The Complete Spectrum ROM Disassembly

This is awesome. I still have a copy of the original book from 1983 on my bookshelf! This looks like a faithful reproduction with some added features to make easier to use.

Looking forward to browsing some of the other code linked to from the site - e.g. Manic Miner which was a particular favourite.

A couple of points about the Spectrum ROM.

- Spectrum BASIC was very slow especially compared to BBC Basic. I think that this was partly due to the constraints of having to fit everything into 16k vs 32k for the BBC - so code was optimised for size rather than speed.

- It included a stack based VM for floating point operations which I think was again mainly to reduce the code size (one byte for each operation rather than three bytes for a subroutine call).

Would seem appropriate to give credit to the main author of the code, Steve Vickers [1], who went on to found Jupiter Computing, makers of the Jupiter Ace.

[1] https://web.archive.org/web/20110516082258/http://www.sincus...

edit: added Steve Vickers credit.

acqq2• 6 years ago

Spectrum BASIC was very slow especially compared to BBC Basic.

Yes. The commercial games were all assembly anyway. The BASIC was just to get the idea what is possible, assembly to achieve the speed.

a stack based VM for floating point operation

Of course, even Intel 8087 FPU chip and all the FPU operations is x86 before the introduction of SSE2 were done based on the stack based code and operations. Stack based calculations indeed allow amazing code density.

rahimnathwani• 6 years ago

Not quite all commercial games used assembly. I remember playing and modifying this game, which was written in BASIC:

https://en.m.wikipedia.org/wiki/Millionaire_(video_game)

tromp• 6 years ago

I just looked at my bookshelf and found it standing next to "Spectrum machine language for the absolute beginner" edited by William Tang, also by Melbourne House Publishers. Good memories...

Theodores• 6 years ago

The Spectrum used the CPU and the ULA for everything. This resulted in a cheaper price for the machine. Meanwhile the BBC had actual hardware. As an example a simple beep would tie up CPU cycles on the Spectrum whereas the BBC micro could define a sound envelope for something more than a beep and hand that off to the AY-8192 sound chip.

The Z80 had plenty of features that the BBC micro's 6502 did not have, so the code should have been quicker with fewer CPU cycles used and fewer lines of code. But the lack of supporting hardware negated this gain.

BBC BASIC was also a more complete implementation of the language. So any code written in BASIC was more efficient just because you didn't have to use your own BASIC workarounds. Being able to drop into assembly language also helped, the Spectrum lacked this option to truly turbo charge your code. The BBC used 16k for BASIC in ROM and had 16k for the OS. This enabled ROMs to be swapped out so you could run BCPL or another ROM, e.g. Fortran.

The Spectrum had BASIC and the OS in 16k with 48k available for RAM. The BBC could use more than 32k of RAM if installed, as per the later Master system that used bank switching.

I am not familiar with the code but I would be surprised if the BBC Basic ROM was anything less than a work of genius concise code. Every byte counted in those days regardless of what home computer you were programming for.

jlarcombe2• 6 years ago

Yes, also no contention for video memory access on the BBC.

klelatti10• 6 years ago

BBC Basic was a work of genius (it was written by Sophie Wilson of ARM fame) but not necessarily of concise code. I remember looking at some of the assembly in the 1980s and it was clear that code size was secondary to speed - there was some significant loop unrolling for example.

Absolutely agree on the hardware point as Sinclair's real talent was squeezing as much out of basic hardware as possible even at the expense of performance (e.g. CPU driving the screen in ZX80/81).

As a commercial decision limiting the ROM to 16k was a very good move in the end for Sinclair. It allowed full colour with 42k of spare RAM whereas the BBC only had c24k after allowing for screen memory, so in the end the Spectrum could support much larger and richer games than the BBC and (later) the Electron.

jhallenworld5• 6 years ago

I recently was curious about the relative performance of the classic CPUs, so I made this table to compare their memcpy performance:

https://github.com/jhallen/joes-sandbox/tree/master/classic-...

pjmlp4• 6 years ago

I only managed to grasp Z80 on the Speccy after getting hold of that book on the local library, unfortunately by then I was already getting into 8086 Assembly, so it felt like a lost opportunity.

Using hexadecimal monitors wasn't that appealing to me, specially trying to find out why a checksum row wasn't correct. Much worse than finding errors on BASIC listings.

rwmj8• 6 years ago

I attended lectures from Steve Vickers on Mathematical Structures in Computer Science, which is to date the hardest course or study I have ever done. I mean, I have studied advanced logic, pure maths, electronic circuit design and those were all a walk in the park compared to this. If you want to get a taste of how difficult and abstract the topic is, have a look at some of his papers, eg: https://www.cambridge.org/core/journals/journal-of-symbolic-... or https://scholar.google.com/scholar_lookup?title=Topical+cate...

Sadly I didn't get him to sign my original copy of the ZX81 Manual :-(

derefr• 6 years ago

Re: that first one, I have no idea what I just read, but it conjures back to mind a philosophical pondering I had once, on the intended formal-relational-algebra meaning of a record not being present in an RDBMS table.

I.e. "Does non-presence of an asserted relation, mean the relation is instead refuted (i.e. is this table a total membership-function for a set whose members are its present rows)? Or does non-presence of an assertion of a relation mean that this system just hasn't been informed of that assertion, and is in a default state of not-knowing the truth value of the relation (i.e. is this table a partial membership function, defined only on asserted relations)?"

Or, to put that another way: if you were to create an RDBMS that "correctly" reflects formal relational algebra, then should a search query looking for P(X), which you haven't asserted to be true, be "satisfied" to say -P(X); or should it be "unsatisfied" / non-halting, because whatever answer it would give could potentially be wrong in the face of later learning?

...does that question have something to do with the concept of "open" vs "closed" sublocales, or am I way off?

beardyw2• 6 years ago

I am now retired and have kept only 4 or 5 technical books. This is one of them. The elegance of this code is in the way it fulfills its role and fits in the space available.

beagle33• 6 years ago

Ha! I couldn’t get this one in the local store in 1984, I had to do with [0], which was inferior although still very helpful. When I sort through my boxes in storage I suspect I’ll find it.

[0] https://www.computinghistory.org.uk/det/41998/The-Spectrum-M...

edit: I originally wrote that the author was Rodney Zacks but was mistaken -- googling found that it's by Richard Ross-Langley

rwmj8• 6 years ago

Rodney Zaks wrote the definitive guide to the Z80: https://en.wikipedia.org/wiki/Programming_the_Z80

stevekemp2• 6 years ago

I have that book, even 30 years later. I've been referring to it a recently, while using an Arduino to drive a Z80 chip (pretending to be I/O ports, and RAM).

canada_dry• 6 years ago

I read this book before I learned to drive... it taught me more about computer basics (e.g. how the address and data bus work) than University did many years later!

samsaga2• 6 years ago

There is no copyright problems with this?

nineteen999• 6 years ago

neilwilson2• 6 years ago

My favourite part of Spectrum basic was that everything was an expression.

Go to 23*N

fasteo2• 6 years ago

Good times indeed !

Off topic. Little quiz:

  RANDOMIZE USR 0

What for ?

klelatti10• 6 years ago

Shortest number of keypresses for a system reset?

fasteo2• 6 years ago

bingo !

rwmj8• 6 years ago

OK, here's one (for the ZX81). Why did I choose ports 16509 and 16514 for the libvirt daemon (https://libvirt.org/remote.html)?

klelatti10• 6 years ago

10 REM TAN

20 PRINT USR 16514

16514

nickt• 6 years ago

I’ve just picked up a copy of “The ZX Spectrum ULA: How to design a microcomputer“ by Chris Smith with makes a good companion read to this, though with the focus (obviously) being on the ULA.

http://www.zxdesign.info/book/

jacquesm3• 6 years ago

I did something like this with a friend for the TRS80 Color Computer / Dragon 32, we found some really neat stuff in there, for instance how to enable the upper 32 K RAM. Those were interesting times in the sense that it was possible to know the function of every instruction present in the memory of your computer.

The Complete Spectrum ROM Disassembly

Related Stories