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From: bayes@hpislx.HP.COM (Scott Bayes)
Newsgroups: comp.sys.mac.misc
Subject: Re: Graphics Questions....
Message-ID: <9590005@hpislx.HP.COM>
Date: 27 Aug 90 18:47:18 GMT
References: <1211@meaddata.mead.UUCP>
Organization: Measurement Systems Operation - Loveland, CO
Lines: 53

> I have a couple of technical questions..
> 
> How does a computer generate the video representation of screen memory?
> 
> 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 or 60 times a second (which if I am not mistaken is
> the speed that a television redraws its picture and monitors are even faster)
> 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.
> Anybody out there who knows something about microprocessors ect..  please
> help me out..........
> 
> Thanks in Advance					- Dan Rubin

An excellent set of questions.  I'll assume that you are concerned about
the process of actually getting the numeric pixel values to be visible
on a screen, and not about doing real-time animation at frame rate (for
which your estimate is probably reasonably valid).  The answers, for
most of the micro computers we deal with these days, are that:

1) a dedicated piece of circuitry scans through a special piece of RAM
called the "frame buffer", turning numeric values into displayed pixels.
A digital-to-analog converter (D to A) converts the numbers to CRT
driving voltages, which control intensity of the flying spot.  All this
happens without the CPU's connivance, once things are set up properly,
and the appropriate pixel data loaded into the frame buffer.

2) the computer has a separate "port" (access path) to the frame buffer
memory (which is often called "dual-ported" because of this) through
which it writes or reads pixel values, almost simultaneously with the
dedicated display hardware's access.

3) the display hardware, being dedicated, can render pixels far faster
than could the typical CPU.  An added bonus is that you don't waste
precious CPU cycles updating a CRT rendering.

Note that some older machines (e.g.  the HP 9830) actually did have the
CPU continually refresh the display.  You could either display or
compute on the '30, but not do both at once.  Your estimate would
probably be close to the mark, were that older type of architecture in
effect today.

Many displays use different architectures than the "shared address space
via dual-ported RAM" we are used to seeing.

Scott Bayes