Questions tagged [zx81]

For questions about the Sinclair ZX81.

Filter by
Sorted by
Tagged with
8 votes
2 answers
2k views

What does Spectrum BASIC have that ZX81 lacks?

The Sinclair ZX81 has 8K BASIC interpreter ROM; the Spectrum has 16K. What extra features does the Spectrum put in the extra 8K? The most obvious thing is commands for color graphics, which are absent ...
rwallace's user avatar
  • 60.3k
8 votes
3 answers
385 views

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

I bought this some time ago, but I did not keep the power supply with it. It does not have any product name on it, and I can't locate the correct information. What is the correct power supply and what ...
Zhro's user avatar
  • 283
14 votes
4 answers
3k views

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

I'm looking over the table of tokens for Sinclair BASIC and I'm struck by two things. One is that it's not ASCII. Some parts are in the same order, but others are not, so it would seem conversion is ...
Maury Markowitz's user avatar
5 votes
1 answer
268 views

How many units of the ZX81 RAM pack were sold?

The Sinclair ZX81 was an extremely popular low-cost home computer, on strength of price, being a computer for less than 100 pounds. It sold over 1.5 million units, according to https://web.archive.org/...
rwallace's user avatar
  • 60.3k
16 votes
3 answers
2k views

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

I'm trying to write some programs from an old magazine that were specified to run on an ZX80/ZX81 but I'm using a Sinclair BASIC interpreter for the Spectrum. Is there any significant changes in BASIC ...
d'Arby's user avatar
  • 161
9 votes
1 answer
368 views

Can you switch the ZX81 ROM to RAM w/o write protecting it and still use ZX81 BASIC?

I built an external 8K RAM circuit that replaced the 8K ROM of Sinclair ZX81 and I also pulled out address lines from the ULA (dogsbody) so that character generation worked. I wrote a small machine ...
mobluse's user avatar
  • 503
19 votes
1 answer
3k views

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

I know you can use different parts of the 8 KB ROM in Sinclair ZX81 as character ROM by changing the I register, but it seems one cannot change the I register to point to RAM [and have character mode]....
mobluse's user avatar
  • 503
9 votes
1 answer
1k views

ZX81 screen corrupted

I am trying to repair a ZX81 that I think was damaged by a lightning since the adaptor and the 7805 were defective, the reset capacitor of the Z80 was blown up and then I realized that Rams were ...
Paul Ghobril's user avatar
  • 1,035
8 votes
1 answer
1k views

On the video signal generated by the ULA of the ZX81

I have two ZX81, one is perfectly working and I am trying to repair the other one. After repairing the external power supply (a wire of the transformer was cut near the connector. I think it was due ...
Paul Ghobril's user avatar
  • 1,035
11 votes
1 answer
728 views

How did ZX81 assembly debuggers implement breakpoints?

What techniques were used for implementing breakpoints on the ZX81? Traditionally for the 8086 int 3h is used, for the 8080 on CP/M with DDT RST 7 as single byte replacements, 6502 has the BRK ...
PeterI's user avatar
  • 5,277
11 votes
3 answers
2k views

Is it possible to PRINT AT 23,0 in ZX Spectrum BASIC?

The ZX Spectrum BASIC doesn't (ordinarily) let you print on lines 23 and 24 to allow for user input. Is there a way around this? I found a fairly recent post that suggested I could POKE 16418,0 but ...
Remy Sharp's user avatar
  • 1,057
13 votes
1 answer
1k views

Why did the ZX81 16K RAM pack use two PCBs?

The ZX80/81 16K RAM pack used two PCBs joined with a connector in the middle. I'd initially assumed this was due to reuse of the original 3K RAM pack case (which has a single PCB and uses static ram)...
PeterI's user avatar
  • 5,277
1 vote
1 answer
282 views

How does the ZX81 ULA generate a full ROM read address?

On the ZX81, when the ULA that functions as the video chip is generating display output, it takes the character cell byte (that the CPU was tricked into fetching as an opcode) and combines it with an ...
rwallace's user avatar
  • 60.3k
5 votes
2 answers
801 views

What does the ZX81 ULA use A14 and A15 for?

As I understand it, the ZX81 video system, which resides in the ULA like all the rest of the machine-specific logic, works by using the CPU as an address generator: during active scan line, it lets ...
rwallace's user avatar
  • 60.3k
2 votes
1 answer
194 views

Did the ZX80 RAM pack pass the expansion bus through?

The ZX80 only came with 1K of RAM, which had to be shared between program and screen memory, so many customers bought the Sinclair 16K RAM pack, which plugged into the expansion bus (the edge ...
rwallace's user avatar
  • 60.3k
9 votes
1 answer
1k views

How did the ZX80 RAM pack use DRAM?

The Sinclair ZX80 was the most popular personal computer in Britain at the beginning of the 1980s, due to its low price which was enabled by extreme minimalism; it consisted of not much more than a ...
rwallace's user avatar
  • 60.3k
5 votes
1 answer
968 views

(How) can I boost my ZX81 video output with an external device?

I bought a small LCD TV today in part to use with my retro computers. My Sinclair ZX Spectrum 48k works perfectly with it - I can manually fine tune Channel 36 as shown in this video. With the ZX81 I ...
harlandski's user avatar
  • 2,943
2 votes
1 answer
775 views

Which retro personal computers, if any, can use a modern day VGA monitor? [closed]

I play around with emulators, but I would love to go back to my roots, with physical ZX81, BBC Micro, Amiga, etc. Unfortunately, those all require a CRT monitor, or a TV, as output. I suppose that I ...
Mawg says reinstate Monica's user avatar
4 votes
1 answer
319 views

How to determine PCB revision of a Sinclair ZX-81

I have a ZX-81 in my collection, but do not know what revision it is. The board text just says "Sinclair ZX81". As far as I know, there were 3 revisions of the ZX-81. What are the landmarks I can ...
Retrograde's user avatar
  • 4,506
8 votes
5 answers
2k views

Largest memory peripheral for Sinclair ZX81?

Back in the early to mid 1980s, 1981-1983, I vaguely recall seeing an advert in a (I think) UK based computer periodical monthly (such as Your Computer, or Computer and Video Games), for a memory ...
Greenonline's user avatar
  • 3,994
11 votes
3 answers
2k views

How can a peripheral work on both the ZX81 and on the ZX Spectrum?

The ZX Printer was made to work with both the ZX81 and the Spectrum (or rather, the Spectrum was made to work with the ZX81's printer!), but these computers have different expansion ports. What I ...
Omar and Lorraine's user avatar
17 votes
3 answers
3k views

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. The first is a floating point format, which consists of one exponent byte, and four mantissa bytes. The first bit of ...
Omar and Lorraine's user avatar
25 votes
2 answers
4k views

How did the ZX80 store both a useful program and screen memory?

I have a vague recollection from my earliest days that the ZX80 only shipped with 1K of RAM. If this RAM was used to store both the program and the contents of the 32x24 screen, wouldn't that mean ...
user avatar
6 votes
2 answers
556 views

Membrane keyboard robustness

Some early computers used a membrane keyboard (ZX80, ZX81, Atari 400), or semi-membrane with minimal keys (ZX Spectrum). This is because it was cheaper than a mechanical keyboard. Intuitively I would ...
rwallace's user avatar
  • 60.3k
19 votes
2 answers
5k views

How do high-resolution graphics work on the ZX81?

The ZX81 has a display file which is treated rather like a serial file. And the CPU generates addresses which the ULA uses to look up the characters' bitmaps from the ROM (Yes, I know the actual ...
Omar and Lorraine's user avatar