Path: utzoo!attcan!uunet!iconsys!caeco!jose!ken
From: ken@jose.UUCP (Ken MacLeod)
Newsgroups: comp.sys.amiga
Subject: Re: How 'Bout HyperCard!
Summary: Common misconception:  "Amiga bus runs at 14MHz"
Message-ID: <194@jose.UUCP>
Date: 20 May 88 14:16:22 GMT
References: <15372@uflorida.cis.ufl.EDU> <31411@linus.UUCP> <5324@cup.portal.com> <5492@cup.portal.com>
Organization: Price Savers, Salt Lake City, UT
Lines: 33

In article <5492@cup.portal.com>, doug-merritt@cup.portal.com writes:
> The thing that ST fans miss when they point out the faster cpu on the
> ST is that the Amiga has *twice* the memory bandwidth...16Mhz. And
> it is used by the blitter, which is itself much more efficient of memory
> bandwidth than the 68000 (at simple pixel operations as used in e.g.
> windowing).

  Whoa!!  Common Amiga owner misconception!  The bus _does_not_ run twice
as fast as the processor clock!  Get "16MHz" out of your mind!

  It goes like this:  The processor clock is 7.14MHz, the 68000 normally
accesses memory only every _fourth_ clock cycle.  Memory can cycle every
_second_ cycle, leaving every other even cycle free for DMA.

  In other words:  Memory does _not_ cycle at twice the processor clock
rate, but at _half_ the processor clock rate.  Since the processor only
uses a _quarter_ of the processor cycles accessing memory, the second
quarter is still free.

  This can all be found in the Hardware Ref., pgs 186-190, and note that
they usually use 'memory cycles' in their descriptions, memory cycles
are two 'processor cycles' each.

  Also, the ST does the same thing, it uses the free cycles (one quarter
of the available processor cycles, one half of the available memory
cycles) to fetch video memory.  The reason you never hear about this
is because video is the only thing on the ST that uses those cycles so
there's nothing really interesting about it.
-- 
Ken MacLeod                                    Price Savers Wholesale Warehouse
uunet!iconsys!caeco!pedro!jose!ken                         Salt Lake City, Utah
(801) 277-6966 (evening)                                  (day)  (801) 264-7224
        Disclaimer:  I don't let anyone share my views, there all mine.