On Tue, Jan 08, 2008 at 01:36:39AM -0800, Bruce Miller wrote: > While my immediate goal is to tell the shop where I bought the > computer which module they must replace, I would also like to > understand the addressing better. Any ideas? Even if you were intimately aware of the 64bit addressing modes you would not be able to determine the module you need to replace without knowing how the motherboard maker wired up the ram slots. You best bet is to test each module individualy. I am not sure if memtest86 fully supports 64bit addressing yet. AMD64 , aka x86_64, addressing is interesting. In native 64bit mode the ram is devided into two halves. One starting at address 0x0 and growing up. The other half starts at address 0xFFFFFFFFFFFFFFFF and grows down. Current AMD chips only support a 48bit physical address space. -- sg