Memory

Data and program instructions are stored in the electronic circuitry of the computer until it is their turn to be processed. This circuitry area is called memory. It is sometimes called primary storage. Many people confuse memory with disk storage. The media that stores information that is not related to the processor is called storage. Memory refers to the circuits that are directly linked to the processor. This circuit board is called a single in-line memory module (SIMM).

The memory chips need to be able to communicate directly with the central

processing unit. In some earlier computers, memory was soldered directly onto the motherboard or the logic board. The need for more and more memory for software solutions, requires that memory now be added onto the board through modules. Most computers come with a standard 8 megabytes (8 million bytes) of RAM with room on the logic board to add more memory. The amount that one can add is dependent upon the machine and the manufacturer of the computer. 

BYTE	(8 bits) A character can be a number, symbol, or letter.
KILOBYTE	(1000 characters) One page of double spaced text.
MEGABYTE	One million (1,000,000) characters or one novel.
GIGABYE	One billion (1,000,000,000) characters or one thousand novels.

 

Different kinds of memory your microcomputer contains:

• Access Memory (RAM) : RAM is the main memory space of your computer. The term RAM means Random Access Memory, and it comes from the early days of computers when mainframes had two types of memory: Random access, in which any bit of memory could be addressed at any moment; and Sequential memory (such as data stored on tape) where bits could only be accessed in a certain order. All of the memory in your computer is random access, so don’t worry about sequential memory. The RAM is the workspace of your computer. This is the area that the computer system uses to temporarily hold data before and after it is processed. Any data that needs to be processed has to have RAM to process it. RAM gives instructions to the central processing unit. Stored in RAM are program instructions, operating system instructions, and data. Each character that is typed into RAM is held in RAM until it is saved to disk. The RAM also holds program instructions. Your computer disk that holds your word processor can suddenly be turned into RAM so that the word processing function can be achieved. The operating instructions which help control basic computer functions are also loaded into RAM each time the computer is turned on. RAM is very immediate. If the computer is turned off or the power has gone out, and you have not saved your work, everything stored in RAM is lost. That is why it is important to save your work as you go along. It is also called temporary memory, because it is constantly being overwritten and updated with new data. RAM consists of banks of microchip transistors that are either on or off (representing a 1 or a zero). RAM chips need constant power to remember what is stored in them; a power interruption of even a fraction of a second (perhaps caused by nearby lightning) can cause the RAM to lose its contents. For this reason, RAM is said to be volatile (from “easily evaporated”) and this is why it is important to save your work often to a more permanent storage such as a hard disk.
• Virtual Memory : It is the computer's ability to use disk storage to simulate RAM. The computer uses space on the hard drive as an extension of RAM. It allows computers to run large programs that do not have enough real memory, and to also run more than one program at a time. The amount of RAM limits how large and how many programs and data files you can have open at once. You normally couldn’t simultaneously open two programs that each requires 70 MB of RAM on a computer that has only 128 MB of RAM. However, modern operating systems use virtual memory to get around this roadblock. If everything won’t fit into RAM at once, the OS can automatically swap out currently unused data to the hard disk, and swap in whatever data are needed. Hard disks (mechanical devices) are much slower than RAM (electronic storage), so your computer will run much slower if using virtual memory.
• Read only Memory (ROM)  : Microcomputers also have some ROM (Read Only Memory) on the motherboard. ROM does not need power to remember its contents, so this is where a computer stores the programs that are needed to start up (boot up) the computer system. (The instructions can’t be stored in RAM, since RAM loses its contents when the computer is off; and they can’t be kept on the hard disk, since just reading data from a hard disk requires programs.) When the computer is first turned on, the program stored in the ROM is feed to the processor. This initial program checks to see that everything is in order and looks for storage devices on which it can locate a copy of the operating system; it then loads the first part of the OS into RAM, then hands control over to that program to finish the boot process. The startup instructions stored on ROM in a PC are part of the BIOS (Basic Input Output System). The BIOS also contains the low-level interface code needed to access the drives, keyboard, and produce simple display output. “ROM” is in most cases nowadays stored on an EEPROM chip (Electrically Erasable Programmable ROM). The motherboard includes special circuitry that allows the “permanent” contents of the chip to be updated if needed, but this is rarely done (EEPROM can only be re-written a limited number of times—but that limit may be 10,000 times).
• CMOS Memory (complementary metal oxide semiconductor)  : The “CMOS memory” of a computer is a small amount of “semi-permanent” storage where changeable data can be stored that needs to remain available while the computer is turned off. A small battery on the motherboard keeps the CMOS ‘alive’ when power is off. The CMOS memory (called PRAM or “Parameter RAM”) can store such information such as what hard drive or copy of the OS you want to boot from, what are your default monitor settings, etc. The BIOS picks up this information and uses it during boot up. The CMOS memory can also hold the time and date so that your computer remembers this even when power its has been off. If your computer can’t remember the proper time, or can’t remember system settings when it’s off, the small battery may need to be replaced. CMOS Memory is a type of transistor memory that requires very little power to store data, so this type of chip was used in the early days for storing semi-permanent data. However, almost ALL of the chips in your computer nowadays (such as the RAM) uses CMOS technology, and (irony) the “CMOS semi-permanent memory” in many modern PCs is NOT stored on CMOS chips, but may use flash memory (a kind of EEPROM). But the terms “CMOS memory” and “the CMOS” are still used to refer to the semi-permanent memory. The CMOS stores the boot data information such as the number of hard drive track and sectors. The computer needs to know this information to find the operating system of the computer during the boot up process. This memory is less permanent than ROM; it can be updated.
• Cache Memory : Cache memory is extremely fast memory that is built into a computer’s central processing unit (CPU), or located next to it on a separate chip. The CPU uses cache memory to store instructions that are repeatedly required to run programs, improving overall system speed. The advantage of cache memory is that the CPU does not have to use the motherboard’s system bus for data transfer. Whenever data must be passed through the system bus, the data transfer speed slows to the motherboard’s capability. The CPU can process data much faster by avoiding the bottleneck created by the system bus. Cache built into the CPU itself is referred to as Level 1 (L1) cache. Cache that resides on a separate chip next to the CPU is called Level 2 (L2) cache. Some CPUs have both L1 and L2 cache built-in and designate the separate cache chip as Level 3 (L3) cache.
• Flash Memory : Flash memory refers to a particular type of EEPROM, or Electronically Erasable Programmable Read Only Memory. It is a memory chip that maintains stored information without requiring a power source. Flash memory differs from EEPROM in that EEPROM erases its content one byte at a time. This makes it slow to update. Flash memory can erase its data in entire blocks, making it a preferable technology for applications that require frequent updating of large amounts of data as in the case of a memory stick. Inside the flash chip, information is stored in cells. A floating gate protects the data written in each cell. Tunneling electrons pass through a low conductive material to change the electronic charge of the gate in "a flash," clearing the cell of its contents so that it can be rewritten. This is how flash memory gets its name. Flash memory is very useful in a variety of applications including:
• Your computer BIOS
• Memory sticks
• PCMCIA memory cards
• MP3 players
• Modems
• Video game cards
• Digital cellular phones
• Digital cameras 

• Flash memory used as a hard drive has many advantages over a traditional hard drive. It's nonvolatile or solid state, meaning there are no moving parts. It's also silent, much smaller than a traditional hard drive, and highly portable with a much faster access time. However, the advantages of a traditional hard drive are price and capacity. Hard drives are many times larger for a price that is many times smaller, comparatively speaking, megabyte for megabyte.