MEMORY Types And Function Computer Memory The interesting

MEMORY Types And Function

Computer Memory The interesting thing about computer memory is that different equipment uses different memory. For example, consider the types of memory you would find used by a computer and other peripheral devices: • • • BIOS Cache Dynamic RAM Flash Memory Sticks RAM Optical Storage (ROM) Static RAM Video Memory Virtual Memory

Computer Memory (cont) • With memory, it can be any for of electronic storage. However, memory is typically used as a means of identifying and completing fast storage. For example, if your computer had to access the hard drive often to retrieve each piece of data, the system would quickly become bogged down. Now, if the memory were used to store the data, the computer’s CPU would be able to draw out the information much quicker. Therefore, while different types of memory do different things, when associated with a computer, most store data on a temporary basis to help speed things up. • For your computer to work, all of the various components must work together. This would include the CPU, operating system, hard drive, and of course, the memory, which is by far the most important. Think of it this way. As long as your computer is turned on, the CPU is using memory.

Flash Memory Flash memory is a form of EEPROM that allows multiple memory locations to be erased or written in one programming operation. Normal EEPROM only allows one location at a time to be erased or written, meaning that flash can operate at higher effective speeds when the systems using it read and write to different locations at the same time. All types of flash memory and EEPROM wear out after a certain number of erase operations. Flash memory is made in two forms: NOR flash and NAND flash. The names refer to the type of logic gate used in each storage cell. NOR flash was the first type to be developed, invented by Intel in 1988. It has long erase and write times, but has a full address/data (memory) interface that allows random access to any location. This makes it suitable for storage of program code that needs to be infrequently updated, as in digital cameras and PDAs. Its endurance is 10, 000 to 100, 000 erase cycles. NOR-based flash is the basis of early flash-based removable media; Compact Flash and Smart. Media are both based on it. NAND flash from Toshiba followed in 1989. It has faster erase and write times, higher density, and lower cost per bit than NOR flash, and ten times the endurance. However its I/O interface allows only sequential access to data. This makes it suitable for mass-storage devices such as PC cards and various memory cards, and somewhat less useful for computer memory. NAND-based flash has led to several much smaller removable media formats, MMC, Secure Digital and Memory Stick. Flash memory forms the core of the removable USB interface storage devices known as key drives.

Memory Sticks Memory Stick is the flash memory format introduced by Sony and is used in a multitude of compatible electronic devices produced by them. One of the great things about Memory sticks is that they are interchangeable amongst host devices, which allows for easy data recording, sharing and transferring of data files between individual pieces of digital equipment. It is able to store this multiple source data simultaneously. Naturally, the range of devices that support Memory Stick is huge and includes car audio & navigation equipment, digital cameras, DVD players, TV's, projectors, PDA's, portable music players, cell phones and much more. As such, it is format that is gaining popularity amongst third party manufacturers that are now designing and producing new products capable of accommodating the Stick.

Random Access Memory (RAM) Similar to a microprocessor, a memory chip is an integrated circuit (IC) made of millions of transistors and capacitors. In the most common form of computer memory, dynamic random access memory (DRAM), a transistor and a capacitor are paired to create a memory cell, which represents a single bit of data. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state. A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.

Optical Storage The optical storage device that most of us are familiar with is the compact disc (CD). A CD can store huge amounts of digital information (783 MB) on a very small surface that is incredibly inexpensive to manufacture. The design that makes this possible is a simple one: The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound spiral. The CD player reads the bumps with a precise laser and interprets the information as bits of data. The spiral of bumps on a CD starts in the center. CD tracks are so small that they have to be measured in microns (millionths of a meter). The CD track is approximately 0. 5 microns wide, with 1. 6 microns separating one track from the next. The elongated bumps are each 0. 5 microns wide, a minimum of 0. 83 microns long and 125 nanometers (billionths of a meter) high. Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1. 2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic bumps that make up the long, spiral track. A thin, reflective aluminum layer is then coated on the top of the disc, covering the bumps. The tricky part of CD technology is reading all the tiny bumps correctly, in the right order and at the right speed. To do all of this, the CD player has to be exceptionally precise when it focuses the laser on the track of bumps. When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off the aluminum layer and hits an optoelectronic device that detects changes in light. The bumps reflect light differently than the flat parts of the aluminum layer, which are called lands. The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player drive interpret the changes as data bits.

Video Memory The traditional, standard DRAM used for video cards typically does not have enough bandwidth to handle the demands of running a card at high resolution and color depths, with acceptable refresh rates. The main reason why is the two competing access factors for the video memory: the processor writing new information to the memory, and the RAMDAC reading it many times per second in order to send video signals to the monitor. To address this fundamental limitation, a new type of memory was created called video RAM or VRAM. As the name implies, this memory is specifically tailored for use in video systems. The fundamental difference between VRAM and standard DRAM is that VRAM is dual-ported. This means that it has two access paths, and can be written to and read from simultaneously. The advantages of this are of course enormous given what the video card does: many times per second a new screen image is calculated and written to the memory, and many times per second this memory is read and sent to the monitor. Dual-porting allows these operations to occur without bumping into each other. VRAM provides substantially more bandwidth than either standard DRAM or EDO DRAM; double in many cases. It is more suited for use in systems requiring high resolution and color depth displays. The only reason that it hasn't replaced standard DRAM entirely is of course: cost. VRAM is more complex and requires more silicon per bit than standard DRAM, which makes it cost more.

Bios The BIOS software has a number of different roles, but its most important role is to load the operating system. When you turn on your computer and the microprocessor tries to execute its first instruction, it has to get that instruction from somewhere. It cannot get it from the operating system because the operating system is located on a hard disk, and the microprocessor cannot get to it without some instructions that tell it how. The BIOS provides those instructions. Some of the other common tasks that the BIOS performs include: A power-on self-test (POST) for all of the different hardware components in the system to make sure everything is working properly Activating other BIOS chips on different cards installed in the computer - For example, SCSI and graphics cards often have their own BIOS chips. Providing a set of low-level routines that the operating system uses to interface to different hardware devices - It is these routines that give the BIOS its name. They manage things like the keyboard, the screen, and the serial and parallel ports, especially when the computer is booting. Managing a collection of settings for the hard disks, clock, etc. The BIOS is special software that interfaces the major hardware components of your computer with the operating system. It is usually stored on a Flash memory chip on the motherboard, but sometimes the chip is another type of ROM.