ZX Spectrum internals (Was: ZX81 kits)

From: Iggy Drougge <optimus_at_canit.se>
Date: Sat Oct 6 20:17:40 2001

Tony Duell skrev:

>> >> What chips would you use? I assume that the Speccy uses one chip for its
>> >> entire video generation, but where would you split the functions?
>> >I should have said _standard_ chips. Once you start allowing ULAs (as the
>> >ZX81, Spectrum, Oric, etc all used) then it's easy to do video in a
>> >couple of chips.
>> Of course I don't expect you to be making your own ULAs. =)

>No, and you know that _I_ wouldn't suggest using an FPGA. Not for a 1-off
>project, certainly....

How in the world does one make those, anyway? I haven't got the slightest how
one could "burn" devices which are capable of even replacing a CPU. Does one
"burn" at all?

>> >Didn't we go through this a couple of weeks back? The 2 chips I would
>> >start with are an SAA5243 Teletext display chip (or an SAA5245 for the
>> >States) and an 8K*8 static RAM (6264). You need to get a 6MHz clock from
>> >somewhere (maybe another chip for that, if you can't also use it for the
>> >CPU clock in some way) and maybe you want to buffer the video output
>> >lines.
>> Teletext? That's a funny idea.

>What's the problem? It gives be a simple 40*25 text display in a handful
>of chips...

No problem really, it's just so simple that it seems a bit unorthodox.
When you think about it, Videotex (Prestel, Micronet et al for you in the UK)
looked very much^H^H^H^H^H^H^H^H^H^H^H^H^H^Hwas identical to teletext. And the
6845-generated eight-colour ABC 800 screen looks very similar to that as well.

>> >If it's another 40 pin chip, then that's the custom ULA chip that
>> >includes most of the glue logic.
>> Is that like a lot of 74138s or something?

>It's rather more than that (I assume that's the address decoder part,
>which is in the ULA). It also handles the DRAM timing, most of the video
>display, and so on.

I thought one of the selling points of the Z80 was its built-in ability to
handle DRAM. But perhaps it's lacking the 6502's ability to easily share
memory with other devices by only doing memory accessing every second or
fourth cycle.

But what does glue logic really mean?

>> >Does the Spectrum really have a separate character generator ROM. Doesn't
>> >sound right to me...
>> I wouldn't know, but this is the chip which somehow enables Swedish
>> characters. Is it a patched BASIC ROM?

>I suspect it is. Certainly if the keyboard layout is different then the
>BASIC ROM must have been modified.

Couldn't Sinclair have socketed the BASIC? It must have been a lot of work to
piggy-back it onto each imported Spectrum, unless the customers had to do it
by themselves.

En ligne avec Thor 2.6a.
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Received on Sat Oct 06 2001 - 20:17:40 BST

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