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The ZX81 Turns 40

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Re: The ZX81 Turns 40

#31

Post by zokier »

rwmj wrote: post #28
trynton wrote: post #19

According to something I read somewhere. The ZX81 floating routines did incorperate some unique math: (2^32) - ((2^32)-1) did not return 1 And the group of Cambridge mathamaticians that wrote the ROM turned out to be one man. "ZX81 ROM Assembly Listing" https://www.tablix.org/~avian/spectrum/rom/zx81.htm

I just tried it on the ZX81 online emulator: http://www.zx81stuff.org.uk/zx81/jtyone.html You have to type the following sequence of keys: P [Shift]I 2 [Shift]H 3 2 [Shift]O - [Shift]I [Shift]I 2 [Shift]H 3 2 [Shift]O - 1 [Shift]O Enter which should appear as: PRINT (2**32)-((2**32)-1) It prints 0 which is not the expected answer! (The 0/0 at the bottom of the screeen is an OK message, the answer is printed in the to…

0 is actually what I'd expect as an answer. Even on very modern Python you get

    In [1]: 2.0**64 - (2.0**64-1.0)
    Out[1]: 0.0
Thats how floats behave like with operands that differ greatly in magnitude. Also this

    In [2]: 2.0**64 == (2.0**64 - 1.0)
    Out[2]: True

Re: The ZX81 Turns 40

#32

Post by canada_dry »

Speaking of modified ZX81's... I re-wired a full sized keyboard (I think I was ~15 at the time) to replace the original [i]

[i] https://i.imgur.com/MpoQP4u.jpg

The ZX81 along with the Z80 microprocessor book from Radio Shack [ii] were the gateway drugs that focused my career direction!

[ii] https://www.cpc-power.com/cpcarchives/index.php?page=article...

I'm retired now, but my career went from jr. programmer (in banking) thru to executive level IT management. A great ride thanks in large part to that inexpensive UK computer!

Re: The ZX81 Turns 40

#33

Post by trynton »

rwmj wrote: post #28
trynton wrote: post #19

According to something I read somewhere. The ZX81 floating routines did incorperate some unique math: (2^32) - ((2^32)-1) did not return 1 And the group of Cambridge mathamaticians that wrote the ROM turned out to be one man. "ZX81 ROM Assembly Listing" https://www.tablix.org/~avian/spectrum/rom/zx81.htm

I just tried it on the ZX81 online emulator: http://www.zx81stuff.org.uk/zx81/jtyone.html You have to type the following sequence of keys: P [Shift]I 2 [Shift]H 3 2 [Shift]O - [Shift]I [Shift]I 2 [Shift]H 3 2 [Shift]O - 1 [Shift]O Enter which should appear as: PRINT (2**32)-((2**32)-1) It prints 0 which is not the expected answer! (The 0/0 at the bottom of the screeen is an OK message, the answer is printed in the to…

According to the link to ""ZX81 ROM Assembly Listing"", it used a "FORTH-like, stack-based language.". Amazing all the same how they managed to squeeze so much out of so little.

Re: The ZX81 Turns 40

#34

Post by [deleted] »

klelatti wrote: post #23
forinti wrote: post #21

Earlier quoted context omitted.

You could, for 170 pounds, get an Acorn Atom. It had 2KB of RAM (twice as much!), a proper keyboard, an excellent BASIC, and a video chip.

Wikipedia says that would be over £700 in 2019 money - too much for a 15 year old! Besides the £170 Atom only had Integer Basic - we had real (sort of) floating point. I'm still jealous of anyone who had an Atom though.

[deleted]

Re: The ZX81 Turns 40

#35

Post by trynton »

zokier wrote: post #31
rwmj wrote: post #28

Earlier quoted context omitted.

I just tried it on the ZX81 online emulator: http://www.zx81stuff.org.uk/zx81/jtyone.html You have to type the following sequence of keys: P [Shift]I 2 [Shift]H 3 2 [Shift]O - [Shift]I [Shift]I 2 [Shift]H 3 2 [Shift]O - 1 [Shift]O Enter which should appear as: PRINT (2**32)-((2**32)-1) It prints 0 which is not the expected answer! (The 0/0 at the bottom of the screeen is an OK message, the answer is printed in the to…

0 is actually what I'd expect as an answer. Even on very modern Python you get In [1]: 2.0**64 - (2.0**64-1.0) Out[1]: 0.0 Thats how floats behave like with operands that differ greatly in magnitude. Also this In [2]: 2.0**64 == (2.0**64 - 1.0) Out[2]: True

> 0 is actually what I'd expect as an answer. Even on very modern Python you get

Given a four byte mantissa, it should be 1 for anything under and equal to 2^32 and 0 for anything over 2^32. Instead it outputted some random number. A bug in checking the flags or some-such. Or maybe a bug in the float to string routine.

Re: The ZX81 Turns 40

#36

Post by andromaton »

klelatti wrote: post #23
forinti wrote: post #21

Earlier quoted context omitted.

You could, for 170 pounds, get an Acorn Atom. It had 2KB of RAM (twice as much!), a proper keyboard, an excellent BASIC, and a video chip.

Wikipedia says that would be over £700 in 2019 money - too much for a 15 year old! Besides the £170 Atom only had Integer Basic - we had real (sort of) floating point. I'm still jealous of anyone who had an Atom though.

And the A in ARM stands for Acorn. And the ARM1 was based on a Berkeley design, now on version 5 (Risc V). Connections, connections everywhere.

Re: The ZX81 Turns 40

#37

Post by spongeb00b »

nickt wrote: post #11

In terms of ROI, the best £69.95 I ever spent (I still have the receipt from WH Smith). It wasn’t the first computer I ever used, but it’s the first one I ever owned at age 9 and it led me down the path that’ll be familiar to many here... I’m glad I still have it, and the Spectrum and Amiga’s that followed. Incredibly, the ZX81 hardware and software scene is still alive. You can get parts for it quite easily, includi…

Even Blu-Tack for holding on the RAM expansion is still available to buy (sorry, too hard to resist)

Re: The ZX81 Turns 40

#38

Post by davidrupp »

I did my first programming on one of these, translating text games (bitmapped graphics were possible with the 16K RAM pack, but I didn't have one of those for a long time) from a book of them written for the TRS-80. The combination of having to translate between dialects of BASIC and having to "translate" game features to match the ZX81's reduced capabilities was a great practical exercise in inventiveness and perseverance. I particularly remember a Star Trek combat game that ended up taking up so much of the onboard RAM (one whole kilobyte!) that it started eating into the screen buffer, reducing the number of lines the computer could display. I think I ended up with one or two lines of text telling me what was going on, and one line of input for my next command.

Today, I work at Amazon as a senior software engineer. I believe having access to this affordable[1] computer as a teenager helped make that career path possible.

[1] The TRS-80 and other contemporaries of the ZX81 were very not-affordable to my family at the time.

Re: The ZX81 Turns 40

#39

Post by sys_64738 »

TMWNN wrote: post #14
nickt wrote: post #13

Earlier quoted context omitted.

> Commodore had the largest warehouse of these in North America by 1984 That sounds like a story, have any more details?

sys_64738 is referring to Commodore running a tradein program for $100 off a C64 with any computer or videogame console. People purchased the Timex Sinclair for $45 solely to trade in ( https://dfarq.homeip.net/wp-content/uploads/2020/08/timex-co... supposedly, Commodore used them as doorstops.

The C128 from 1985 had a Z80 built into it. Where did those come from I wonder?

Re: The ZX81 Turns 40

#40

Post by vram22 »

cesaref wrote: post #25
TacticalCoder wrote: post #24

Earlier quoted context omitted.

> I’m glad I still have it, and the Spectrum and Amiga’s that followed. I wonder how many of us still have them all... I probably sound like a broken record at this time but I still have my Atari 600 XL, Commodore 128 (which I was only using in C64 mode) and Commodore Amiga. The 600 XL and 128 are still working and the Amiga is probably easy to fix.

I ditched almost all my old computers as I upgraded, but i've still got my ZX-81 with a dk'tronics graphics rom, that was a killer combination in 1982 or whenever! The ones that have left the building include C64, C128D, Amiga 1000, Amiga 500, before I got my first 'proper' computer, a Mac IIcx which I had for many years running Logic. I'd like to say those were the days, but actually i get much more done and have mo…

I agree about the nostalgia, and will also add (differing), that at least in some ways, those were more fun times ... because the overhead of all this modern complexity (although it sure does confer greater features and power), does kill off some of the fun.
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