Computer Memory

In a computer, cache memory is a special type of fast access memory that is between the CPU and the main memory. If the CPU always had to access main memory, it would spend most of its time idle waiting for that memory to respond. But because memory accesses statistically tend to cluster around each other in real programs instead of completely randomly scattering across memory, a single CPU memory access can cause the cache memory controller to perform a fast burst access of main memory including that address to load an entire "line" of cache memory. If a following CPU memory access is in this same "line" of cache memory that has already been loaded, it will not have to wait for the main memory to respond, instead the cache responds first providing the copy it has of that address' contents.

Cache memory was originally invented in the late 1950s by IBM for their 7030 Stretch supercomputer (a machine built entirely using discrete germanium transistors, no integrated circuits at all). However all the 7030 documents use the term "virtual memory" for what is now universally called cache memory, and "virtual memory" means something entirely different now.

Each cache memory unit is composed of three sections:

1. cache controller
2. content addressable memory (CAM)
3. fast access static random access memory (SRAM)

Cache memory is organized into levels (L1, L2, L3, etc.) with the L1 Cache closest to the CPU and each additional level further away until the final Cache level connects directly to the memory. The L1 Cache uses the fastest speed SRAM but has the smallest amount of SRAM and each additional level uses slower speed SRAM but has more SRAM than the previous level. The L1 Cache is usually divided into two independent Caches (L1 Instruction Cache and L1 Data Cache) and is dedicated to supporting only one CPU but each additional level is usually a single Cache (shared by both instructions and data) and in systems having multiple CPUs may be shared by two or more CPUs. In systems having multiple CPUs the cache controllers must implement special bus protocols to coordinate line invalidations and updates to prevent some of the CPUs from accessing obsolete Cache contents from its Caches when a different CPU that does not use those Caches has modified memory that is mapped to both its local Caches and the other CPUs local caches.

Proper design of a Cache memory system for a computer requires extensive simulation of typical real code expected to be used on the system. Any problems found must be corrected, so that the Cache usage will remain even and balanced, and anticipated performance verified.

Read-Only Memory (ROM) One major type of memory that is used in PCs is called read-only memory, or ROM for short. ROM is a type of memory that normally can only be read, as opposed to RAM which can be both read and written. There are two main reasons that read-only memory is used for certain functions within the PC: * Permanence: The values stored in ROM are always there, whether the power is on or not. A ROM can be removed from the PC, stored for an indefinite period of time, and then replaced, and the data it contains will still be there. For this reason, it is called non-volatile storage. A hard disk is also non-volatile, for the same reason, but regular RAM is not. * Security: The fact that ROM cannot easily be modified provides a measure of security against accidental (or malicious) changes to its contents. You are not going to find viruses infecting true ROMs, for example; it's just not possible. (It's technically possible with erasable EPROMs, though in practice never seen.) Read-only memory is most commonly used to store system-level programs that we want to have available to the PC at all times. The most common example is the system BIOS program, which is stored in a ROM called (amazingly enough) the system BIOS ROM. Having this in a permanent ROM means it is available when the power is turned on so that the PC can use it to boot up the system. Remember that when you first turn on the PC the system memory is empty, so there has to be something for the PC to use when it starts up. See this section for a description of the system BIOS ROM; see here for a description of the system boot sequence. While the whole point of a ROM is supposed to be that the contents cannot be changed, there are times when being able to change the contents of a ROM can be very useful. There are several ROM variants that can be changed under certain circumstances; these can be thought of as "mostly read-only memory". :^) The following are the different types of ROMs with a description of their relative modifiability: * ROM: A regular ROM is constructed from hard-wired logic, encoded in the silicon itself, much the way that a processor is. It is designed to perform a specific function and cannot be changed. This is inflexible and so regular ROMs are only used generally for programs that are static (not changing often) and mass-produced. This product is analagous to a commercial software CD-ROM that you purchase in a store. * Programmable ROM (PROM): This is a type of ROM that can be programmed using special equipment; it can be written to, but only once. This is useful for companies that make their own ROMs from software they write, because when they change their code they can create new PROMs without requiring expensive equipment. This is similar to the way a CD-ROM recorder works by letting you "burn" programs onto blanks once and then letting you read from them many times. In fact, programming a PROM is also called burning, just like burning a CD-R, and it is comparable in terms of its flexibility. * Erasable Programmable ROM (EPROM): An EPROM is a ROM that can be erased and reprogrammed. A little glass window is installed in the top of the ROM package, through which you can actually see the chip that holds the memory. Ultraviolet light of a specific frequency can be shined through this window for a specified period of time, which will erase the EPROM and allow it to be reprogrammed again. Obviously this is much more useful than a regular PROM, but it does require the erasing light. Continuing the "CD" analogy, this technology is analogous to a reusable CD-RW. * Electrically Erasable Programmable ROM (EEPROM): The next level of erasability is the EEPROM, which can be erased under software control. This is the most flexible type of ROM, and is now commonly used for holding BIOS programs. When you hear reference to a "flash BIOS" or doing a BIOS upgrade by "flashing", this refers to reprogramming the BIOS EEPROM with a special software program. Here we are blurring the line a bit between what "read-only" really means, but remember that this rewriting is done maybe once a year or so, compared to real read-write memory (RAM) where rewriting is done often many times per second! Note:One thing that sometimes confuses people is that since RAM is the "opposite" of ROM (since RAM is read-write and ROM is read-only), and since RAM stands for "random access memory", they think that ROM is not random access. This is not true; any location can be read from ROM in any order, so it is random access as well, just not writeable. RAM gets its name because earlier read-write memories were sequential, and did not allow random access.
Finally, one other characteristic of ROM, compared to RAM, is that it is much slower, typically having double the access time of RAM or more. This is one reason why the code in the BIOS ROM is often shadowed to improve performance.

Random Access Memory (RAM) The kind of memory used for holding programs and data being executed is called random access memory or RAM

. RAM differs from read-only memory (ROM) in that it can be both read and written. It is considered volatile storage because unlike ROM, the contents of RAM are lost when the power is turned off. RAM is also sometimes called read-write memory or RWM. This is actually a much more precise name, so of course it is hardly ever used. :^) It's a better name because calling RAM "random access" implies to some people that ROM isn't random access, which is not true. RAM is called "random access" because earlier read-write memories were sequential and did not allow random access. Sometimes old acronyms persist even when they don't make much sense any more (e.g., the "AT" in the old IBM AT stands for "advanced technology" :^) ). Obviously, RAM needs to be writeable in order for it to do its job of holding programs and data that you are working on. The volatility of RAM also means that you risk losing what you are working on unless you save it frequently. RAM is much faster than ROM is, due to the nature of how it stores information. This is why RAM is often used to shadow the BIOS ROM to improve performance when executing BIOS code. There are many different types of RAMs, including static RAM (SRAM) and many flavors of dynamic RAM (DRAM).

Sources:www.pcguide.com

bit

4=nybble

2=byte

1024=kilobyte

1024=megabyte

1024=gigabyte

1024=terabyte

1024=petabyte

1024=exabyte

1024=zettabyte

1024=yottabyte

1024=kiloyottabyte

1024=megayottabyte - No server is bigger than this.

1024=gigayottabyte

1024=terayottabyte

1024=petayottabyte

1024=exayottabyte

1024=zettayottabyte

1024=yottayottabyte

1024=kiloyottayottabyte

and so on.

Memory is made in powers of 2.

Like 1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384

and so on.