hardware solution for direct access to video ram
Brian D. Botton
botton at laidbak.UUCP
Fri Jul 28 15:46:48 AEST 1989
Hi netlanders. I am posting this to both unix-pc.general and comp.sys.att,
even though there seem to be some people that would disagree.
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. The driving force behind this board was the desire to port
Mgr to the 3B1 while NOT making any hardware modifications to the actual
mother board. The idea for the board came from a friend and co-worker,
Brad Bosch, who is a Mgr fanatic. Brad and I have been running our modified
3B1's continuously for over a month without a problem and I am confident that
there are no problems lurking to bite us later. We have also gotten the
portable bit blit routines for Mgr working. So, without further delay, I will
begin.
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:
BERREN = BGC* X PA22* X SPA23* X LPS0* X LPS1* X T90 X PAS
+ BGC* X SUPV* X SPA23* X PA22* X KADDR X T90 X PAS
+ BGC* X SUPV* X SPA23* X PA22* X RW X LWE* X T90
+ BGC* X SUPV* X SPA23* X PA22 X IODTACK
+ BGC* X SUPV* X SPA23* X DTACK
The signal BERREN is what causes a bus error on pin 22 of the 68010.
Address 0x42XXXX means that SPA23 will be low and PA22 will be high, and thus
the first 3 terms of the equation will be false and have no effect. Therefore,
in order to stop a bus error from occuring when addressing the video ram, the
last two terms must be made false also. Looking at the other MMU pal equations,
and the available inputs, 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 to decode video ram access and forces
the mother board, and MMU pal, to think that it is in supervisor, a.k.a. kernel,
mode. Yes, this means that ANY user level process can access the video ram, but
when you examine the consequences of this, most of them are trivial, especially
when you now have direct access to the screen. I suspect the people porting X
to the 3B1 will be very happy to have this direct video ram access.
To continue, if you look at sheet 4 of the CPU schematic, you will see that
IC 26G does the actual decoding. Address lines A16, A17, A18, A19, and GATE1*
are inputs, where GATE1* is A24* X A23. Notice that A20 and A21 are not used
at all to decode device addresses. What this means is the memory map for the
3B1 has several places where addresses are aliased, i.e., video ram is at
0x42XXXX, 0x52XXXX, 0x62XXXX, and 0x72XXXX. Therefore, the following equation
expresses what needs to be done:
SUPV = FC2 + (A23* X A22 X A19* X A18* X A17 X A16*)
Changing the SUPV signal in any manner is risky, too many propagation delays
could easily cause the MMU pal to function incorrectly. To keep delays down,
I decided to use a pal with a 10nsec maximum delay time. Using a pal also makes
manufacturing the daughter board much easier because there are less components
to worry about. After I verified that my ideas were correct, I programmed a
25nsed UV pal and it worked also.
The following is the listing of the program file used by abel, the pal
programming tool that I have access to, to burn the pal. As you can see, it is
a simple program that anyone can follow and duplicate/modify for his/her pal
programmer.
*** CUT HERE - vidpal.abl ****** CUT HERE ************
module VIDPAL
title 'Map video ram into user space on 3B1.
Brian D. Botton June 27, 1989'
"
" Permission is granted to use this file in any way as long as:
" 1. No profit is made off of my design.
" 2. Credit is given where it is due.
"
"
" U1 - MC68010L10
" U2 - PAL16L8DCN
U2 device 'P16L8';
" Constants:
x = .X.;
" Pass FC2 through, unless addressing 0x42xxxx, 0x52xxxx,
" 0x62xxxx, or 0x72xxxx, then force SUPV high.
A23, A22, A19, A18 pin 1, 2, 3, 4;
A17, A16, FC2 pin 5, 6, 9;
SUPV pin 12;
" Tie all unused inputs to Vcc, pins 8, 9, & 11.
equations
SUPV = FC2 # (!A23 & A22 & !A19 & !A18 & A17 & !A16);
test_vectors 'SUPV test'
([A23, A22, A19, A18, A17, A16, FC2] -> SUPV)
" A A A A A A F S
" 2 2 1 1 1 1 C U
" 3 2 9 8 7 6 2 P
" V
[x, x, x, x, x, x, 1] -> 1;
[0, 1, 0, 0, 1, 0, x] -> 1;
[x, 0, x, x, x, x, 0] -> 0;
[x, x, x, x, 0, x, 0] -> 0;
end VIDPAL
*** CUT HERE ****** CUT HERE ************
REQUIRED SUPPLIES
1. Double sided PC board blank, approximately 3 3/8 x 1 5/8 inches.
2. One roll of 4 pt. and one roll of 2 pt. artist tape.
3. One 64 pin low insertion force socket, P/N 44F8066, page 769.
4. Three 40 pin carrier with socket pin terminals, P/N 46N6790,
page 736.
5. One .1 micro farad tantalum capacitor, 7 volts or greater.
6. One MMI16L8DCN pal (10nsec), see text.
7. 2 inches of 28 gauge solid wire, no insulation.
8. Solder, etchant, one XXX drill bit, one drill bit smaller than
XXX, drill, hobby knife, diagonal cutters, a jewelers file.
9. Rosin core solder and a soldering iron, small wattage.
10. A spare 20 pin IC socket, needed only for layout purposes.
The part and page numbers are from the Newark Electronics catalog #110.
The XXX drill bit should be sized to allow the socket pin terminals to be easily
inserted into the PC board. Sorry, I don't know the size, I just happened to
have one that worked and the catalog doesn't give the diameter.
STEP-BY-STEP INSTRUCTIONS
The purpose of the daughter board is to allow the FC2 signal to be intercepted
and to provide a mounting point for the pal. You will want to orient the PC
board such that pin 1 of the 68010, U1, is at the top left corner of the board
with the pal, U2, to the right of U1, with U2's pin 1 horizontal to pin 49 of
U1. While you may certainly make the daughter board using a bread-board and
point-to-point wiring, I liked the etched PC board approach better. All
referenced to the board are made assuming the long axis of the board is vertical
and the short axis is horizontal. So let's begin.
1. First we want to determine the size of the daughter board. To do this, lay
the 64 pin IC socket along the left side of the PC board and the 20 pin IC
socket to the right. You want to allow about 3/8 of an inch clearance
between the two sockets to allow room for the traces to be laid down. Mark
the square formed by the two sockets, allowing about 1/8 inch of overhang on
each side to give the board strength. After marking the square, cut it out,
smooth the edges with a file or sandpaper, and clean both surfaces until
shiny.
2. Carefully place the 64 pin socket on the left side of the PC board and mark
where the pin 1 and 32 are. Remove the socket, and draw a line connecting
the two marks with a straight edge. Replace the socket and mark where the
other 30 pins touch the line. Needless to say, this is very critical, you
cannot be too accurate, so TAKE YOUR TIME. Using the point of the hobby
knife, make starting indentations where the pins are marked. Use a smaller
than XXX drill bit and drill 32 starter holes, then switch to the XXX drill
bit and drill the finish holes. Make sure the holes are not too tight, this
will make inserting the pin sockets more difficult and it will be easier to
break some pins. Again, be very patient drilling these holes, especially
the starter holes.
3. Place the socket on the board, aligning the left holes with the drilled
holes, and mark where pin 33 and 64 should go. Again, draw a line between
the marks and use the socket to mark where the other 30 holes go.
CAUTION: You will be running traces between pins 38 & 39, 40 & 41, and
41 & 42. Make sure you get good, equal spacing around these
pins, it will make your life much easier.
As in step two, drill the holes.
4. Place the 20 pin IC socket along the right edge of the board and align it
so that it's pin 1 is directly to the right of pins 16 and 49 of U1. Using
the same mark and drill procedure described above, drill the 10 left holes
and then the 10 right holes. We now have to drill holes to allow signals to
pass from one side of the board to the other.
5. Drill a small hole half way between pins 1 and 20 and about 1/4 inch above
U2, call this hole A. Drill hole B 2/3 of the way from U1 to U2 and between
pins 41 & 42. Drill hole C 1/3 of the way from U1 to U2 and between pins
40 & 41. Drill hole D 2/3 of the way from U1 to U2 and between pins 38 &
39. Drill hole E 1/4 inch to the left and center of pins 40 & 41, and
finally, drill hole F 1/4 inch to the right and center of pins 26 & 27.
6. The next step is to make solder pads for the pin sockets and feed through
holes. To do this we will use the 4 pt. artist tape and hobby knife, and
make sure the board is clean. It is quite likely you will need to clean
both sides again. Pads are made by placing the tape across the hole
horizontally so the hole is in the center of the tape and trim the tape so
that one widths worth of tape is to the left and right of the hole. After
the tape has been applied to the hole, carefully use the blunt end of the
hobby knife to burnish the tape to the board. This is important because
otherwise the etchant will undercut the tape and ruin the board. Mash it
down good, but don't distort the tape. If needed, don't be afraid to remove
a piece of tape and try again. On the top side of the board, put pads on
all feed through holes and on pins 26 and 49 of U1. On the bottom side of
board, put pads on all holes.
NOTE: While at the art supply store, pick up a set of small
donut pads. While optional, they make the feed throughs,
A through F, a lot easier to do and look nicer.
7. Now we start to put down the signal traces using the 2 pt. artist tape.
Starting on the top side, run a trace from U1, pin 49 to hole A above U2,
this will provide +5 volts to the pal. Run a trace from U1, pin 26 above
hole E (don't touch E!), between holes 41 & 42 to hole B, this is FC2. Run
a trace from hole E between holes 40 & 41 to hole C, this is GND to the pal.
And run a trace from hole F to hole D between holes 38 & 39, this is the new
SUPV signal. It is easiest if you press the tape down at one hole and
burnish as you lay the tape. If you do it this way your will find it is
easy to control the path of the tape and make any required curves. Be
careful when laying out curves, any kinks in the tape will allow etchant
under the tape.
8. Turn the board over and start laying out the bottom traces. For those of
you who are unfamiliar with doing this kind of work, take your time so you
don't make any mistakes. Remember, the top is still at the top, but the
right and left sides are reversed. Run a trace from U1, pin 16 to hole E,
and from hole C to U2, pin 10, this completes the ground. Run a trace from
hole A to U2 pins 20, 7, 8, and 11. I recommend running the trace down the
middle of U2, this completes the +5 volts. U2 pins 7 & 8 are unused and are
tied to +5 as per the spec. sheets for the pal. Run traces from U1, pin 52
to U2, pin 1, from U1, pin 51 to U2, pin 2, from U1, pin 47 to U2, pin 3,
from U1, pin 46 to U2, pin 4, from U1, pin 45 to U2, pin 5, and from U1, pin
44 to U2, pin 6. These are address lines A23, A22, A19, A18, A17, and A16,
respectively. Run a trace from hole B to U2, pin 9, this completes FC2.
Finally, run a trace from hole D, around the bottom of U2 to U2, pin 12,
this completes the SUPV signal and all of the traces.
WARNING: NEVER allow any metal to touch the etchant, violent
chemical reactions can occur.
9. Double check all of your work, making sure traces go where they belong, and
that all of the tape is securely attached to the PC board. Pour the etchant
into a class or plastic bowl and place the card into the etchant. To etch
the board evenly, you wand to arrange things so the card remains on edge
while emersed in the solution. This allows both sides to etch at the same
rate and greatly increases the quality of the etching. I find that it is
helpful to have a couple of plastic sticks to rock the board around and to
stir the etchant. After all of the excess copper has been removed, remove
the board from the etchant and run under hot water for several minutes,
while scrubbing the board to remove any etchant. Remove the tape and
carefully shine up the copper traces. If you have used a fiberglass PC
board, and you should, you can now hold up the board to a strong light and
see all the traces. Double check to make sure they go where they belong and
that there are no shorts, especially where traces go between pins.
10. Insert 64 of the pin sockets all the way into the holes for U1. If any
happen to break, save them, we'll use them in a minute. Solder the top
of pins 16, 26, and 49 to the pads beneath the shoulders of the pin sockets.
Turn the board over and solder all 64 pins to their pads. Make sure that
the traces on the top side of the board don't short to any of the pin
sockets. You may need to use the hobby knife to trim a trace or two. Next
insert the 20 pin sockets into U2, if you happened to break any of the pins
off of other pin sockets, you may use them on U2. Solder the 20 pin sockets
to their pads and using diagonal cutters, cut off all 20 pins of U2 only.
Take the 28 gauge wire and strip off a couple of inches of insulation. For
each hole, A through E, pass the end of the wire through about 1/16 of an
inch and bend it over 90 degrees. Holding the bent wire to the pad beneath
it, cut the wire off on the other side leaving 1/16 of an inch. Bend this
end over, leaving the wire in a U shape and solidly in the hole. Solder the
wire on both sides of the board.
NOTE: When I say cut off the pin of the pin sockets, I mean the small
piece below the larger cylindrical socket.
11. Cut off U1, pin 26, we don't want to pass FC2 to the mother board. Using a
jewelers file, file off the bottom part of the pin socket so it will have no
chance of shorting when inserted into the 64 pin IC socket. Take that 64
pin IC socket and file the socket (top half) of pin 26, again, so it has no
chance of shorting to the filed down pin 26 on the daughter board. This
should leave a small trough in the plastic of the socket that is more then
big enough for a piece of 28 gauge wire to lie in without sticking out above
the top of the socket.
12. Strip 1 inch of insulation off of the 28 gauge wire and stick the end of the
wire into what is left of IC socket pin 26. Very carefully solder the wire
to the pin 26 socket. The plastic of the IC socket melts very easily so you
must work fast and accurately. When the solder cools, cut the wire, leaving
about 1/2 inch and bend the wire at a 90 degree angle into the trough and
toward the center of the socket.
13. Very carefully insert the daughter board U1 pin sockets into the 64 pin IC
socket. Don't bend any of the pin sockets, or you'll have to unsolder the
pin and replace it. Likewise, don't bend any of the IC socket pins or you
will have to replace it also. If you were careful when drilling U1's
holes, it should be fairly easy to get the daughter board into the socket.
When the socket is completely on, pass the 28 gauge wire solder to IC
socket pin 26 through hole F. Make sure there is no contact with daughter
board pin 26, solder the wire to the top pad of hole F, and clip the excess
wire off.
14. Take the .1 micro farad capacitor and position it on the top of the daughter
board between U1 and U2 and solder one leg to hole A and the other to hole
C. This will provide power supply decoupling to the pal. I ran my board
for several days before I put a scope on the +5 and saw all the spikes.
While the board works without the capacitor, it's good practice to use it
anyway, and it did clean up the +5 quite a bit.
15. Ohm out all traces to make sure there are no shorts. Also, check to make
sure that none of the pins short to any traces. If there is a short, fix
it with the hobby knife.
The crowd roars as they realize you have finished your
daughter board, :-). I have made three boards, the first
took about 5 hours because I had to find all my PC board
junk. The second and third took about 3 3/4 hours each
to complete.
INSTALLING THE DAUGHTER BOARD
1. Install the pal on the daughter board.
2. Take the cover off your machine, this has been described elsewhere on the
net.
3. Carefully remove the 68010 from the mother board.
4. Insert the 68010 into the pin sockets on the daughter board.
5. Carefully insert the IC socket pins of the daughter board into the IC socket pins on the mother board.
6. Re-assemble your machine.
7. You're done!!!
TESTING
Declare an unsigned sort such as:
unsigned short *video = (unsigned short *)0x420000;
Don't forget the cast, or the pointer will point to the wrong place,
namely 0x0000. Now you just place a value where video points to:
*video = 0xffff;
HOW DO I GET A PAL?
I got my pals from Hamilton Avnet Electronics, but they have a $50 minimum
order, and unless you are going to make several boards this may not be the best
answer. Jameco Electronics lists a 16L8 pal which I suspect is either 25 or
35nsec.
To add further controversy, I don't know what the affects other upcoming
hardware projects will have, so I don't know if you should use the 10nsec or
a 25nsec pal. The 10nsec pal uses 180 ma of current, or .9 watts! You
can get a 25nsec pal that uses 45 ma of current, or .225 watts. It may be
that some people will find they have a current or heat problem and the slower
pal is better. On the other hand, having as short a propagation delay as
possible may be more important. All I can say is that the 10nsec pal works
fine in my machine, a 3B1 with 1 Meg on board ram, a two port combo card
with an additional .5 Meg, and a 40 Meg hard disk. Brad's 3B1 has 1 Meg of
ram and an 80 Meg drive and hasn't had any problems either.
Finally, I'm not so sure I want to get into the pal burning business. For
one, I don't know how much interest this will generate. For another, I don't
have a whole lot of free time to dedicate to this kind of thing. So, what I
propose is that all of those people who are interested in this little project
send me mail at !laidbak!botton.
What I would like to know is how many people are interested in this project,
even if you can get and burn your own pal. Also, tell me if you would like me
to provide a programmed pal, which speed, and if you would like the sockets
provided as well. Let's set a cutoff date of August 15 for this survey, and
we'll see what happens.
MISCELLANEOUS
Again, there is no reason why you must use a double sided PC board, that was
just my method. It can easily be done with point-to-point wiring or on a single
sided PC board, but each has it's own set of problems.
Enjoy your newfound video freedom.
DISCLAIMER - Junk to cover my ___.
This project was done on my own time, and even though it works for
me, there is no guarantee that it will work for you.
Also, do with this as you may, as long as:
1. You do not profit financially from it.
2. Credit is given where it is due.
--
... ___
_][_n_n___i_i ________ Brian D. Botton
(____________I I______I laidbak!botton
/ooOOOO OOOOoo oo oooo
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