DDR SDRAM. Generic DDR-266 memory in the 184-pin DIMM form Corsair DDR-400 memory with heat spreaders Double data rate synchronous dynamic random-access memory (DDR SDRAM) is a class of memory integrated circuits used in computers. DDR SDRAM, also called DDR1 SDRAM, has been superseded by DDR2 SDRAM and DDR3 SDRAM, neither of which is either forward or backward compatible with DDR1 SDRAM -meaning that DDR2 or DDR3 memory modules will not work in DDR1-equipped motherboards, and vice versa. With data being transferred 64 bits at a time, DDR SDRAM gives a transfer rate of (memory bus clock rate) × 2 (for dual rate) × 64 (number of bits transferred) / 8 (number of bits/byte).
"Beginning in 1996 and concluding in June 2000, JEDEC developed the DDR (Double Data Rate) SDRAM specification (JESD79) Specification standards[edit] Comparison of memory modules for desktop PCs (DIMM). Physical DDR layout Comparison of memory modules for portable/mobile PCs (SO-DIMM). Chips and modules[edit] Chip characteristics[edit] Dynamic random access memory. Dynamic random-access memory (DRAM) is a type of random-access memory that stores each bit of data in a separate capacitor within an integrated circuit. The capacitor can be either charged or discharged; these two states are taken to represent the two values of a bit, conventionally called 0 and 1.
Since even "nonconducting" transistors always leak a small amount, the capacitors will slowly discharge, and the information eventually fades unless the capacitor charge is refreshed periodically. Because of this refresh requirement, it is a dynamic memory as opposed to SRAM and other static memory. The main memory (the "RAM") in personal computers is dynamic RAM (DRAM). It is the RAM in desktops, laptops and workstation computers as well as some of the RAM of video game consoles.
The advantage of DRAM is its structural simplicity: only one transistor and a capacitor are required per bit, compared to four or six transistors in SRAM. History[edit] Operation principle[edit] Write Refresh rate[edit] Xeon. Hard disk drive. A disassembled and labeled 1997 HDD laying atop a mirror. Overview of how an HDD functions The two most common form factors for modern HDDs are 3.5-inch in desktop computers and 2.5-inch in laptops. HDDs are connected to systems by standard interface cables such as SATA (Serial ATA), USB or SAS (Serial attached SCSI) cables.
As of 2012[update], the primary competing technology for secondary storage is flash memory in the form of solid-state drives (SSDs). HDDs are expected to remain the dominant medium for secondary storage due to predicted continuing advantages in recording capacity and price per unit of storage;[4][5] but SSDs are replacing HDDs where speed, power consumption and durability are more important considerations than price and capacity.[6][7] History[edit] Video of modern HDD operation (cover removed) HDDs were introduced in 1956 as data storage for an IBM real-time transaction processing computer[3] and were developed for use with general purpose mainframe and minicomputers.
RAID. RAID (originally redundant array of inexpensive disks; now commonly redundant array of independent disks) is a data storage virtualization technology that combines multiple disk drive components into a logical unit for the purposes of data redundancy or performance improvement.[1] History[edit] Although not yet using that terminology, each of the five levels of RAID named in the paper were well established in the art prior to the paper's publications, for example: Around 1983, DEC began shipping subsystem mirrored RA8X disk drives (now known as RAID 1) as part of its HSC50 subsystem.[3]Around 1988, the Thinking Machines DataVault used error correction codes (now known as RAID 2) in an array of disk drives.[4] A similar approach was used in the 1970s on the IBM 3330.[5]In 1977, Norman Ken Ouchi at IBM filed a patent disclosing what was subsequently named RAID 4.[6]In 1986, Clark et al. at IBM filed a patent disclosing what was subsequently named RAID 5.[7] Concept[edit] Standard levels[edit]
SCSI. Single Ended Parallel SCSI icon/logo. SCSI is an intelligent, peripheral, buffered, peer to peer interface. It hides the complexity of physical format. Every device attaches to the SCSI bus in a similar manner. Up to 8 or 16 devices can be attached to a single bus. There can be any number of hosts and peripheral devices but there should be at least one host. History[edit] SCSI was derived from "SASI", the "Shugart Associates System Interface", developed c. 1978 and publicly disclosed in 1981.[2] A SASI controller provided a bridge between a hard disk drive's low-level interface and a host computer, which needed to read blocks of data.
Larry Boucher is considered to be the "father" of SASI and SCSI due to his pioneering work first at Shugart Associates and then at Adaptec.[4] Until at least February 1982, ANSI developed the specification as "SASI" and "Shugart Associates System Interface;"[5] however, the committee documenting the standard would not allow it to be named after a company.