CODEWITHSUNDEEP
memory-managementx86-64memory-address
wangt13
wangt13

Reputation: 1215

What if accessing a non-existing physical address in X86 system?

I am working on a Linux kernel module, which maps a physical address range to a process virtual address space, by playing with process's page tables.
Then, I have a question in my head, what will happen if a PTE points to a non-existing physical address?
For example, my X86 laptop has 8GB DRAM, and if a PTE has the value of 0x8000000400001227, will the CPU generate some exception for this invalid address accessing?
I did a quick a test with that, but there is NOthing unusual happened, and I got confused totally.

Please help clarifying the reason behind, or let me know if I really need to read some X86 documents.

Upvotes: 2

Views: 810

Answers (1)

prl
prl

Reputation: 12434

Typically a memory read to non-existent memory will return all FF's and a memory write will be discarded. (With some platforms and/or some address ranges, reads may return 0. It depends on how the address range is decoded by the chipset.)

Page table entry bits 51:M are reserved (where M is the physical address width supported by the processor), so if you map and try to access an address greater than the physical address width, you will get a page fault due to a reserved bit violation. I think M is typically 39 bits for clients; more for servers. You can find out the value for your system using CPUID with eax=80000008 and examining bits 7:0 of eax.

Upvotes: 3

Related Questions