Computer Architecture – External Memory & Storage (Lecture Summary)

These 70 Q&A flashcards cover the key points from the lecture on external memory, including magnetic disks, performance metrics, internal components, floppy history, removable storage, RAID concepts and levels, optical media (CD/DVD), and magnetic tape standards.

Magnetic (e.g., disks, tape), Optical (e.g., CD, DVD), and Solid-state/other removable media.

What are the three major categories of external memory discussed?

Name three types of magnetic disk storage called out in the lecture.

Standard fixed hard disks, RAID arrays, and removable magnetic disks/cartridges.

Which optical media formats were covered?

CD-ROM, CD-R, CD-RW, and DVD (including writable and multi-layer versions).

What magnetic medium is still popular for large-scale backup despite slow speed?

Magnetic tape (e.g., DAT, DLT, 8-mm).

When were hard disks first invented?

In the 1950s.

Why were early hard disks called "Winchesters"?

It was IBM’s code-name for a popular sealed-disk product that caught on as a generic term.

Hard disks became known as "hard" to distinguish them from which earlier medium?

Floppy disks, whose platters are flexible.

At a basic level, what technology do hard disks share with cassette tapes?

Both use magnetic recording that can be erased, rewritten, and retain data for years.

On a desktop PC, typical hard-disk capacity (per the slides) ranges between what sizes?

10 GB and 40 GB (note: capacities have since grown, but these were lecture figures).

Define the hard-disk performance metric ‘data rate’.

The number of bytes per second the drive can stream to the CPU (commonly 5-40 MB/s).

Define the hard-disk performance metric ‘seek time’.

Time from a CPU’s file request to when the first byte is delivered, usually 10-20 ms.

What material now often replaces aluminum as a disk substrate, and why?

Glass; for better surface uniformity, fewer defects, lower fly heights, more stiffness, and better shock resistance.

What component turns all platters in unison?

The spindle and its integrated spindle motor.

Typical spindle rotational speeds for PCs are __ and __ RPM.

3,600 RPM and 7,200 RPM (higher for high-end drives).

How many read/write heads does a platter face normally have?

One head per surface (one per side).

Why must hard drives be assembled in clean rooms?

The head-to-platter gap is so tiny that a single dust particle or fingerprint could crash the heads.

Modern drives use what actuator type instead of a stepper motor?

Voice-coil actuator with servo control.

What is meant by ‘flying’ heads?

When platters spin, air pressure lifts the heads microscopically above the surface to avoid contact.

Multiple aligned tracks on different platters form what logical structure?

A cylinder.

During a write, how is data actually recorded?

Current through the inductive coil of the head produces a magnetic field that flips bits on the platter below.

Contemporary drives separate read and write elements; what sensor is used for high-frequency reads?

A magneto-resistive (MR) sensor.

How is a disk physically organized for storage?

Concentric tracks divided into sectors; a sector is the minimum addressable block.

What is low-level formatting?

Writing the track and sector boundaries onto the platter, preparing it to store data blocks.

Explain Constant Angular Velocity (CAV).

The platter spins at one speed, so outer tracks travel faster; sectors are pie-shaped, causing wasted outer space.

What technique increases capacity by varying bits per track in regions?

Zoned recording (multiple zone CAV).

What are the three components of disk access time?

Seek time + rotational latency + data transfer time.

Fixed-head versus movable-head: what’s the difference?

Fixed head has one head per track (rare); movable head uses one per surface that slides across all tracks.

List two reasons removable disks are useful.

Backup/archiving, transporting data, software distribution, off-site security, sharing with others.

What was the highest widely used capacity of the 3.5-inch floppy?

1.44 MB.

Who invented the original floppy disk drive and in what year?

Alan Shugart at IBM, 1967.

State two drawbacks that led to the decline of floppy disks.

Low capacity and vulnerability of the recording surface to contamination.

Define a Winchester hard disk.

A sealed hard drive design by IBM (1973) where heads fly on an air cushion extremely close to the platter.

Removable magnetic cartridge that could store up to 750 MB and used a higher-quality coating than floppies.

Iomega Zip disk.

Iomega’s larger removable cartridge that contains platters inside a plastic case is called what?

Jaz cartridge.

Portable external drives became popular mainly because of which interface?

USB.

What equation defines total disk access time?

Access time = Seek time + Rotational latency.

Why was RAID developed?

To provide large capacity, higher performance, and fault tolerance beyond a single disk’s limits.

What does RAID stand for?

Redundant Array of Inexpensive (or Independent) Disks.

List four general benefits of RAID.

Higher data security, fault tolerance, improved availability, and increased performance.

Which two core techniques enable RAID redundancy and performance?

Mirroring (redundancy) and striping (performance); parity adds redundancy without full duplication.

RAID 0 provides what and lacks what?

Provides striping for speed; lacks redundancy (no fault tolerance).

In RAID 1, data is __ on two drives.

Mirrored (duplicated).

Biggest disadvantage of RAID 1?

Cost—50 % of total capacity is used for duplication.

RAID level that uses Hamming-code ECC across bit-level stripes.

RAID 2.

RAID level with byte-level striping and a single dedicated parity disk.

RAID 3.

RAID level with block-level striping and a dedicated parity disk.

RAID 4.

RAID level with block-level striping and distributed parity across all disks.

RAID 5.

How does RAID 5 remove the parity bottleneck of RAID 4?

Parity blocks are spread (round-robin) across all disks instead of one dedicated disk.

Which RAID level can tolerate failure of any two drives in the array?

RAID 6 (dual distributed parity).

Striping can occur at which two granularities?

Byte-level or block-level (stripe size).

Mirroring always requires what number of drives?

An even number (pairs).

CD-ROM has a typical storage capacity of about __ MB and over __ minutes of audio.

650 MB; 70-74 minutes.

What material forms the main substrate of a CD or DVD?

Clear polycarbonate plastic.

On a CD, data is stored as what physical feature?

Microscopic pits (bumps) and lands in a single spiral track.

Describe how a CD player reads data.

A laser passes through plastic, reflects off aluminum; changes in reflectivity between pits and lands are detected by a photodiode and decoded into bits.

Why do CD drives use Constant Linear Velocity (CLV) during audio playback?

To keep data rate constant despite the spiral, the spin speed varies so track passes the laser at constant linear speed.

What are typical multiples used to market CD-ROM speed?

1× (150 kB/s) baseline; higher numbers (e.g., 24×) indicate multiples of that baseline maximum.

Two erasable optical formats that succeeded CD-ROM?

CD-R (write-once) and CD-RW (rewritable using phase-change material).

DVDs achieve higher capacity primarily due to what three factors?

Smaller pits/track spacing, less error-correction overhead, and multiple layers/sides.

Capacity of a single-sided, single-layer DVD vs a CD.

4.7 GB vs 0.65 GB.

Maximum capacity of a double-sided, dual-layer DVD stated in lecture.

17 GB.

Why can early DVD drives have compatibility issues with DVD-W or CD-RW?

Multiple competing writable standards emerged before full consolidation; early hardware may not support newer media reflectivity or file systems.

Magnetic tape drives are described as what type of access device?

Serial (sequential) access.

Main advantage and main disadvantage of tape backup.

Advantage: very low cost per GB; Disadvantage: slow access/retrieval times.

Exabyte’s 8-mm tape drives transfer up to roughly _ MB/s.

6 MB/s.

What does DLT stand for, and why is it popular in networks?

Digital Linear Tape; offers high capacity, high speed, and robust reliability.

DAT stands for __ and typically stores up to __ GB uncompressed.

Digital Audio Tape; about 4 GB (8 GB compressed).

Which optical technology uses phase-change material for erase/rewrite capability?

CD-RW (also DVD-RW/DVD-RAM variants).

In DVD region coding, what prevents a disc from playing in another region’s player?

Firmware in the player checks region code bits on the disc and refuses playback if mismatched.

What homework resources were recommended for deeper study of hard disks and storage?

PCGuide’s HDD reference pages, and HowStuffWorks articles on hard disks, removable storage, floppy drives, and SCSI.