🚀 go-pugleaf

Someone correct me if I'm wrong, but...  {just waiting for the flames about
math and spelling errors!}

All timing in the Macintosh operates at a master clock rate of 15.67 MHz.
This is divided by two to get the CPU clock, 7.83 MHz.  The 68000 requires
a minimum of 4 clock cycles (8 master clock cycles) to complete a memory
access.  The video/sound/disk speed circuitry also takes 8 master clock
cycles per memory access.  The catch is, when the 68K tries to access the
RAM, if a video access is in progress, or is about to start, it has to wait.

The Mac displays active video on 342 out of 370 scan lines, and on each scan
line there are 32 (512/16) video accesses or 256 master clock cycles out of
704 master clock cycles total per scan line (44*16, where 44 is the total
number of words (16-bit) per scan line).  Audio and disk speed take 1 video
access on *every* scan line (all 370).  This means that the total number of
clock cycles eaten by video, audio, and disk speed per video frame is
(32*342+370)*8, or 90,512, out of 704*370, or 260,480 clock cycles, for an
overhead of only 34.75%.  The only remaining problem is that out of the
remaining 65.25% of the time, the 68K has to waste some of the available
cycles for synchronization, for instance if a memory access is initiated one
cycle before a video access begins, that cycle is wasted.
-------------------------------------------------------------------------------
Eric L. Smith  (801) 581-8100  e-smith@utah-cs.arpa  ...decvax!utah-cs!e-smith
3118 Merrill Engineering,  University of Utah,  Salt Lake City, UT  84112

The opinions expressed herein do not necessarily represent those of the
University of Utah, my friends, enemies, computer, or even me.  :-)

If everyone would dare to be stupid, what a stupid world this would be.
				-- Wierd Al Yankovic
-- 
-------------------------------------------------------------------------------
Eric L. Smith  (801) 581-8100  e-smith@utah-cs.arpa  ...decvax!utah-cs!e-smith
3118 Merrill Engineering,  University of Utah,  Salt Lake City, UT  84112

The opinions expressed herein do not necessarily represent those of the
University of Utah, my friends, enemies, computer, or even me.  :-)