milazzo@RICE.EDU (Paul Milazzo) (12/22/85)
In <382@pedsgd.UUCP>, Bob Halloran <bobh@pedsgd.UUCP> ventures:
"Granted, the graphics chips relieve the [Amiga's] 68000 from
dealing with the display, as it must in the Mac, but the 68000
and the various co-processors must still contend for access to
memory. YES, I/O can proceed independently, etc., but only ONE
of the processors can use memory at a time. This HAS to impact
the throughput of the 68000."
This turns out not to be the case. First, the Amiga's coprocessors can
*only* access the first 512k of RAM. Any additional RAM resides on a
completely separate bus to which only the 68000 has access. The loader
allows a program developer to specify that those program segments
containing graphics structures should reside in the shared RAM, and the
remainder of the program in "fast" RAM, if available.
Second, the shared bus is run at twice the CPU speed, and the CPU and
coprocessors use alternate bus cycles. Thus, under "normal" conditions
the CPU sees no bus contention whatsoever. Only when the coprocessor
bus cycles are inadequate for a particular task do the coprocessors
begin to preempt the CPU's bus cycles. Most commonly, this happens
whenever a 640-pixel-wide display with more than 3 (I think) bit planes
must be displayed. Default text windows use the 640-wide mode with two
bit planes, and so do not slow the CPU.
N.B. I own neither of the machines being compared. Much of the above
information comes from INFO-AMIGA postings by Amiga technical
personnel, and any inaccuracies therein are no doubt due to my
faulty recollection of those messages. I welcome corrections.
Paul G. Milazzo
Dept. of Computer Science
Rice University, Houston, TX
Domain: milazzo@rice.EDU
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