hardware solution for direct access to video ram

[Steve Simmons]

botton at laidbak.UUCP (Brian D. Botton) writes:

>  Hi netlanders.

Hi yourself.  Nice introductory article.

>  This article is fairly long as is gives the theory and step-by-step method
>for constructing a daughter board that allows user level code to access the
>video ram on a 3B1. . . .

Usually these hardware article make me nod my head sagely and wish I
wasn't such an idiot with a soldering iron.  However, there are some
interesting implications....

>THEORY OF OPERATION

>  The 3B1 memory map allocates addresses 0x420000 to 0x42ffff for video ram,
>even though all of these addresses are not displayed on the CRT.  As most of
>you know, addressing beyond 0x3fffff causes a memory fault, which is why you
>cannot access the video ram.  If you take a look at the equations for the MMU
>pal, page A-10 of the UNIX PC Reference Manual, you will find the following:
> [details removed]
> . . . it becomes clear that simply reprogramming the pal
>will not do you any good.  However, if you follow the SUPV signal back to it's
>origin, you will see that it is just FC2 on pin 26 of the 68010.  This is the
>key to my solution.  
>  Which is, a daughter board is placed between the 68010 and the mother board,
>that takes off the address lines needed . . .

There might be a very interesting implication here.  He's talking
about direct access to the video ram (wouldn't this be easier and safer
with an appropriate new device driver for fb?  never mind, it's irrelevant
to the implication).  But it's clear he's found and removed one bottleneck
to addressing >4MB!  With ram prices coming down (and down, and down) I
see an interesting idea.

Using a similar scheme is it possible put >4M in the Unix-PC?  This
should be of great interest to the mondo combo people and the X people.
At first glance I though a killer gating item might be the number of lines
on the backplane, but it looks like this might enable one to completely
bypass the backplane for system memory expansion!  Mod the mondo combo to
use SIMM modules (1Megabyte, 100ns are $180 and still dropping), and think
about a 8 or 16MB 3b1.  Brrrr!  You'd still need the backplane, but only
for power and accessing the non-memory parts of the board (serial ports,
etc).  This also has the benefit that when your 3b1 finally dies, you can
possibly put the SIMMs into the replacement.  Yes, I know SIMMs stick up
too high to fit thru the openings into the backplane.  Didn't a large
chunk of the mondo combo stick out the back?  Anyway, it's an idea.
-- 
Steve Simmons		          scs at vax3.iti.org
Industrial Technology Institute     Ann Arbor, MI.
"Velveeta -- the Spam of Cheeses!" -- Uncle Bonsai