My M571 motherboard reports 640kB of RAM when 256MB of SDRAM is installed.
I tried 16x8 8-chip 133MHz SDRAM and 18-chip 100MHz SDRAM. Both in 128x2 banks.
Also updated the BIOS to newest version. It seems to do this when 128MB only is inserted as well, while 64MB 133MHz works fine.
I had to google your board. Apparently it came with a TX Pro chipset (dunno what that is exactly) and it apparently can handle 128MB RAM sticks maximum.
Also keep in mind the density of your RAM sticks. I'm not really surprised your 256MB sticks are doing weird stuff.
Do you have any other 128MB double sided SDRAM modules laying around? Do these function normally?
Your 64MB stick is probably 8 chips all on one side (this was the most common configuration) and should operate normally.
I found this site, should have lots of information for you.
Should be a SIS5591 chipset with 64 MB L2 cacheable range
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