why only 1k of ram?

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why only 1k of ram?

Post by Crayon21 »

even the Vic 20 had more. the about zx80/81 section calls it massive. what's so massive about 1024 kb? curious to know :?:
zx81: great yet flawed
Zx spectrum: masterpiece that keeps on giving

(wish it weren't so expensive) :o
Lardo Boffin
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Re: why only 1k of ram?

Post by Lardo Boffin »

I suspect to keep costs down. The ZX81 was built to a cost more so than functionality level.
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Re: why only 1k of ram?

Post by 1024MAK »

Sinclair’s aim with the ZX80, was to produce the lowest cost, but fully functional home computer that was possible. Hence every single part of the design and manufacturing was geared towards this.

Apart from the Z80, the ROM chip and the RAM chips, all the remaining chips were inexpensive standard 74LSxxx series parts.

The ZX81 was designed to lower the cost further. Hence the ULA replaced ALL the standard 74LSxxx series parts. A keyboard made of a plastic membrane also lowered the cost.

This left the ZX81 with a smaller (and cheaper) PCB, with only four chips or only five chips. The chips are:
  • Z80A or more likely a NEC unlicensed clone (NEC D780C-1) CPU
  • ULA
  • Mask ROM
  • 1K byte SRAM chip
  • Two 1K by 4 bit SRAM chips (combined giving 1K byte)
So why 1K? Because this was the most cost effective SRAM chip available at the time that provided the minimum functionality that suited Sinclair’s requirements.

At that time, all RAM was very expensive. SRAM was very simple to use, and did not need any support chips. Whereas DRAM chips for larger amounts of RAM were cheaper per byte, but needed support chips. Complexity that Sinclair did not want.

To see an indication of how much DRAM cost, look through adverts in the magazines of the time and compare the cost of a ZX81 and that of a 16K RAM pack for it.

Rich Moore
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Re: why only 1k of ram?

Post by Rich Moore »

The limitations of the machine, particularly the 1k ram, is part of the charm of the zx81. It forces better, at least more efficient, programming. Quite clever to sell the ram expansion as an 'optional' (!) extra. When they released the zx80 and 81 what was their market? There were plenty of utility programs and I remember most ads were aimed at serious, grown up markets ie for office use. The home gaming market hadn't really been invented yet and I wonder if the birth of this market was partly a happy accident. Was it initially developed by Sinclair as just a very programmable calculator?
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