#1 Meg Agnus?
10 messages in this thread
:Ladies and Gentlemen, I have a question regarding the Agnus 1 and 2 meg
chip upgrades. I have an A500, purchased in 1990, January 1. I have a
Spirit add on A501 type board, for a 1 meg configuration. I have checked,
and found that the Agnus chip I have is 512K. I'd like to upgrade the Agnus
to the 1 meg chip, and would like to know if there are any problems with
one of these upgrades on a system such as my own. I know that there is no
actual RAM on the Agnus chip, (or so I've been told), but in conjunction
with one of the A501 type boards that 1 meg of chip ram is possible. I'd
appreciate any comments or help on this subject before I purchase the
chipset. Thanks very much,
Randy
You can plug a 1-meg ECS Agnus (#8372a) straight into your A500,
replacing the current FAT Agnus [#8370 (NTSC), or #8371 (PAL)].
You will need a special chip puller, to prevent damaging the Agnus
socket.
At this point, the ECS Agnus will still be in 512K mode.
Check the revision number of the motherboard, if it is rev.6 or higher,
you can upgrade to 1-meg CHIP with no problems. IF it is rev.5, then
it should work, BUT there is a chance it won't.
As for the Spirit A501-slot expansion, I don't know if it can be
re-configured as CHIP.
Problems:
Some programs are "dumb". If they detect "more than 512K" of memory in
the Amiga, they will assume that part of it is FAST. They will try to
utilitize the >512K memory as FAST, but in an Amiga that has only
1-meg (ALL CHIP) this leads to a system crash. If some FAST memory is
also availible – there will be no problem. Some other programs, can't
function correctly in an Amiga that has more than 512K, regardless of
there being FAST memory availible.
Both types are rare today, but there is some older programs that have
this problems.
Also, expanding the A500 to 1-meg CHIP is semi-permanent. Once the Agnus
is configured to use 1-meg of memory, the A501 *must* be configured to
be accessed as CHIP RAM and the A501 must be availible. (The A500 won't
boot without the A501 expansion.)
Ron.
Ron, thanks for your help. I appreciate all the info, particularly on the
aspect of potential problems. Far better to get all the info before I
commit to this. Thanks again,
Randy
Ronald,
Not quite correct regarding the A501 needing to be installed if you
have an A500 configured for 1Meg or more of CHIP RAM. The system is quite
capable of dealing with only 512K of CHIP RAM even if the A500 has a 1Meg
Agnus installed and configured for 1Meg.
Dean
DKB Software
It was my understanding, that when the Agnus is in 1-meg (with an A500
or A2000) that there must be 1-meg of CHIP for the Agnus.
I figured it was due to testing of the CHIP memory at bootup, or
something simular. I know there are programs that can "remove"
CHIP memory to simulate a 512K CHIP system, but I thought the A501
memory had to still be there physicially.
Ron.
Ronald,
The board changes needed to make the A500 a 1Meg CHIP machine is only
to change where the A501 addresses at in the machine. The OS goes out at
boot time and determines exactly how much CHIP RAM is available and
adjusts itself accordingly. As an example, I can jumper an A500 for 1Meg
of CHIP, install a MegAChip board and an A501 and get 2Meg of CHIP RAM. If
I remove the MegAChip and replace it with a 1Meg Agnus I get 1Meg of CHIP,
If I reinstall the MegAChip but remove the A501 I end up with 1.5Meg of
CHIP RAM. With just the 1Meg Agnus without the A501 the machine shows 1/2
Meg of CHIP.
The only configuration you really need to watch out for is a 512K Agnus
in a machine (A500 or A2000) configured for 1Meg of CHIP RAM. If this is
the case the OS sees 1Meg of CHIP but the Agnus can only use 512K of it.
Then when you use all of the real CHIP RAM, the OS tries to use the 'fake'
memory as CHIP with catastrophic results.
Dean
DKB Software
Dean,
I'd like to know how you can jumper an A500 with a 1 Meg Agnus so that you
can remove the A501 board and still boot. On my machine, (rev 5
motherboard, 1.3/2.0 switcher) I did the cut trace, solder jumper pad mod
to get 1 Meg chip, which works fine as long as I do not remove the A501.
If I do, I
definitely can NOT boot the machine. All I get is a pretty strange blue
striped display and an audible buzzing sound. Plug the A501 back in, and
everything works normally. Is there a way to avoid this?
//
\X/ Amiga -Steve ——> On AutoPilot from Silicon Valley
Steve,
I'd suggest getting a good memory test program and testing all your
CHIP ram. What you are describing sounds like a memory failure at the top
of the 512K of CHIP RAM on your motherboard.
If everything is setup correctly, the machine should have no problems
setup for 1Meg of CHIP without the A501 installed. Another thing to check
is to see if the trace cut is really a trace cut, and not just a pin
lifted out of it's socket on the Gary chip.
Dean
DKB Software
Dean,
After posting my question, I got to nosing around in my machine again
(I'm an E.T.), 'cause I've been getting intermittent gurus (or should I
start calling them 'software failures' now?), and I'd suspected something
funny in this area before, just couldn't get it to break long enough to
find it.
I'd run various memory checkers for long periods of time with no
failures. I did the 1 meg mod myself, and I checked the cut trace with a
DMM 2 or 3 times (I'm nothing if not thorough). To make a long story
short, I finally found the problem — a cold solder joint on the ground
pin of one of the data path buffers (244's, if I recall properly).
Whenever the A501 was plugged in, it would bend the motherboard just
enough to make contact. Without the board in, well, you already know what
happened!
After whipping out and using my trusty soldering iron, I've since been
able to boot with or without the A501 installed. And I haven't had a
weird 'software failure' yet. (Keeping fingers crossed).
All this while I thought it was normal operation to have to have the
A501 installed after the mod. Thanks for getting my curiosity aroused!
And for the help.
//
\X/ Amiga -Steve ——> On AutoPilot from Silicon Valley
Steve,
Glad I could help! 😉 Hopefully that was your problem so that now your
machine will be stable. At least now its acting like it should.
Dean
DKB Software