Windows uses the standard logical boot-time prefetcher if the system has less than 512 MB of memory, but if the system has 700 MB or more of RAM, it uses an in-RAM cache to optimize the boot process. The size of the cache depends on the total RAM available, but it is large enough to create a reasonable cache and yet allow the system the memory it needs to boot smoothly.
After every boot, the ReadyBoost service (see Chapter 9 for information on ReadyBoost) uses idle CPU time to calculate a boot-time caching plan for the next boot. It analyzes file trace information from the five previous boots and identifies which files were accessed and where they are located on disk. It stores the processed traces in %SystemRoot%\Prefetch\Readyboot as .fx files and saves the caching plan under HKLM\SYSTEM\CurrentControlSet\Services\Ecache\Parameters in REG_BINARY values named for internal disk volumes they refer to.
The cache is implemented by the same device driver that implements ReadyBoost caching (Ecache.sys), but the cache’s population is guided by the boot plan previously stored in the registry. Although the boot cache is compressed like the ReadyBoost cache, another difference between ReadyBoost and ReadyBoot cache management is that while in ReadyBoot mode, other than the ReadyBoost service’s updates, the cache doesn’t change to reflect data that’s read or written during the boot. The ReadyBoost service deletes the cache 90 seconds after the start of the boot, or if other memory demands warrant it, and records the cache’s statistics in HKLM\SYSTEM\CurrentControlSet\Services\Ecache\Parameters\ReadyBootStats.
Source of Information : Microsoft Press Windows Internals 5th Edition