Memory Hierarchy

Topics to be discussed…. Memory Hierarchy Main Memory Random-Access Memory (RAM) Read only memory(ROM) Nonvolatile Memories Auxiliary Memory

Memory Hierarchy The memory unit that directly communicate with CPU is called the main memory Devices that provide backup storage are called auxiliary memory The memory hierarchy system consists of all storage devices employed in a computer system from the slow by high- capacity auxiliary memory to a relatively faster main memory, to an even smaller and faster cache memory

Memory Hierarchy The main memory occupies a central position by being able to communicate directly with the CPU and with auxiliary memory devices through an I/O processor A special very-high-speed memory called cache is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate

Memory Hierarchy CPU logic is usually faster than main memory access time, with the result that processing speed is limited primarily by the speed of main memory The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations The typical access time ratio between cache and main memory is about 1 to 7~10 Auxiliary memory access time is usually 1000 times that of main memory back

Main Memory Most of the main memory in a general purpose computer is made up of RAM integrated circuits chips, but a portion of the memory may be constructed with ROM chips RAM– Random Access memory Integrated RAM are available in two possible operating modes, Static and Dynamic ROM– Read Only memory back

Random-Access Memory (RAM) Static RAM (SRAM) Each cell stores bit with a six-transistor circuit. Retains value indefinitely, as long as it is kept powered. Relatively insensitive to disturbances such as electrical noise. Faster (8-16 times faster) and more expensive (8-16 times more expensive as well) than DRAM. Dynamic RAM (DRAM) Each cell stores bit with a capacitor and transistor. Value must be refreshed every 10-100 ms. Sensitive to disturbances. Slower and cheaper than SRAM. back

ROM ROM is used for storing programs that are PERMENANTLY resident in the computer and for tables of constants that do not change in value once the production of the computer is completed The ROM portion of main memory is needed for storing an initial program called bootstrap loader, witch is to start the computer software operating when power is turned off

ROM Data is programmed into the chip using an external ROM programmer The programmed chip is used as a component into the circuit The circuit doesn’t change the content of the ROM Can be used as lookup tables to implement various functions Used by PCs to store the instructions that form their Basic Input/Output System (BIOS) When power is removed from a ROM chip, the information is not lost, so it is a nonvolatile type of memory It has a OE (Output Enable) specific control pin. Both OE and CE must be enabled in order for the ROM to output data; otherwise its data output is tri-stated.

ROM Types Masked ROM – programmed with its data when the chip is fabricated PROM – programmable ROM, by the user using a standard PROM programmer, by burning some special type of fuses. Once programmed will not be possible to program it again EPROM – erasable ROM; the chip can be erased and chip reprogrammed; programming process consists in charging some internal capacitors; the UV light (method of erase) makes those capacitors to leak their charge, thus resetting the chip EEPROM – Electrically Erasable PROM; it is possible to modify individual locations of the memory, leaving others unchanged; one common use of the EEPROM is in BIOS of personal computers. back

Nonvolatile Memories back DRAM and SRAM are volatile memories Lose information if powered off. Nonvolatile memories retain value even if powered off. Generic name is read-only memory (ROM). Misleading because some ROMs can be read and modified. Types of ROMs Programmable ROM (PROM) Eraseable programmable ROM (EPROM) Electrically eraseable PROM (EEPROM) Flash memory Firmware Program stored in a ROM Boot time code, BIOS (basic input/ouput system) graphics cards, disk controllers. back

Auxiliary Memory The main memory construction is costly. Therefore, it has to be limited in size. The main memory is used to store only those instructions and data which are to be used immediately. However, a computer has to store a large amount of information. The bulk of information is stored in the auxiliary memory. This is also called backing storageor secondary storage. They include hard disk, floppy disks, CD-ROM, USB flash drives, etc. When the electricity supply to the computer is off, all data stored in the primary storage is destroyed. On the other hand, this is not true for secondary storage. The data stored in secondary storage can be stored for the desired time. back

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