30.5.1. Basic structure and operation of magnetic disks

The disk substrate is coated with magnetic materials. The surface of this circular plate is coated with magnetizable material, which allows for it to be magnetized in different polarities. Originally, substrates were made of aluminum, but now glass is more common due to its better uniformity and reliability.

Magnetic disks consist of a base layer known as the substrate, which is coated with materials that can be magnetized. This coating is crucial as it holds the data in the form of magnetic charges. Glass substrates are preferred in modern disks because they result in fewer errors during the reading and writing processes thanks to their smoothness and strength.

Think of the way refrigerator magnets hold notes on your fridge door. Just like these magnets can hold onto something based on their polarity, magnetic disks store data using magnetic charges, which are like tiny magnets that can be turned on or off to represent data.

The read operation retrieves information from disks and sends it to the processor, while the write operation stores data from the main memory into the hard disk.

When the computer needs to access data, it performs a read operation using the read/write head, which translates the magnetic data into a format the computer can understand. Conversely, when data is modified in the main memory, a write operation is triggered to save this updated data back onto the disk, ensuring it persists even when the computer is turned off.

Picture a library where books are stored by subject. When you need to remember a detail from a book, that’s like a read operation—you go to the shelf, grab the book, and find the information you need. When you finish reading and take notes, you’re participating in a write operation, ensuring your thoughts are captured on paper.

The write head creates a magnetic field by passing current through a coil, which is then recorded on the magnetic material. During reading, the magnetic field moves relative to the coil, producing a current that translates into data.

The head of the magnetic disk operates much like a speaker or a microphone. When writing data, electricity flows through the coil to create a magnetic field that alters the polarity of the disk's surface. During reading, this same head detects the magnetic charges that create an electrical current, allowing the computer to interpret it as data.

Imagine a record player. The needle moves over the grooves in a vinyl record, reading the music encoded in those grooves. Similarly, the read/write head moves over the magnetic disk's surface, reading or writing data, depending on whether it's in play mode (reading) or record mode (writing).

Data is stored in concentric rings called tracks, and each track is further divided into sectors. The minimum block size for storage on a disk is typically a sector.

Tracks on a disk allow for organized data storage. Each track comprises several sectors, with each sector capable of holding a fixed amount of data. By organizing data in this manner, the disk can efficiently retrieve it when requested. The size of sectors can vary, but they often hold around 512 bytes, which determines how data is chunked together.

Think of a parking lot with different rows for various types of vehicles. Each row can hold a certain number of cars (sectors), and rows represent the tracks. Just like how you would find and park your car in designated spaces, the system locates and organizes data in designated sectors.

--