Re: Z80 CPU Speeds?

[Eric Smith <eric-no-spam-for-me@brouhaha.com>]

BAD <badavis@nbnet.nb.ca> writes:
> I am wondering what the speed differences are between the Z80, Z80A,
> Z80B, and any other Z80 versions?

The ratings of the chips are 2.5 MHz with no letter suffix, 4 MHz for A,
6 MHz for B, and 8 MHz for H.  After that, Zilog started using two
digit numeric speed grades as part of the base part number.

Note that the rating of the chip does NOT tell you what speed it actually
is operating at on the board.  A part with a B suffix will run just fine
at 2 MHz.

The Microsoft Softcard, and its clones, run the Z80 at twice the Apple's
bus frequency using an asymmetric clock.  The effective speed is thus
2.046 MHz, independent of any suffix on the chip.

If the card doesn't have on-board RAM, it's almost certainly a Softcard
clone.