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Oct 28 '19 at 8:46 history edited john_e CC BY-SA 4.0
I think 'ALU' must have been a typo for 'ULA'.
Oct 27 '19 at 22:44 comment added tahrey In fact, even the IBM PC used 200 (250?) ns parts. Sinclair could have used much slower memory (300ns or more) and pushed against the specs somewhat, which was a common tactic used by a lot of manufacturers... likely, the 200ns "half broken" chips were the cheapest, and offered sufficient overhead "just in case" whatever fault knocked out the unused half also affected the speed of the "working" half. Using faster stuff and prebuffering it may have worked, but it'd be way too complex for a machine of its type. Anyway, like you say, they already had performance stratification - the 16K vs 48K.
Oct 27 '19 at 22:41 comment added tahrey IE once the need for the extra screen memory and per-line bandwidth is considered, as well as rejecting more marginal parts, it'd have been a zero sum outcome for performance, or at least performance per pound, and Sinclair's aim was the absolute cheapest computer meeting certain goals; in the case of the Spectrum, being a ZX81 (also running at 3.5MHz, with 32x24 text), but with lowercase, colour, sound, and more internal memory. Similarly the RAM speed was neither here nor there. 200ns is more than enough for 5MHz, let alone 3.5; they were just the cheapest chips. 150ns is Amiga/ST grade.
Oct 27 '19 at 22:39 comment added tahrey All of those things would again have increased cost though. They tended to use B-grade parts in order to get them for the rock bottom wholesale price (it may only be legend, but apparently the 16K chips were all "half broken" 32Ks wired to only use the working half), so underclocking everything was somewhat necessary to ensure reliability. Also, like most old computers, the pixel clock was tied to the rest of the system; increasing that would mean changing the whole screen structure, and probably increasing resolution to lessen the "windowbox" effect. It'd have likely counterbalanced...
Sep 20 '18 at 10:58 history edited Jules CC BY-SA 4.0
added 58 characters in body
Sep 20 '18 at 10:12 history answered Jules CC BY-SA 4.0