ZX80/ZX81 with a 6502 ...

[marste]

From article like this one https://retrocomputing.stackexchange.co ... -computers seem Clive selected Z80 not to invest too much on the video part, but reading other ones (that I found while researching about KIM-1 and video generation) I found out that it could have been even easier, as for example using the scungy circuit described here: https://www.tinaja.com/ebooks/socv1.pdf ...

With its architecture seems to me that the 6502 would have been even more fitting for managing 1K RAM, and for sure the CPU would have been cheaper!...

How many of you (that know decently both CPUs) agree? (polling is open...)

[Moggy]

1k was never the endgame though was it?

The ZX81 may have come with only 1k as standard but extra memory was always there as per the ZX80 which renders the 6502 moot on that particular point and I tend to believe that the thinking at Sinclair research was that the double register set, the index registers, 16 bit maths instructions and the ability to use the refresh function of the Z80 for other purposes may have swayed their views somewhat as well as other technical considerations.

As for the price of the CPU,without knowing what deals Sinclair struck with their suppliers we can never know which was the cheaper of the two to Sinclair and knowing Clive's thriftiness I suspect the Z80 was cheaper to him than the 6502. Perhaps Commodore International who became the owners of the 6502 very early on were more keen on supplying Apple and Atari etc than doing a sweet deal with a small UK company, which I think also makes this price point argument moot and whilst the 6502 may have been cheaper in 1976 I don't believe that was the case in 1980/81 and onwards.

Just a few musings on my part.

[1024MAK]

Some points…

• There were in 1980, rather a lot of second sources for the Z80A. Including the unlicensed NEC clone, the μPD780C-1 (many Sinclair computers contain these rather than Zilog parts, and yes, Sinclair did use the other second sources, e.g. Mostek MK3880 amongst others). Yes, there were also second sources for the 6502 (and the other versions). With all these companies producing microprocessors, it’s likely the cost when bought in bulk would not be much different between the two. The initial price in the mid 1970s is not really relevant. Especially as the 6502 was designed to undercut everything else on the market at the time of its design, especially its main competitor from Motorola, the 6800. That does not mean the situation was the same in 1980…
• Industry used both the Z80 and the 6502 microprocessors, but I believe the Z80 was more popular, because it was code compatible with the Intel 8080. Hence many CP/M systems actually used a Z80 processor rather than a Intel 8080.
• The use of the Z80s built in memory refresh system is fundamental to the operation of the ZX80 and ZX81 video display system. The 6502 has absolutely no support for any memory refresh. All 6502 systems that need a memory refresh system (for use with DRAM chips) have to either have a DRAM controller (special chip or equivalent circuitry) or use the video system chip(s) to do this for it.
• In terms of processing power, the Z80 and the 6502 are rather similar in overall performance. There are of course differences depending on what tasks you want them to do. And of course, the Z80 has a higher frequency clock compared to the 6502. But don’t let that fool you, as with both, it’s the memory bus bandwidth that is far more important.
• One area where the Z80 definitely wins is with interfacing input/output (I/O) systems. The Z80 has a separate address space and separate control lines, this makes the hardware a lot simpler for inexpensive computers like the ZX80 and ZX81. In comparison, 6502 systems have to have all I/O as part of the normal memory map. This increases the complexity of the memory decoding unless you have a very simple address decoder and are happy to have 8 to 16k bytes of the memory map allocated to the I/O area.

Also of note, the Amstrad CPC range, the Amstrad NC100, NC150 and NC200, plus the MSX range all use the Z80. Okay, all these were much later machines.

Just to point out that although I am not completely independent or neutral on this, I do have various computers based on the 6502 (or other versions), including various Acorn computers and some Commodore 64 and 128 computers.

I’m far better at hardware than writing assembler code. But I have written Z80 and learned 6502 when at college when I was a teenager (that was on a Rockwell AIM 65).

Mark

[David G]

Great posts, very interesting information!

One question, doesn't the 16K RAM PAK have its own refresh circuit?

[Paul]

If the Sinclair Computer would have been based on a 6502 I would never have bought one

[Paul]

One question, doesn't the 16K RAM PAK have its own refresh circuit?

Please don't abuse this thread

[David G]

One question, doesn't the 16K RAM PAK have its own refresh circuit?

Please don't abuse this thread

I was replying to this thread which said the Z80 was superior for supporting DRAM refresh. But did the ZX81 make use of that feature? ZX81 did not use DRAM, only the add-on memory packs used DRAM

[1024MAK]

I was replying to this thread which said the Z80 was superior for supporting DRAM refresh. But did the ZX81 make use of that feature?

No, in this topic, the important point is that the Z80 refresh system is used by the video system.

See the PM I sent you about RAM packs.

Mark

[Moggy]

QUOTE...
"I was replying to this thread which said the Z80 was superior for supporting DRAM refresh. But did the ZX81 make use of that feature? ZX81 did not use DRAM, only the add-on memory packs used DRAM."


If you took the trouble to read things through before replying you would have seen that nobody, myself included, said any such thing.

Just to make it a bit clearer for you...

QUOTE FROM MY POST.

".. the ability to use the refresh function of the Z80 for other purposes may have swayed their views somewhat as well as other technical considerations..."

The other purpose in this case being for video generation, as Mark has kindly pointed out.