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Amiga 500 ECS, 1MB Chip RAM reported as 1018K

deanimator

Experienced Member
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Apr 25, 2020
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Hi everyone,
does anyone know why an A500 with a 1MB-capable "Fat Agnus" (CSG 8372A) cannot actually address full 1MB (1024K) of RAM ?
It seems to be around 6 kilobytes short, reporting in SysInfo as a 1018K total, with an upper address bound of $000fe7ff. In the Workbench CLI, "avail" returns yet an interesting value of 1040152 bytes total RAM. Who's telling the truth now, toss a coin :)
I have verified both behaviors on my Rev6 A500 and also on the WinUAE emulator, attempting to rule out chipset issues. Also when the RAM upgrade is set as slow RAM, then there's 512K of chip and 506K of slow RAM reported by SysInfo 4.4 on both real hardware and the emulator; 1018K in total.
It's not that I would care about a couple of missing K's in the year of our lord 2023, but this in turn leads to issues for software requiring at least a proper 1MB of chip RAM, as in an A500+ or A1200. Think Worms/FLT (crashes after intro) and OTT (left channel samples sound corrupt). Out of curiosity, what happens with the last 6K - anyone knows, or is it an issue of the "Fat Agnus" addressing chip?

Regards
 
Could be a malfunctioning RAM chip. Is your motherboard populated with 1MB, or do you have the typical 512K with additional 512K on the trapdoor slot? Is this NTSC or PAL?
 
deanimator said:
I have verified both behaviors on my Rev6 A500 and also on the WinUAE emulator, attempting to rule out chipset issues

I was kind of wondering why they did it like that. Ah well, it's not the "1MB Fat Agnus", but rather a 0.9941MB one. :)
In both tests, I have configured it chip RAM and then also slow RAM (in here, it was reported as 506K in SysInfo, both in WinUAE and real hardware; PAL version.)
 
Do you boot straight to SysInfo? I don't know how SysInfo works but some memory might be getting eaten by device drivers like the floppy drive or a hard drive if you have it. One would think SysInfo would report the total system RAM though... but SysInfo is not very accurate (never knew why people love it so much)
 
I have verified both behaviors on my Rev6 A500 and also on the WinUAE emulator, attempting to rule out chipset issues. Also when the RAM upgrade is set as slow RAM, then there's 512K of chip and 506K of slow RAM reported by SysInfo 4.4 on both real hardware and the emulator; 1018K in total.
If the WinUAE emulator does the same, there's no issue. It's just how it works in this hardware configuration.

It's very unlikely that the "missing" 6k are causing any game to fail (except bad cracks), since what you have was common hardware back then.
 
It's very unlikely that the "missing" 6k are causing any game to fail (except bad cracks), since what you have was common hardware back then.
This is not entirely correct, there were plenty of games that would fail on "common" hardware.
For example, Heimdall 2 would not run on 1MB systems that had a hard drive connected (like an A600HD), you had to -physically- remove the hard drive to have it boot.
I bet they didn't really test that config out.

So, I wouldn't be totally surprised if a game would fail with 6K missing, especially if you have a bad memory detecting routine. It would help to know what games are behaving weird in the OP's config and knowing their full configuration. Every Amiga is its own thing usually, too many combinations of configs.

Still, SysInfo is probably misreporting the total RAM exemplifying with the "avail" juxtaposition that things can go awry depending on how you detect that available RAM. it could be a type mismatch or conversion/math error.
 
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