MTTR介绍

原文：http://baike.baidu.com/view/1493498.htm

MTRR是Memory Type Range Register的缩写，可翻译为存储区域类型寄存器，它规定了读写某段范围物理内存的策略，用于优化CPU数据传送性能。例如可将MTTR设为在显存的地址范围上使用“write-combining”策略，CPU能够在PCI/AGP总线上，将许多次少量的数据写入集合成一次大的数据写入，这样能获得 2.5倍以上图像传送速度的提升。

　　Memory Type Range Registers (MTRRs) are control registers that are used as a control mechanism which provides system software with control of how accesses to memory ranges by the CPU are cached. It uses a set of programmable model-specific registers (MSRs) which are special registers provided by most modern CPUs.

　　Possible access modes to memory ranges can be:

　　* uncached

　　* write-through

　　* write-combining

　　* write-protect

　　* write-back

　　Additional bits which are provided on some computer architectures such as AMD64 allow the shadowing of ROM contents in system memory (Shadow ROM) and the configuration of memory-mapped I/O.

　　In write-back mode, writes are written to the CPU's cache and the cache is marked dirty, so that its contents are written to memory later)

　　Write-combining allows bus write transfers to be combined into a larger transfer before bursting them over the bus to allow more efficient writes to system resources like graphics card memory. This often increases the speed of image write operations by 2.5 times, at the cost of losing the simple sequential read/write semantics of normal memory.