39.Computer Memory 2

The CPU requests the data it needs from RAM, processes it and writes new data back to RAM in a continuous cycle. In most computers, this shuffling of data between the CPU and RAM happens millions of times every second.

When an application is closed, it and any accompanying files are usually purged (deleted) from RAM to make room for new data. If the changed files are not saved to a permanent storage device before being purged, they are lost. Fast, powerful CPUs need quick and easy access to large amounts of data in order to maximize their performance.

If the CPU cannot get to the data it needs, it literally stops and waits for it. Modern CPUs running at speeds of about 1 gigahertz can consume massive amounts of data — potentially billions of bytes per second.

The problem that computer designers face is that memory that can keep up with a 1-gigahertz CPU is extremely expensive — much more expensive than anyone can afford in large quantities.

Computer designers have solved the cost problem by “tiering” memory — using expensive memory in small quantities and then backing it up with larger quantities of less expensive memory.

The cheapest form of read/write memory in wide use today is the hard disk. Hard disks provide large quantities of inexpensive,permanent storage. You can buy hard disk space for pennies per megabyte, but it can take a good bit of time (approaching a second) to read a megabyte off a hard disk. Because storage space on a hard disk is so cheap and plentiful, it forms the final stage of a CPUs memory hierarchy, called virtual memory.

The next level of the hierarchy is RAM. The bit size of a CPU tells you how many bytes of information it can access from RAM at the same time. For example, a 16-bit CPU can process 2 bytes at a time (1 byte = 8 bits, so 16 bits = 2 bytes), and a 64-bit CPU can process 8 bytes at a time. Megahertz (MHz) is a measure of a CPU’s processing speed, or clock cycle, in millions per second.

So, a 32-bit 800-MHz Pentium III can potentially process 4 bytes simultaneously, 800 million times per second (possibly more based on pipelining)!

The goal of the memory system is to meet those requirements. A computer’s system RAM alone is not fast enough to match the speed of the CPU. That is why you need a cache (discussed later). However, the faster RAM is the better.

Most chips today operate with a cycle rate of 50 to 70 nanoseconds. The read/write speed is typically a function of the type of RAM used, such as DRAM, SDRAM, RAMBUS. System RAM speed is controlled by bus width and bus speed.

Bus width refers to the number of bits that can be sent to the CPU simultaneously, and bus speed refers to the number of times a group of bits can be sent each second. A bus cycle occurs every time data travels from memory to the CPU.

For example, a 100-MHz 32-bit bus is theoretically capable of sending 4 bytes (32 bits divided by 8 = 4 bytes) of data to the CPU 100 million times per second, while a 66-MHz 16-bit bus can send 2 bytes of data 66 million times per second.

If you do the math, you’ll find that simply changing the bus width from 16 bits to 32 bits and the speed from 66 MHz to 100 MHz in our example allows for three times as much data (400 million bytes versus 132 million bytes) passing through to the CPU every second.