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@goz one point. you say "With the speed increase, [the 1200XL] would be the fastest 6502 based computer available at the time." not so; the BBC Micro was available in 1981 (just!), and ran its 6502 at an uncontended 2MHz!

@millihertz Ah, I didn't know that about the BBC Micro! Was it able to run at the full 2MHz? The Atari loses 30% of it's 1.79MHz to the ANTIC under normal conditions.

@goz it was; the 6502 and 6845 interleaved memory accesses, taking advantage of the fact that they both used two-phase clocks and only accessed memory on one of them. getting 250ns DRAMs in 1981 was quite a challenge, apparently ;-) also the 6845 touched every row frequently enough to refresh it, which removed another obstacle from the 6502's memory path. so the 6502 could basically pretend it was the only thing on the memory's data bus, and did

(the Electron, unfortunately, wasn't so well equipped; in the interests of cheaper manufacturing, they only used a single 64Kx4-wide DRAM, and let the ULA double it up to 8 bits - which reduced the effective speed of the 6502 to 1MHz when accessing RAM. but worse, now it was in direct competition with the video circuit, so any mode that needed to get 16Mbit/s out of memory - anything above 320x256 at 1bpp, basically - locked the 6502 out of RAM completely)

@millihertz Being in the US, I didn't get much access to BBC Micros, but the specs are impressive.

Article updated! Thanks!

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