add 1 MB onboard ram to A500 Revision 6A MB

[Tenacious]

There is a hack floating around (I can't find it yet) to replace your 1 Meg Agnus with a 2 Meg Super Agnus (the one used in the A3000).  IIRC, the 4 existing ram chips are pulled and 8 new 1Meg chips are installed.  With just a few trace mods you get 2 Megs of chip on the motherboard.  You must then disable the memory on the A501 but you can still use the clock.  I did this modification almost 15 years ago and that machine is still running strong.

BTW: I thought I saw 2 meg Agnus for sell on AmigaKit.

If you have 1 Meg of chip (512K on the mb and 512K on the A501), whats the advantage of moving it all to the mb?  You still need the RTC.  If you add another 512K (1.5 meg total) with some kind of glue logic, that 512K wont be true chip nor true fast memory.

You probably know this, but Supra's 500 RX fast ram expansion is one of the best first moves you can make (if you're not going for a HD/ram combo).   I wish I had had one of these back in 1989!

[Tenacious]

I think I remember getting the plans from a magazine as well.  I did find a file on my HD that details a similar hack for the A2000. The URL was still in the file note.   I never tried this one since acquiring A2000s only a few years ago.

I didn't test this link: http://main.aminet.net/hard/hack/A2K_agnus_hack.lha

[Tenacious]

It would be interesting if rkauer's link will convert a NTSC A500 into a defacto A500+ (I came away from the link a little in doubt).  If it works, (and you have 2-Meg Agnus) then all you need is a European (1 meg) A501 mapped to the chip memory address space.

The mod I mentioned above puts all 2 megs on the motherboard, the A501 is retained for the RTC only and its memory is disabled.

The Aminet links are broken because the directory structure was consolidated.  The hardware hacks were moved under the docs.  aminet.net/docs/hard/A2K_agnus_hack.lha

Good Luck

[Tenacious]

@klx300r

You have 1 Meg of chip ram, right? Not 512K of chip and 512k of fast when the sidecars are all pulled off, right?

[Tenacious]

@klx300r

I caught on too late, sorry.  The A500 technically has 3 kinds of memory, but, only 2 labels to call them by.  Pretty confusing, huh?

Chip Ram:  memory that is shared by the CPU AND all of the coprocessors.  This memory is managed by the Agnus chip.  The CPU must waite for its turn to use it.  The A500s (not A500+) all shipped with 512K of chip memory on the MB.

Fast (slow) Ram:  This originally was the memory on the A501.  Commodore mapped this memory to a different block address than the 2 Meg address space reserved for Chip.  It is not shared with the coprocessors, however, it is still managed by the Agnus chip.  The CPU must wait to use it as well.  This address space is called Fast by Commodore, but it's not true fast nor true chip. This address range has worst attributes of both kinds and should not be used.

Fast (true) Ram:  This memory comes off the side CPU connector and is never shared or slowed by the coprocessors.  It comes with its own refresh circuitry (not managed by Agnus) and so the CPU can run full speed through these addresses.  8 megs is the max. for the stock 68000.

This strange situation came about because of the evolution of the Agnus chip, which could originally address only 512K
of memory (used in A1000 and, I think, some A500s). The next version could address 1 Meg of chip, used in most A500s and A2000s.  The Super Agnus could address 2 megs and was used in A500+, A600, A3000, etc.

Commodore provided jumpers on later A500 MBs to select the address range where the A501 could live, but, set it to Fast/Slow by default.  Really stupid.  There are several docs on Aminet that detail moving the A501 to the true chip ram address range.  Probably, most A500s that remain have had this simple modification.  I'm surprised that yours didn't.

As a practical matter, 1 Meg of chip is usually enough for an A500, 3 of my A500s are configured this way.  Without a 2 Meg Agnus, I don't know how you can utilize the extra chips you bought.  Maybe someone else does.

I'm gonna sign off, I'm missing "Polar Express" with the kids.

[Tenacious]

@ klx300r

Most of us have been telling you the same thing. An ECS (or OCS) Amiga isn't likely to need more than 1 Meg of chip for any 1 application. I don't recall ever running out.  Going beyond the limits of your Agnus will require great knowledge on your part and the return on your investment deminishes very quickly after you acheive 1 Meg of chip.

What Jumpers?

Do you have access to Aminet?  There are a number files in Aminet/docs/hard/ describing the mod.  Get them all and decide which matches your MB revision, version of Agnus, and your sense of doing it 'right'.

You sucked the solder out of the MB?

Certainly, solder the chips back in.  Try to alternate between chips to prevent overheating any one of them.

Good Luck!

[Tenacious]

I checked Aminet and found the following files:

http://us2.aminet.net/docs/hard/A5001MChip.lha
http://de5.aminet.net/docs/hard/a500chip.lha

All talk about the Rev 6 MB and give a lot of detail.  There are even ascii diagrams of the jumpers.

To answer your question:

1. Remove the jumper (trace) between the middle and bottom pads of JP7.

2. Remove the trace between the middle and bottom pads of JP2 AND bridge a wire between the middle and top pads of that same JP2.

Look the files over.  There is another file in the same directory, 500Hacks, I think.  It details this and other modifications with switches so you can boot into Pal or NTSC.

Good Luck again!