Computer Memory Types and Characteristics Quiz

Internal Memory

• RAM technology is divided into two types: Dynamic RAM (DRAM) and Static RAM (SRAM)
• DRAM is made with cells that store data as charge on capacitors
• The presence or absence of charge in a capacitor is interpreted as binary 1 or 0
• DRAM requires periodic charge refreshing to maintain data storage
• The term "dynamic" refers to the tendency of the stored charge to leak away, even with continuous power application
• SRAM uses the same logic elements as a processor
• Binary values are stored using traditional flip-flop logic gate configurations
• SRAM holds its data as long as power is supplied

SRAM versus DRAM

• Both SRAM and DRAM are volatile, meaning power must be continuously supplied to preserve bit values
• DRAM cells are simpler to build and smaller, resulting in more cells per unit area, making them less expensive
• DRAM requires supporting refresh circuitry
• DRAM is favored for large memory requirements and used for main memory
• SRAM is faster and used for cache memory (both on- and off-chip)

Read Only Memory (ROM)

• ROM contains a permanent pattern of data that cannot be changed or added to
• No power source is needed to maintain bit values
• Data or programs are permanently stored in main memory and never need loading from secondary storage
• Data is wired into the chip during fabrication
• Disadvantages include no room for error, and a relatively high fixed cost for data insertion

Programmable ROM (PROM)

• A less expensive alternative to ROM
• PROM is non-volatile and can be written into only once
• The writing process is electrical and can be performed by a supplier or customer after the original chip fabrication
• Special equipment is required for programming
• PROM provides flexibility and convenience, and is attractive for high-volume production runs

Read-Mostly Memory

• This section includes categories of memory that fall in between EPROM and EEPROM in both cost and functionality
• EPROM: Erasable Programmable Read-Only Memory. Erasure can be performed repeatedly. More expensive than PROM but has multiple update capabilities.
• EEPROM: Electrically Erasable Programmable Read-Only Memory. Can be written into at any time without erasing prior contents. More expensive than EPROM.
• Flash: Combines advantages of non-volatility with the flexibility of being updatable in place. Uses electrical erasing. Does not provide byte-level erasure. Uses only one transistor per bit, achieving high density.

Error Correction

• Hard Failures: Permanent physical defects in a memory cell. Cells may become stuck at 0 or 1, or switch erratically between 0s and 1s. Can be caused by environmental abuse, manufacturing defects, or wear.
• Soft Errors: Random, non-destructive events that alter memory cell contents. Cause includes power supply problems and alpha particles.

Advanced DRAM Organization

• One of the most critical system bottlenecks using high-performance processors is how the memory interface works with the processor.
• Traditional DRAM chips are constrained by both their internal architecture and their interface with the processor’s memory bus
• Improvements to the basic DRAM architecture have been explored and schemes that currently dominate the market are SDRAM and DDR-DRAM

Synchronous DRAM (SDRAM)

• SDRAM exchanges data with the processor synchronized with an external clock signal
• It runs at the full speed of the processor/memory bus without imposing wait states.
• The processor (or other master) initiates the instruction and address which the DRAM latches
• DRAM responds after a set number of clock cycles
• The master can safely perform other tasks while the SDRAM processes

Double Data Rate SDRAM (DDR SDRAM)

• Developed by JEDEC
• Companies make DDR chips which are widely used in desktop computers and servers.
• DDR achieves higher data rates via:
  • Synchronization to both the rising and falling edges of the clock.
  • Using a higher clock rate on the bus.
  • Using a buffering scheme.

Flash Memory

• Used for both internal and external memory applications.
• Introduced in the mid-1980s.
• Intermediate between EPROM and EEPROM in cost and functionality
• Uses electrical erasing technology,
• Erasing is possible for blocks, not entire chips.
• Does not provide byte-level erasure, but achieves high density because of transistor use
• Microchip is organized to erase sections of cells ("flash").

Memory Hierarchy

• A pyramid shows a hierarchy of NVMMs (Non-volatile memory modules) from highest performance (SRAM), decreasing cost per bit, and increasing capacity/density to the lowest performance (hard disk).
• Includes SRAM, STT-RAM, DRAM, NVM flash, PCRAM, ReRAM, hard disk

External Memory

Magnetic Disk

• A disk is a circular platter of non-magnetic material coated with a magnetizable material
• Traditionally, substrates are aluminum or aluminum alloy.
• Glass substrates are also used to improve uniformity of the magnetic film and reduce read/write errors, stiffness for reduced dynamics, higher shock resistance
• The write mechanism relies on the magnetic field generated by electricity flowing through a coil to magnetize regions on the disk.
• An electric current changes direction to reverse magnetization on the medium

Magnetic Read and Write Mechanisms

• Data is recorded on or retrieved from the disk via a conducting coil called the head, often a read head and write head
• Electric pulses sent to the write head create magnetic patterns on the disk
• Different patterns correspond to positive and negative currents
• Magnetizing a small area on the disk reverses the current direction

Disk Data Layout

• Data is organized into sectors, tracks, cylinders, and a read-write head
• The heads are on an arm, which can move to specific locations.
• The platter rotates under the stationary arm/head during reading/writing
• The arrangement (figure) shows a representation of how data is stored on the disk (i.e., tracks/sectors).

Physical Characteristics of Disk Systems

• Head motion types: Fixed (one per track) or movable (one per surface).
• Disk portability: Non-removable (e.g., hard disk) or removable (e.g., floppy).
• Sides: Single or double-sided.
• Head mechanism: Contact (floppy), fixed gap, or aerodynamic gap (Winchester).
• Physical characteristics of disk types include platter types, head position, etc.

Disk Classification

• Winchester heads: Used in sealed assemblies, free from contaminants for close operation to the disk and high data density
• The mechanism uses an aerodynamic foil that moves on the platter's surface even when the disk is motionless.
• The air pressure of the spinning disk prevents the foil from touching the surface, thus allowing for near-surface operation and high data density.
• The head must generate or sense an electromagnetic field of sufficient magnitude to write/read properly.
• Narrower heads require closer operation to the platter surface, which leads to tracks and higher data density

Typical Hard Disk Drive Parameters

• Table shows specifications for different types of hard disk drives (including capacity, average seek time, spindle speed, average latency, maximum transfer rate, byte/sectors, tracks, cache size)

Timing of a Disk I/O Transfer

• Shows sequence of events in a disk transfer (wait for device, wait for channel, seek operation, rotational delay, data transfer)

Disk Performance Parameters

• Seek time: time required for the head to move to the correct track
• Rotational latency: time required for the desired sector to reach the head
• Access time: total time required for the read/write head to position at the correct track and sector, including seek time and rotational latency
• Transfer time: time for the data transfer

RAID

• RAID is an acronym for Redundant Arrays of Independent Disks.
• Redundant array of independent disks (RAID) consists of multiple hard drives, combined to perform as a single volume.
• Provides data redundancy and improved performance over a single hard drive

RAID Levels

• RAID Level 0: Data striping without redundancy (higher throughput). Data striping across multiple disks. Data lost if one drive fails.
• RAID Level 1: Data mirroring (redundancy). Creates a redundant copy of all data. High data availability as data recovery can be done from a backup copy if a drive fails.
• RAID Level 2: Error correction using Hamming code (highest performance). Uses a parallel access technique, where data is distributed across multiple disks
• RAID Level 3: Data striping with parity (moderate cost and availability). Employs parallel access, with data distributed across the drives, and parity data dispersed in a distributed manner. Only one dedicated parity drive is needed
• RAID Level 4: Data striping with parity on a block (moderate/reduced cost/availability). Data operations handled independently to satisfy I/O request in parallel
• RAID Level 5: Data striping with distributed parity (high/increased cost/availability). Parity strips distributed across all disks to avoid a bottleneck .
• RAID Level 6: Data striping with dual parity (higher/increased cost/availability). Two different parity calculations are performed and stored in separate blocks on different disks. Very high data availability

SSD Compared to HDD

• SSDs have advantages over HDDs, including high-performance input/output operations per second (IOPS), durability, and longer lifespan. Also have lower power consumption, quieter and cooler running, and lower access times.

NAND Flash Drives

• Comparison table for NAND flash drives and HDDs, including file copy/write speed, power draw, storage capacity, and cost.
• Differences in speed and power consumption with HDDs. .

Optical Disk Products

• Includes CD-ROM, CD-R, CD-RW, DVD, DVD-R, DVD-RW, and Blu-Ray
• CD-ROM is non-erasable, and good for distribution of large data amounts
• CD-R can be written to only once
• CD-RW is rewritable
• DVD products have larger capacities than CD products
• Blu-Ray uses a shorter wavelength laser, allowing for higher storage density

Compact Disk Read-Only Memory (CD-ROM)

• CD-ROM technology similar to audio CDs
• Different in that an error correction device ensures that transfers are properly transferred

CD-R, CD-RW

• CD-Recordable (CD-R) - One time writing
• CD-Rewritable (CD-RW) – Multi-time writing

Magnetic Tape

• Tape systems use similar read/write techniques as disk systems
• Medium is flexible polyester coated with a magnetizable material
• Data is stored on parallel tracks
• Data stored in blocks called "physical records"
• Records separated by gaps("inter-record gaps").