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From: midkiff@portia.Stanford.EDU (Neil Midkiff)
Newsgroups: comp.sys.mac.misc
Subject: Re: Graphics Questions....
Message-ID: <1990Aug24.045311.21371@portia.Stanford.EDU>
Date: 24 Aug 90 04:53:11 GMT
References: <1211@meaddata.mead.UUCP>
Organization: AIR, Stanford University
Lines: 37

In article <1211@meaddata.mead.UUCP> mead!rubin@uccba.uc.edu writes:
>How does a computer update the memory that is used for the screen?
>
>I was thinking and I got stuck when I thought about a 25Mhz (fast!)
>processor.  It seems to me that a the most memory that this processor can
>update at 60Hz is only .417 Megabytes???
>
>Is this correct - the most graphical data that you can malipulate is less
>than 1/2 of a Meg at the normal speeds of a typical television when you
>are using a 25Mhz processor dedicated to graphics?  Is there something that
>I am missing - perhaps there is some sort of co-processor or something.

You're thinking along the right lines, but the "coprocessor" is really
just some dedicated logic on the video board.  The memory associated
with the video display is usually arranged so that the computer's
processor can write into it (and maybe read out of it) whenever NEW
data is put on-screen, while more or less independently, the video board is
continually reading out the data 60 or more times a second, in a
pattern that is timed to synchronize with the monitor's sweeping
electron beam.  Since this pattern is basically repetitive reading,
you don't need a processor that can do computations and stuff.  There
are complications I'm not mentioning (like preventing flicker when
both the processor and the video board want to access the same memory
location at once) but this is the general idea.

In most instances, there is very little new data written to the screen
for each 1/60th second update, so the main processor doesn't spend
most of its time writing to video memory.  Of course, if you want to
do "full animation" with completely new pictures each screen, then the
calculations you made do apply, and you find that you need a VERY fast
processor just to get the data written to the screen (let alone
generating the pictures in the first place).  This tells you, among
other things, why fully digital TV is not yet commercially viable.

End of basic lesson...let me know if you want more detailed
information.
-Neil