpg ...
Matthew Barber's user avatar
6 votes

Why does Sinclair BASIC have two formats for storing numbers in the same structure?

Doing integer arithmetic is by some orders of magnitude faster as floating point. So if there is a statement like LET a = b + c the interpreter would first check if both a and b are integers. If yes,...
followed Monica to Codidact's user avatar
6 votes
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How many units of the ZX81 RAM pack were sold?

I guess it will be next to impossible to get tangible sales figures for the number of RAM packs sold. I did, however find a quote from Geoff Boyd (one of the owners of Memotech, who was a well-known ...
tofro's user avatar
  • 35k
6 votes

What is the correct power supply for this SD card/cassette reader for Sinclair computers?

That looks to be a MaxDuino tape emulator. According to the website, it wants to see a 12V power supply. Polarity is always a bit of a gamble with Sinclair computers and accessories. I'd rather not ...
tofro's user avatar
  • 35k
5 votes
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Which retro personal computers, if any, can use a modern day VGA monitor?

A few 'home' computers from the '80s could output a VGA signal, including:- Acorn Archimedes Commodore Amiga (with Flicker Fixer) PC clones that had VGA on board or could take a VGA card (eg. ...
Bruce Abbott's user avatar
  • 6,683
5 votes
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How to determine PCB revision of a Sinclair ZX-81

The easiest way to tell is by looking at the traces on the board. If they look curvy, as though hand-drawn, it's an issue 1. If they're in more or less straight lines, it's a 2 or 3. Seeing as ...
Matthew Barber's user avatar
5 votes

Why does Sinclair BASIC have two formats for storing numbers in the same structure?

I bought and read 'Complete (ZX) Spectrum ROM Disassembly' (Ian Logan, 1983, very out of print) many years ago, here are the answers, from my memory. (Btw, the ZX80 ROM is insanely tight Z80 assembler ...
smci's user avatar
  • 178

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