computer designers have solved the value trouble through "tiering" memory -- the use of high-priced reminiscence in small portions and then backing it up with large portions of much less high-priced reminiscence.

The most inexpensive shape of examine/write reminiscence in huge use today is the tough disk. hard disks provide large quantities of cheaper, everlasting storage. you can buy tough disk space for pennies per megabyte, but it can take a great bit of time (approaching a 2nd) to examine a megabyte off a tough disk. because storage area on a difficult disk is so reasonably-priced and abundant, it paperwork the very last degree of a CPUs memory hierarchy, called virtuals reminiscence.

the following level of the hierarchy is RAM. We speak RAM in detail in How RAM Works, but several points approximately RAM are crucial right here.

The bit length of a CPU tells you how many bytes of facts it is able to get right of entry to from RAM on the equal time. for instance, a 16-bit CPU can process 2 bytes at a time (1 byte = 8 bits, so 16 bits = 2 bytes), and a sixty four-bit CPU can manner eight bytes at a time.

Megahertz (MHz) is a degree of a CPU's processing velocity, or clock cycle, in thousands and thousands in keeping with second. So, a 32-bit 800-MHz Pentium III can probably system four bytes concurrently, 800 million times in step with second (possibly extra based on pipelining)! The goal of the reminiscence machine is to satisfy those requirements.

A laptop's system RAM by myself isn't always rapid sufficient to healthy the rate of the CPU. this is why you need a cache (mentioned later). however, the quicker RAM is, the higher. most chips these days function with a cycle rate of fifty to 70 nanoseconds. The examine/write speed is typically a characteristic of the sort of RAM used, which includes DRAM, SDRAM, RAMBUS. we are able to communicate about those numerous styles of reminiscence later