The Apple IIe was designed to be timing-compatible with the Apple II+ so that timing-depndent software (such games and copy-protection systems) and hardware designed for the Apple II and II+ would, for the most part, work in the IIe.

However, the Apple IIe used a (presumably more expensive) 6502A, rated for 2 MHz operation. Why did they do this? Can I substitute a 1 MHz 6502 for the 6502A?

  • From what I recall, the only popular 6502 based systems that ran at 2 Mhz were the Atari 8 bit systems, 400 | 800 | 65XE | 130XE . – rcgldr Mar 18 at 5:20

No, you can't substitute; the 6502A was used precisely because it is faster for some things, even when not run at a higher clock rate.

Apple IIe Technical Note #2: Hardware Protocol for Doing DMA (starting on page 2 of that PDF) explains this. On page 4 of 9 of the note it says:

In the Apple IIe a 6502A, a 2 MHz part is used instead of the 1 MHz 6502 used in the Apple ][+. Since the custom chips in the Apple IIe are MOS and slower than the TTL in the Apple ][+, the faster 6502A was used to guarantee better margins. For example, the 6502A sets up the address bus faster on the Apple IIe than the 6502 does in the Apple ][+.

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  • 2
    Manufacturing-wise there was no difference between the 6502 and the 6502A devices. They were sorted (binned) at final test when 6502A chips were needed. Often, there were no orders for the "A" versions so all chips just had to pass the 6502 timing tests. So it was not uncommon to find 6502 marked chips that ran at 2MHz. If you cherry picked through them you could often find a few that would run at 3 or even 4MHz. – jwh20 Mar 18 at 1:32

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