Understanding RAM, Hard Disks, and I/O Modules
RAM Technology: DRAM vs. SRAM
RAM technology is divided into two primary types:
- Dynamic RAM (DRAM)
- Static RAM (SRAM)
DRAM
- Made with cells that store data as a charge on capacitors.
- The presence or absence of charge in a capacitor is interpreted as a binary 1 or 0.
- Requires periodic charge refreshing to maintain data storage.
- The term *dynamic* refers to the tendency of the stored charge to leak away, even with power continuously applied.
DRAM vs. SRAM: Key Differences
- Both are volatile, meaning power must be continuously supplied to the memory to preserve the bit values.
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Dynamic cell:
- Simpler to build, smaller.
- More dense (smaller cells = more cells per unit area).
- Less expensive.
- Requires supporting refresh circuitry.
- Favored for large memory requirements.
- Used for main memory.
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Static:
- Faster.
- Used for cache memory (both on and off-chip).
Magnetic Disk Technology
A disk is a circular platter constructed of nonmagnetic material, called the *substrate*, coated with a magnetizable material.
- Traditionally, the substrate has been an aluminum or aluminum alloy material.
- Recently, glass substrates have been introduced.
Benefits of Glass Substrates
- Improvement in the uniformity of the magnetic film surface to increase disk reliability.
- A significant reduction in overall surface defects to help reduce read-write errors.
- Ability to support lower fly heights.
- Better stiffness to reduce disk dynamics.
- Greater ability to withstand shock and damage.
Disk Performance Parameters
When the disk drive is operating, the disk is rotating at a constant speed.
To read or write, the head must be positioned at the desired track and at the beginning of the desired sector on that track.
Track selection involves moving the head in a movable-head system or electronically selecting one head on a fixed-head system.
Once the track is selected, the disk controller waits until the appropriate sector rotates to line up with the head.
Seek Time
On a movable-head system, this is the time it takes to position the head at the track.
Rotational Delay (Rotational Latency)
The time it takes for the beginning of the sector to reach the head.
Access Time
The sum of the seek time and the rotational delay.
The time it takes to get into position to read or write.
Transfer Time
Once the head is in position, the read or write operation is performed as the sector moves under the head.
This is the data transfer portion of the operation.
Seek Time
On a movable-head system, the time it takes to position the head at the track.
Rotational Delay (Rotational Latency)
The time it takes for the beginning of the sector to reach the head.
Access Time
The sum of the seek time and the rotational delay. The time it takes to get into position to read or write.
Transfer Time
Once the head is in position, the read or write operation is performed as the sector moves under the head. This is the data transfer portion of the operation.
Inter-Track Gap in Hard Disks
Why do we need an inter-track gap in a hard disk?
To avoid interference from neighboring data.
Hard Drive Read and Write Mechanism
Describe the hard drive read and write mechanism.
The write mechanism exploits the fact that electricity flowing through a coil produces a magnetic field. The traditional read mechanism exploits the fact that a magnetic field moving relative to a coil produces an electrical current in the coil.
Read/Write Heads in Fixed vs. Movable Head Systems
In a disk having *n* tracks, how many R/W heads are needed in the following:
- Fixed head: One per track.
- Movable head: One per surface.
Major Functions of an I/O Module
The major functions of an I/O module fall into the following categories:
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Control and timing:
- Coordinates the flow of traffic between internal resources and external devices.
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Processor communication:
- Involves command decoding, data, status reporting, and address recognition.
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Device communication:
- Involves commands, status information, and data.
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Data buffering:
- Performs the needed buffering operation to balance device and memory speeds.
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Error detection:
- Detects and reports transmission errors.
