C volatile variables and Cache Memory

Question

Cache is controlled by cache hardware transparently to processor, so if we use volatile variables in C program, how is it guaranteed that my program reads data each time fro

Answer 1

Wikipedia has a pretty good article about MTRR (Memory Type Range Registers) which apply to the x86 family of CPUs.

To summarize it, starting with the Pentium Pro Intel (and AMD copied) had these MTR registers which could set uncached, write-through, write-combining, write-protect or write-back attributes on ranges of memory.

Starting with the Pentium III but as far as I know, only really useful with the 64-bit processors, they honor the MTRRs but they can be overridden by the Page Attribute Tables which let the CPU set a memory type for each page of memory.

A major use of the MTRRs that I know of is graphics RAM. It is much more efficient to mark it as write-combining. This lets the cache store up the writes and it relaxes all of the memory write ordering rules to allow very high-speed burst writes to a graphics card.

But for your purposes you would want either a MTRR or a PAT setting of either uncached or write-through.