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I have an 500+ rev8A, I added the missing 0.5 MB RAM and the U32 to the motherboard. I also have 512 KB in the trapdoor. This results in 1.5 MB chip RAM; but I want to have 1 MB chip RAM and 0.5 MB slow RAM.

How can I arrange for this? That is, for the expansion to be used as slow RAM? What are the right jumper settings (JP2, JP3, JP4A, JP4B, JP7A — no U32) for this configuration?

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    Note I'm not an Amiga expert, but don't understand why someone would want to have slow RAM over chip RAM. AFAIK, slow RAM has the same speed restrictions as chip RAM, but not the benefits (can be used by the chipset)
    – tofro
    Sep 27, 2018 at 15:30
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    It's the U32 addition that is causing the trapdoor memory to be decoded as CHIP RAM, I think. Without that modification the motherboard RAM should be CHIP RAM and the trapdoor RAM CPU only.
    – Brian H
    Sep 27, 2018 at 17:59
  • @tofro Rumours are there are badly written games that expect RAM on a given address space. Chip and "slow" are on different addresses so it could be something like that.
    – pipe
    Sep 28, 2018 at 0:42
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    You want 0.5MB 'slow fast' RAM at C00000-C7FFFF? amigacoding.com/index.php/Amiga_memory_map
    – Bruce Abbott
    Sep 28, 2018 at 10:10
  • @Bruce, yes exactly. some demos, and games needs memory at C0...
    – lasertank
    Sep 28, 2018 at 11:02

1 Answer 1

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You need to have a 8372A Fatter Agnus chip, and have to modify the mainboard to enable the 1mb chip ram mode. I am not really familiar with the A500+, but you can do a search to find out what needs to be done.

On an A500 with a ver 5 mainboard, you need to modify JP2 by cutting the trace between the two bottom pads, and join the top and center pads with a small blob of solder. You also need to cut the trace going to a pin on the trapdoor (exram)and pin 32 of the gary chip. And have an a501 or equivalent in the door. That will enable the fatter agnus to 1mb chip ram.

I have seen pictures where a switch was placed across the jp2 and the exram trace to enable and disable the 1mb mode.

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  • I think you're misunderstanding the question. The orignal question suggest that the A500+ is already set up with a 2MB Agnus (with 1MB on board) and the trap door is currently showing as 0.5MB of chip ram, but the OP wants it to show as 0.5MB of slow ram instead for compatibility with old software that expects slow RAM instead of CHIP. Current setup: 1.5MB CHIP. OP wants: 1MB CHIP, 0.5MB slow.
    – mnem
    Feb 25, 2019 at 8:47

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