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Geoff Wearmouth @warmtoffee.bsky.social · 9h

In both the ZX81 and ZX Spectrum the division routine misses a shift and the 34th bit is lost.
As Dr. Frank O’Hara explained a jump to DIV-34TH must be made and not to DIV-START. After division the mantissa is not normalised. Normalisation pulls in the 33rd unset bit from A and bit 34 moves to 33.

This is wrong. Rounding never takes place as the 33rd bit is unset. It was originally bit 34 and should have been set if Dr. Frank O’Hara’s change had been made. On the ZX Spectrum bits 33 and 34 are both unset. The scenario in the Vickers manual is an enigma.

The division routine as documented by Dr Frank O’Hara in The Complete ZX Spectrum Disassembly.
After division the mantissa is
O1100110 01100110 01100110 01100110 00(A
and needs normalisation.

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Geoff Wearmouth @warmtoffee.bsky.social · 4d

The ZX Spectrum BASIC on a single page but don’t type it in. You can download it from Spectrum Computing.

spectrumcomputing.co.uk/entry/17727/...

Spherical Objects - Spectrum Computing

Spherical Objects for ZX-Spectrum 16K (?) by Geoff Wearmouth

spectrumcomputing.co.uk

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Geoff Wearmouth @warmtoffee.bsky.social · 4d

Here is the Z80 source code from my ZX Spectrum ZEUS assembler.

Z80 source code which uses the internal FORTH like language of the ZX Spectrum

The final page of Z80 source code from 40 years ago. It takes 15 minutes to transpose a picture. The Spectrum correctly spends most if the time checking free memory.

The program uses routines in the ZX Spectrum ROM and some of the six 5-byte memory locations reserved for floating point operations.

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Geoff Wearmouth @warmtoffee.bsky.social · 4d

And on Page 44, yours truly with a short ZX Spectrum type-in “Spherical Objects”. No image or Z80 source code but that is fixed now.

A BASIC PROGRAM to transpose an image onto the surface of a sphere. I wrote this on a ZX Spectrum using the ZEUS assembler published by Sinclair Research. The image shows a well known loading screen for DaleyThompson’s Decathlon.

The image of Daley Thompson transposed onto the side of a sphere. For a globe I would transpose half the image.

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Reposted by Geoff Wearmouth

Webb-site @webb-site.com · 7d

The Webb-site server will shut down on 31-Oct-2025

So is this the last that you will hear from me? Hopefully not! Read on. Source: Webb-site.com | Source date: 13-Oct-2025 20:15

dlvr.it

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Reposted by Geoff Wearmouth

Kevin Edwards ( Retro Videogame development ) @kevedwardsretro.bsky.social · 11d

One of the most difficult BBC Micro books to obtain. Is there anyone out there that has a copy? I managed to get an ex-library copy that was re-bound. Curious to know how many are actually around. It was withdrawn from sale for legal reasons - it contained an annotated disassembly of BBC BASIC.

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Geoff Wearmouth @warmtoffee.bsky.social · 12d

This is the most exciting ZX81 BASIC program ever. It runs on any Sinclair ZX81 model - TS1000 etc. It compares the upcoming LG81 ROM with the host ZX81 ROM.
This BASIC program should be downloadable soon but if you want to see how bad your ZX81 is get typing. Developed on iOS ZX81 by Kevin Palser.

$My BASIC program generates a stream of random decimal fractions and then compares them to my faultless ZX81 ROM LG81 (Looking Glass ROM). The ZX81 numbers are half wrong, often by only one or two bits. When the screen is full, enter CONT to continue.$ The last few lines of my program. It took me less than an hour and I had to look up the address of VARS and the code for CHR$ “0”.

The last few lines of my program. It took me less than an hour and I had to look up the address of VARS and the code for CHR$ “0”.

Sample output.
Only the Looking Glass ROMs can be all right all of the time.

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Reposted by Geoff Wearmouth

Geoff Wearmouth @warmtoffee.bsky.social · 15d

Download The Looking Glass ZX Spectrum ROM V1.08 from

spectrumcomputing.co.uk/entry/44586/...

speccy4ever.speccy.org/_CMS.htm

ZX Spectrum ROMs and games.

Looking Glass - Spectrum Computing

Looking Glass for ZX-Spectrum 16K/48K (Geoff Wearmouth/UK, ?) by Geoff Wearmouth

spectrumcomputing.co.uk

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Geoff Wearmouth @warmtoffee.bsky.social · 12d

A nice Page 179 on BIN, ATTR and pure colours.
Amstrad messed up that text in all their manuals but 30 years later in 2014 Steve’s penguin had the last word on BIN 010 (It’s 2 not 4).

The 1982 ZX Spectrum manual by Steve Vickers explains BIN format.

Amstrad has BIN 10 representing 4 decimal not 2.

30 years later in 2014 Steve has the last word and his penguin has BIN 010 representing two.

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Geoff Wearmouth @warmtoffee.bsky.social · 12d

This is the most revealing ZX Spectrum BASIC program ever. It runs on any Sinclair model - Spectrum, TS2068, or any clone.
It compares the error-free Looking Glass ROM v1.08 against the host ZX ROM. It should be downloadable by the weekend but if you want to see how bad it your Speccy is get typing.

$The ZX Spectrum Introduction by Steven Vickers was an important step in computing around the world. There was however an error in the processing of binary fractions where half the fractions are out by one or two bits. My Looking Glass ROM version 1.08 perfects numbers by ditching an inappropriate BCD algorithm and using instead more of Steven Vickers’ Z80 code. This BASIC program compares the routine in my ROM with the routine in the computer that is hosting the program.$ The first part of my BASIC program.

The last few lines of BASIC. The program generates a continuous stream of random numbers which are tested against my algorithm. If you see something interesting press BREAK.

Example output. ZX numbers can be too high, too low or just right. Even then some of the perfect numbers occur where two wrong components cancel each other out.
I am submitting this short program to download sites.

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Geoff Wearmouth @warmtoffee.bsky.social · 15d

It’s all happening in 1982. Dr. Ian Logan reveals three ZX Spectrum bugs on the letters page and on Page 19 there is a competition to win a newly launched Jupiter Ace.

Win a Jupiter Ace by writing a non-BASIC program.

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Geoff Wearmouth @warmtoffee.bsky.social · 15d

Download The Looking Glass ZX Spectrum ROM V1.08 from

spectrumcomputing.co.uk/entry/44586/...

speccy4ever.speccy.org/_CMS.htm

ZX Spectrum ROMs and games.

Looking Glass - Spectrum Computing

Looking Glass for ZX-Spectrum 16K/48K (Geoff Wearmouth/UK, ?) by Geoff Wearmouth

spectrumcomputing.co.uk

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Geoff Wearmouth @warmtoffee.bsky.social · 15d

Remember in the 1980s when Sinclair and Amstrad dropped the price of The ZX Spectrum. Well RGL have continued the tradition and you can buy The Spectrum for £84.99 - 5% off. Just download the Looking Glass ROM V1.08 from Spectrum Computing or speccy4ever

www.smythstoys.com/uk/en-gb/gam...

speccy4ever.com

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Geoff Wearmouth @warmtoffee.bsky.social · 22d

I have seen a YouTube video of Federico Faggin referring to the processor inside as the “Zee-Eighty”. He cleverly designed it so no objections there.

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Geoff Wearmouth @warmtoffee.bsky.social · 22d

If, as I expect, those ZX Spectrum machines have sold out then you can go to Amazon Prime,
smythstoys.com , Argos etc. and have a much more exciting model for £89.99 😊. With my LG18 v1.08 ROM installed it will run all 48K software. You can get it all on one SD Card too.

The Spectrum in stock at smthystoys.com, Amazon Prime, Argos and others

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Geoff Wearmouth @warmtoffee.bsky.social · 22d

Isometric ZX Spectrum game by Chris Smith who designed my “The Spectrum” machine and has been a leading light on the Spectrum scene for over thirty years. Author of the book about “The ZX Spectrum ULA”.

An in-game scene from Cybex for the ZX Spectrum by Chris Smith who also designed my “The Spectrum” machine.

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Geoff Wearmouth @warmtoffee.bsky.social · 24d

That is a great issue with Nigel Searle explaining that the QL might have a Z80 co-processor and, after his Beijing visit, ZX Spectrums will be assembled there.
Best bit is this letter from Paul Ockenden whose type-in reveals all the ZX Spectrum Scrabble words.
That is my weekend taken care of.

This looks like a great program from Paul Ockenden that reveals all the ZX Spectrum Scrabble keywords. Load the ZX Printer with a new roll of paper. If I get it working I will submit it to Spectrum Computing

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Geoff Wearmouth @warmtoffee.bsky.social · 24d

My virtual computer written in ZX Spectrum BASIC highlights some more errors in the original 1982 ZX Spectrum. My latest ROM is error free.

Continuing the sequence, 123.457 is wrong as highlighted in red.

The incremented number 123.458 is wrong.

Number 123.459 lacks precision but the next number 123.460 is right.

Although out by only one bit the fault with 0.5 is well known. My Looking Glass ROM has total accuracy and corrects millions of faulty conversions. Happily by using three code segments written by Steve Vickers back in 1981/1982. None of my Z80 code.

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Geoff Wearmouth @warmtoffee.bsky.social · 26d

Inspired by a S. McCarthy ZX Spectrum f.p. program my Virtual Computer has a flexible exponent and mantissa and will convert a decimal number to perfect binary.
The only data from the ZX Spectrum is the bright yellow exponent and mantissa. Wrong digits will be highlighted in red.

The number 138.375 is correct on all computers ZX Spectrum, C64, PC and Looking Glass ROM

On the standard ZX Spectrum this number is not rounded correctly but on the Looking Glass ROM it is perfect.

This is the largest whole number that a ZX Spectrum, C64 and PC can hold. No problems.

The original ZX Spectrum gets this wrong. The LG18 ROM is prefect.

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Geoff Wearmouth @warmtoffee.bsky.social · 26d

Back in 2014 Dr. Steve Vickers made some lecture slides available on his University website. I used his decimal to binary conversion routines to expand on a floating point display program I had typed in. This created a virtual computer with correct exponent and mantissa to test my ZX Spectrum ROMs.

The start of a comprehensive set of lecture slides from the ever fascinating Dr. Steve Vickers, author of the ZX81 and ZX Spectrum ROMs who was then teaching at Birmingham University. The entire lecture was instructive but the number conversion routines just visible at the bottom would be invaluable in building my virtual computer with always correct exponent and mantissa.
Alas all his uplifting files have now been deleted following his retirement. ☹️

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Geoff Wearmouth @warmtoffee.bsky.social · 26d

I uploaded it to spectrum computing

spectrumcomputing.co.uk/entry/40367/...

Where Thomas had reserved a place for the ZX Spectrum program.
I could not get it to repeat the exact
Results but I intend to return to it.
It inspired me.

Floating Point Numbers - Spectrum Computing

Floating Point Numbers for ZX-Spectrum 48K (?)

spectrumcomputing.co.uk

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Reposted by Geoff Wearmouth

Quazar @samcoupe.com · 28d

The very brief mention of MGT's ZX Spectrum Emulator in the #SAMCoupe user guide here, being called "a special program". For the initial emulator MGT had to create a compatible #ZXSpectrum ROM from scratch, as the actual ZX ROM's copyright was held by Amstrad. The compatible ROM wasn't fully...

ZXVintage @zxvintage.bsky.social · 28d

SAM Coupe User Guide

READ: https://archive.org/stream/NH2021_SamCoupe_UsersGuide.pdf/page/n25/mode/2up #zxspectrum #retrogaming

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Geoff Wearmouth @warmtoffee.bsky.social · 27d

The Looking Glass ROM V1.08 for the 48K ZX Spectrum is now available for download with documentation.

speccy4ever.speccy.org/_CMS.htm

This is the most accurate ROM ever available for the ZX Spectrum and is the equal to Microsoft Binary Format 9-digit BASIC. Say goodbye to dodgy figures.

Some of the numerous ZX Spectrum ROMs available from the site of Davide Barlotti

Microsoft 9-digit BASIC examples from Wikipedia. LG18 matches them all except for that red logarithm.

Original 1982 square roots don’t match Microsoft’s GW-BASIC hex values $35 $04 $F3 $34. The Looking Glass ROM now does.

The new LG18 ROM matches the C64 with 138.375

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Geoff Wearmouth @warmtoffee.bsky.social · 29d

It’s all happening in 1982. The ZX Spectrum is launched in April. By September the Jupiter Ace makes its debut.

Steven Vickers and Richard Altwasser reveal details of the Jupiter Ace to be launched at the Personal Computer World Show.

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Geoff Wearmouth @warmtoffee.bsky.social · Sep 21

Two pages of Colin’s paperback give some love for my favourite ZX Spectrum emulator, the PC version of Spectaculator by Jonathan Needle. It is so accurate with the ZX Microdrives and the ZX Printer. Well worth the small payment for so many models emulated.

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