Primary storage

1. Primary storage • Primary storage is directly connected to the central processing unit of the computer. It must be present for the CPU to function correctly, just as in a biological analogy the lungs must be present (for oxygen storage) for the heart to function (to pump and oxygenate the blood). As shown in the diagram, primary storage typically consists of three kinds of storage: • In contemporary usage, memory usually refers to a form of solid state storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage. • Random-access memory (usually known by its acronym, RAM) is a type of data store used in computers that allows the stored data to be accessed in any order — that is, at random, not just in sequence. In contrast, other types of memory devices (such as magnetic tapes, disks, and drums) can access data on the storage medium only in a predetermined order due to constraints in their mechanical design.

3. Memory / USB Flash Memory • Flash memory is a form of non-volatile computer memory that can be electrically erased and reprogrammed. • In contemporary usage, memory usually refers to a form of solid state storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage. • Random-access memory (usually known by its acronym, RAM) is a type of data store used in computers that allows the stored data to be accessed in any order — that is, at random, not just in sequence. In contrast, other types of memory devices (such as magnetic tapes, disks, and drums) can access data on the storage medium only in a predetermined order due to constraints in their mechanical design. • Unlike EEPROM, it is erased and programmed in blocks consisting of multiple locations (in early flash the entire chip had to be erased at once). • Flash memory costs far less than EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid-state storage is needed.

4. Flash memory is a form of non-volatile computer memory that can be electrically erased and reprogrammed. • Unlike EEPROM, Flash Memory is erased and programmed in blocks consisting of multiple locations (in early flash the entire chip had to be erased at once). • Flash memory costs far less than EEPROM and therefore has become the dominant technology wherever a significant amount of non-volatile, solid-state storage is needed. • Flash memory is also used in USB flash drives (thumb drives , handy drive), which are used for general storage and transfer of data between computers. It has also gained some popularity in the gaming market, where it is often used instead of EEPROMs or battery-powered SRAM for game save data.

5. Comparison with EPROM (Erasable Programmable Read Only Memory) EPROM has two significant limitations: it has to be taken out of the circuit and put under an ultraviolet light source to erase it higher (non-TTL) voltages are generally required to program it. These voltages are normally not available in regular computer circuit boards. EEPROM can be programmed and erased electrically. The difference between EPROM and EEPROM lies in the thickness of the insulating layer — in an EPROM it is about 3 nanometers thick whereas in an EEPROM it is much thinner — typically around 1 nanometer. This thinner insulating layer allows for lower voltages to be used for programming. An EEPROM(also called an E2PROM) or Electrically Erasable Programmable Read-Only Memory, is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. When larger amounts of more static data are to be stored (such as in USB flash drives) other memory types like flash memory are more economic. SEEPROM, meaning Serial EEPROM, is an EEPROM chip that uses a serial interface to the circuit board.

6. While RAM has no limitations on rewrites to memory, EEPROMs are limited in that repeated write and erase cycles eventually damage the thin insulating layer, a process called 'wear out'. New models specify 100,000 erase-write cycles or more. Flash memory is cheaper than EEPROM but will wear out faster, typically after 10,000 erase-write cycles. Another reason why EEPROM is more effective for storing configuration data is that Flash memory has to erase multiple memory locations at a time. Changing a single byte is only possible by rewriting a whole block, which causes the flash memory to wear out more quickly than one might otherwise expect.

7. Principles of operation Flash memory stores information in an array of floating gate transistors, called "cells", each of which traditionally stores one bit of information. Newer flash memory devices, sometimes referred to as multi-level cell devices, can store more than 1 bit per cell, by using more than two levels of electrical charge, placed on the floating gate of a cell. NAND Flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB interface storage devices known as USB flash drives. Tunnel injection is the quantum tunneling effect, also called Fowler-Nordheim tunnel injection, when charge carriers are injected to an electric conductor through a thin layer of an electric insulator. It is used to program NAND flash memory. (Cf.tunnel release, used for its erasing.) An alternative to tunnel injection is the spin injection.

8. Essential components There are typically three parts to a flash drive: Male type-A USB connector - provides an interface to the host computer. USB mass storage controller - implements the USB host controller and provides a linear interface to block-oriented serial flash devices while hiding the complexities of block-orientation, block erasure, and wear balancing, or wear levelling, although drives that actually perform this in hardware are rare. The controller contains a small RISC microprocessor and a small amount of on-chip ROM and RAM. NAND flash memory chip - stores data. NAND flash is typically also used in digital cameras. Crystal oscillator - produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.

10. USB Flash Memory USB flash drives are NAND-type flash memorydata storage devices integrated with a USB interface. They are typically small, lightweight, removable and rewritable. Memory capacity typically ranges from 128 megabytes up to 64 gigabytes NAND Type: NAND flash memories cannot provide execute-in-place due to their different construction principles. These memories are accessed much like block devices such as hard disks or memory cards. The blocks are typically 512 or 2048 bytes in size. Associated with each block are a few bytes (typically 12–16 bytes) that should be used for storage of an error detection and correction block checksum. NAND devices typically have software-based bad block management. This means that when a logical block is accessed it is mapped to a physical block, and the device has a number of blocks set aside for compensating bad blocks and for storing primary and secondary mapping tables.

11. Strengths and Weaknesses • Like all flash memory devices, flash drives can sustain only a limited number of write and erase cycles before failure. • Mid-range flash drives under normal conditions will support several hundred thousand cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a flash drive to run application software or an operating system. To address this, as well as space limitations, some developers have produced special versions of operating systems (such as Linux) or commonplace applications (such as Mozilla Firefox) designed to run from flash drives. These are typically optimized for size and configured to place temporary or intermediate files in memory rather than store them temporarily on the flash drive.