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Old 25 May 2020, 23:23   #1
cbmeeks
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Amiga 500 memory upgrades

I have an Amiga 500 REV6A that has the trapdoor memory expansion. On the board itself, it says 512K/1M. It has four soldered RAM chips and four empty spaces. I assume for bumping the onboard memory to 1 MiB.


Now, if I did that, and used the trapdoor memory, how would the memory be arranged?


Would it be 1 MiB of CHIP RAM and 1 MiB of FAST RAM?

Or something else?


Thanks
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Old 26 May 2020, 02:49   #2
torsti76
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Originally Posted by cbmeeks View Post
I have an Amiga 500 REV6A that has the trapdoor memory expansion. On the board itself, it says 512K/1M. It has four soldered RAM chips and four empty spaces. I assume for bumping the onboard memory to 1 MiB.


Now, if I did that, and used the trapdoor memory, how would the memory be arranged?


Would it be 1 MiB of CHIP RAM and 1 MiB of FAST RAM?

Or something else?


Thanks
If you populate the empty spaces on the mainboard along with the decoupling capacitors for the chips, I believe you'll have to set some solder jumpers differently for the RAM to be recognised. Also, you need the ECS Agnus chip of the A500+. Your machine would then have 1 MB of chip mem.

The expansion RAM would still be only 512kB and (as far as I remember) would cease to work, because your A500 with 1MB chip mem basically behaves like an A500+. You could even fit it with an ECS Denise to enjoy the extra Screenmodes. The only thing that would be missing in comparison to a real A500+ would be the onboard clock circuitry.
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Old 26 May 2020, 03:26   #3
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Originally Posted by torsti76 View Post
If you populate the empty spaces on the mainboard along with the decoupling capacitors for the chips, I believe you'll have to set some solder jumpers differently for the RAM to be recognised. Also, you need the ECS Agnus chip of the A500+. Your machine would then have 1 MB of chip mem.
.



No you dont need the Agnus form the A500+. it wont work in a rev 6 Amiga because of a different Pinout of the 8375 Agnus.


the Normal Agnus for a REV 6 amiga (should be an8372A ) can handle up to 1 MB Chip.


If you upgarde the a500 onboard you will have 1 MB Chip.


For the jumper setting see here: http://eab.abime.net/showthread.php?t=88689


After modding a 512 KB extension will not work anymore.
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Old 26 May 2020, 04:34   #4
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Ah, thanks for the clarification.

So, as it stands now...with 512K onboard and another 512K in the trapdoor, do I have 1MB of chip RAM now?

I have four A500's total. Three of them are REV5 boards and this one is a REV6. Out of them, I'd like at least one of them to have maxed out chip RAM and maybe a little fast RAM.

Thanks!
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Old 26 May 2020, 07:59   #5
TomXP411
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Ah, thanks for the clarification.

So, as it stands now...with 512K onboard and another 512K in the trapdoor, do I have 1MB of chip RAM now?

I have four A500's total. Three of them are REV5 boards and this one is a REV6. Out of them, I'd like at least one of them to have maxed out chip RAM and maybe a little fast RAM.

Thanks!
I have a Rev 6, as well, along with a 512K trapdoor expansion. When I install the trap door expansion RAM, it shows up as slow RAM.

So in my experience, the answser is... "no", you have 512K of Chip RAM and 512K of slow RAM. But other people may have figured out how to make the trap door RAM act like Chip RAM, in which case I'd be wrong. =)
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Old 26 May 2020, 08:19   #6
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So, as it stands now...with 512K onboard and another 512K in the trapdoor, do I have 1MB of chip RAM now?
This depends on the position of JP2 and JP7 and also whether you have the 8372 Agnus. If it is as it came from the factory, the machine will have 512k chip 512k slow when you install a trapdoor expansion.

If you toggle JP2 and cut JP7, you will get 1MB of chip (if no previous owner stole your 1MB Agnus and replaced it with a 512k one)
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Old 26 May 2020, 14:19   #7
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No you dont need the Agnus form the A500+. it wont work in a rev 6 Amiga because of a different Pinout of the 8375 Agnus.
Ok, my bad.

Quote:
the Normal Agnus for a REV 6 amiga (should be an8372A ) can handle up to 1 MB Chip.
That can't be true universally. I had a Rev 6 back in the day, bought sealed and fresh from the factory. I had to upgrade Agnus to be able to use the trapdoor memory as ChipMem. I remember distinctly changing Agnus, because it was the first time I had to pull a large PLCC chip out of its socket (without proper tools), and the socket was cracked in one corner in the process.
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Old 26 May 2020, 15:03   #8
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I thought slow RAM was Chip RAM?
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Old 26 May 2020, 15:37   #9
torsti76
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I thought slow RAM was Chip RAM?
No, while it is true that ChipRAM is slow, SlowRAM is basically FastRAM... ;-)

Jokes aside, SlowRAM is just a sloppy word for something called Ranger Memory. It's the kind of special memory you can put in the trapdoor expansion of the A500 (or on the A2000's mainboard).

While this kind of RAM is controlled and refreshed by the chipset, it is not ChipRAM per se (only if you set the aforementioned jumpers it can become ChipRAM). Because it's not controlled by the CPU directly, it is also not FastRAM, hence the term SlowRAM.

And since the A500 chipset, i.e. Agnus, can only control a total of 1MB, this also explains why you can't use 1MB of ChipRAM AND 512kB of SlowRAM.
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Old 27 May 2020, 03:05   #10
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That can't be true universally. I had a Rev 6 back in the day, bought sealed and fresh from the factory. I had to upgrade Agnus to be able to use the trapdoor memory as ChipMem. I remember distinctly changing Agnus, because it was the first time I had to pull a large PLCC chip out of its socket (without proper tools), and the socket was cracked in one corner in the process.

Only the early Rev6 had a Agnus 8317.This Agnus can only adress 512 KB chip Ram so you had to replce it.
But the most common Agnus in the Rev 6 is the 1 MB Agnus.


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Old 27 May 2020, 04:06   #11
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I thought slow RAM was Chip RAM?

Slow RAM is memory on the Chip RAM bus that the custom chipset can't access. You can't use it for sound/gfx etc. but since it's on the same bus as Chip RAM, the CPU still has to wait for cycles on the bus to use the memory (because the chipset is still using the bus to talk to Chip Memory).

In the OS it shows up as "Fast Memory" but it's not true fast memory because it has all the same restrictions as Chip RAM.
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Old 27 May 2020, 04:23   #12
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I just confirmed my Agnus is the 8372. I also just ordered some more RAM for onboard. Of course, I forgot to order sockets. lol So that will be next.

After I add this, and do the jumper configs, I will have 1 MiB of CHIP RAM. At which point I will take the expander and use in another Amiga.

Thanks guys for the information.
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Old 27 May 2020, 12:39   #13
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I forgot to order sockets. lol So that will be next.
Just a heads up, you might want to source ultra-flat sockets because it gets tight with keyboard clearance once you put standard 20 pin dip sockets in there. Check out the (sticky) thread "Amiga 500 mod to Amiga 500+" in the Hardware mods section, I added some pics a while back in the end of that thread.
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Old 27 May 2020, 17:31   #14
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Good idea. I didn't think of that. I actually found some sockets I had that are 20 pin (dual wipe). I then searched for low profile sockets and they honestly look the same size as what I got. Didn't seem any shorter than normal.

Do you have any recommendations or know where I can find some?

I also have some machine headers that might be lower.

Thanks
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Old 28 May 2020, 05:49   #15
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Well, you were correct. I put a standard socket in there with a chip and the keyboard definitely was pressing down on the chip.

Fortunately, I found some ultra-low profile sockets pretty cheap so they should be here soon.

Thanks!
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Old 28 May 2020, 12:11   #16
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No Problem. Glad it helped. With a Rev 6A motherboard I would rather use the trapdoor expansion port though, and not populate the four empty slots on mobo. Most memory expansion cards has a RTC chip (real-time clock) onboard which is useful if you prefer to have correct timestamp on files created/modified in wb. Anyway, socketing the DRAMs makes them easily removable should you change your mind later on.
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