Stupid question about Fat Agnus

[Generale]

I know that there are 1mb chip mods. The thing that confuses me is this:

Do you have to replace Agnus with another version to do the 1mb alteration?

Was it only the a500+ that had the 1mb version, or does the 6a have it too, because I know that Agnus has a different label.

Sorry about the stupid question. I've just had a lot of trouble gaining clarification from the internet at large.

[Ilwrath]

Do you have to replace Agnus with another version to do the 1mb alteration?

Depends on which Agnus you have.  As I understand it, you need at least an 8372A.  I think some A500s with 6a motherboards were sold stock with that chip and just need a little jumper modification to enable 1MB, while others need the chip and the jumper.  As far as I know, no Rev 5 or earlier A500s had the 1MB Agnus stock.

[analogkid]

the A500s containing a 8375 doesn't need the modification, that are A500+ or A500 with those boards downgraded to Kick 1.3. But you can't use this Agnus and the 8272B (A3000) in any other Rev 6a or older board. As said before, Rev. 6a has got an 8372A, the 1 MB Chip-RAM Agnus, you have to cut some lines to get the full Chip-RAM. Rev. 5 or earlier got a 8371, which must be replaced with 8272A und some more modifications must be done.

[Damion]

Ilwrath wrote:

Do you have to replace Agnus with another version to do the 1mb alteration?

Depends on which Agnus you have.  As I understand it, you need at least an 8372A.  I think some A500s with 6a motherboards were sold stock with that chip and just need a little jumper modification to enable 1MB, while others need the chip and the jumper.  As far as I know, no Rev 5 or earlier A500s had the 1MB Agnus stock.

I remember being disappointed that my Rev 6 A500 didn't have the 1MB Agnus. Installed the chip, cut 2 traces (IIRC) and along with the 512k "trapdoor" board I had one full meg of chipram...whoooooooeeeeeeeeee, all the games I could play then:-D

[Generale]

Ahh nuts. I just took the case off and it's an 8371. Heh. I thought that it was a rev6a. Both of them are rev5.
I hate to ask, but how much is a 1mb agnus worth these days?

An odd thing to note, the Datel ram expansion in it has blanks for a clockchip. Hmm..My dead ra5-512k has a clockchip. Wonder if it'll fit.

[Jeff]

It's been a LONG time, but I had a Rev 6A 500 that I installed an 8372B 2Meg agnus in.  As I recall it involved populating the motherboard with 4 more (256x4) memory chips, changing some soldered jumpers, and possibly a change to the A501.  Then you had an A500 with 2 Meg Chip!

Sorry if this was OT.  The hack came from Amazing Computing in the early 90's I think.  They also had a version for the A2000. I would like to see that, BTW if anyone still has it.

Jeff

[Ilwrath]

Then you had an A500 with 2 Meg Chip!

Yeah, I've seen A500's with 2MB chip.  I thought there was a custom 68000 socket board that needed to be installed along with the 8372B, though.  Maybe that just added the extra RAM sockets if your A500 didn't have them, though.

My original A500 was a 6a with the 8372A.  I modified it up to 1MB chip and then had 4MB fast on my DataFlyer HD/RAM card.  Assembled, the whole unit was so wide it barely fit on my desk.  :lol:

[Generale]

It would have been massive  :lol: Still, that would have been so cool too.
I guess a 1mb agnus is out of my reach  :-( After the other mods including shoving the a500 in a tower, I think I'll do the 8meg hack. I need the RAM. I don't seem to have enough for the hard drive, CD-ROM and TCP/IP stack at the same time.

[zipper]

The 2 MB Agnus addons usually have 1 MB memory built-in.
I have Megachip 2 in my A500T, it uses 8375 Agnus. Of course I had to make those mobo hacks to enable full 2 MB just like for 1 MB Chip.

[mjg59]

Then you had an A500 with 2 Meg Chip!

For 2 megs of Chip, you need an 8372A/B (different to an A or a B) or an 8375. The 8372A/B is the same core as the 8372A, but has a slightly different pin-out to give it an extra address line. I'm not sure what (if any) differences there were with an 8375.

500 (rather than 500+) motherboards simply aren't laid out for 2MB of chip RAM. In order to get it, the extra RAM needs to be on a small board plugged into the Agnus socket, along with an 8372A/B or 8375. There's no other way of doing it - the chips have different pinouts and the lines just aren't present on an A500.

Slightly strangely, my A3500 has an Agnus that's stamped 8372A and has had the /B drawn in by hand. Early part, I guess.

[TheMagicM]

you mean like this:

http://www.bartonekdragracing.com/webfilez/ebay/dkbmegachip2000.jpg

[fillosauru]

Yes you have to.
my 6a has a 8372A

8371 is the original Agnus, can adress 512K of CHIP RAM.
8372 probabilly just like 8371
8372A can adress up to 1 meg (1024k)
So, you have to install a 8372A in order to get 1024k CHIP RAM. :-)

[Ilwrath]

Is there a plain 8372?  If so, I imagine it's just a bugged version of the 8372A.

8370 is NTSC only 512k Agnus.
8371 is PAL only 512k Agnus.
8372A is NTSC/PAL switchable 1MB Agnus (will work as a drop-in replacement for an 8370/8371.)
8372B is NTSC/PAL switchable 2MB Agnus.
8375 is another NTSC/PAL switchable 2MB Agnus.

[Jeff]

Hey!

The Amiga 500 rev 6a and Amiga 2000 rev 6.x 2Meg Chip hacks were in Amazing Computing in February 1993 with a correction a couple months later. (May?)

I was mistaken. I had to remove the 4 (256x4) dram chips on the motherboard and socket all 8 dram locations.  Then you install 8 (1Meg x 4) drams for a total of 2 megs on the motherboard when used with the 8372B Agnus. I don't think the 8375 would work.

There are also some other simple mods that must be done. Address line changes via jumpers I think, it's been years.  My 500 worked great like that.  You can also cut something on the A501 to disable the ram and still allow the Battery Back-up clock to function.

I would like to do this to one of my Amiga 2000's if I can find the mod again and the time to do it.

Jeff

[Azryl]

@Jeff

That would be a very interesting hack to find again!
Anything like that would be appreciated

Az