Saturday, May 30, 2009

DIMM, SDRAM, FAT, Disk controllers and Archival storage devices

(i) DIMM

Dual In-line Memory Module
Small circuit boards carrying memory integrated circuits, with signal and power pins on both sides of the board, in contrast to single-in-line memory modules (SIMM).
The individual gold or lead connectors (pins) on SIMMs, although they are on both sides of the chip, are connected to the same memory chip, while on a DIMM, the connections on each side of the module connect to different chips. This allows for a wider data path, as more modules can be accessed at once. DIMM pins are arranged in a zigzag design to allow PCB tracks to pass between them.
The 8-byte DIMM format with dual-sided contacts can accommodate 4- and 16-megabit dynamic RAM chips, and is predicted to handle 64- and 256-Mbit devices. The 8-byte DIMM will hold up to 32 megabytes of memory using 16-Mbit DRAMs, but with the 256-Mbit future-generation DRAM, it will be able to hold a 64-Mx64 configuration. Another variation, the 72-pin SO-DIMM, is designed to connect directly to 32 bit data buses, and is intended for use in memory-expansion applications in notebook computers.
A Dual in-line memory module (DIMM), as opposed to SIMMs (used by the majority of the PC industry) allows for a 128-bit data path by interleaving memory on alternating memory access cycles. SIMMs on the other hand, have a 64-bit data path. Suppliers are unanimous in their belief that the DIMM will eventually replace the SIMM as the market's preferred memory module.

(ii) SDRAM

(SDRAM, Synchronous DRAM) A form of DRAM which adds a separate clock signal to the control signals. SDRAM chips can contain more complex state machines, allowing the m to support "burst" access modes that clock out a series of successive bits (similar to the nibble mode DRAM).





(iii) FAT

File Allocation Table
(FAT) The component of an MS-DOS or Windows 95 file system which describes the files, directories, and free space on a hard disk or floppy disk.
A disk is divided into partitions. Under the FAT file system each partition is divided into clusters, each of which can be one or more sectors, depending on the size of the partition. Each cluster is either allocated to a file or directory or it is free (unused). A directory lists the name, size, modification time and starting cluster of each file or subdirectory it contains.
At the start of the partition is a table (the FAT) with one entry for each cluster. Each entry gives the number of the next cluster in the same file or a special value for "not allocated" or a special value for "this is the last cluster in the chain". The first few clusters after the FAT contain the root directory.
The FAT file system was originally created for the CP/M[?] operating system where files were catalogued using 8-bit addressing. MS DOS's FAT allows only 8.3 filenames.
With the introduction of MS-DOS 4 an incompatible 16-bit FAT (FAT16) with 32-kilobyte clusters was introduced that allowed partitions of up to 2 gigabytes.
Microsoft later created FAT32 to support partitions larger than two gigabytes and pathnames greater that 256 characters. It also allows more efficient use of disk space since clusters are four kilobytes rather than 32 kilobytes. FAT32 was first available in OEM Service Release 2 of Windows 95 in 1996. It is not fully backward compatible with the 16-bit and 8-bit FATs.



(iv) Disk controllers

(Or "hard disk controller", HDC) The circuit which allows the CPU to communicate with a hard disk, floppy disk or other kind of disk drive.

The most common disk controllers in use are IDE and SCSI controllers. Most home personal computers use IDE controllers. High end PCs, workstations and network file servers mostly have SCSI adaptors.

(v) Archival storage devices

Digital information stored on magnetic tape or compact disc.

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