The Hard Disk Drive (HDD)
In this paper we’ll consider the secondary memory to be the data storage components in which the presence of the media element is not dependent on its insertion by the user in the device. In this case, the hard drives are always present in the computer. Always present is the correct term to use, in all circumstances.
The Hard Disk Drive, to be designated with the acronym HDD, is always defined as secondary memory. It is so under the operating perspective of the CPU.
Actually, the CPU operates directly with data or instructions at a level 1 cache, thus on the main memory. Hence the name Main. The CPU was conceived to read and execute instructions in addresses in the Main Memory. The CPU processes only what is there, in its permanent state cycle of fetch and execution.
But the Main Memory, as well as the superior hierarchy levels, is volatile, i. e., the values inputted into it only remain there as long as the computer is being powered. If the power is cut, everything is lost.
All the information, programs and necessary data for the CPU to work are found in a permanent and non volatile form in the HDD. In order for the CPU to process, the MMU (Memory Management Unit) finds in the HDD what needs to be on the Main Memory. The information needed to boot the computer is stored in the HDD. The Operating System (OS), which once installed in the Main Memory is what runs the computer, is also stored in the HDD.
All this to state that the real memory of the computer is the HDD. Far from the deprecating definition of data storage device. Programs are not stored in the HDD. What is stored are the program installers and data files. Programs are installed in the HDD, so that the OS can place them on the Main Memory so the CPU can run them.
And by the way, remember that the HDD is also a part of the Main Memory, constituting an extension of it, to which we call Virtual Memory, matter that will be addressed along with the OS.
This approach is meant to raise the importance of the HDD to its real standard in context.
All of these subjects will be treated with much greater depth.
It’s only when the HDD comes along that we first speak about OS, for it was its function to interact between the CPU and the HDD, creating a mean for mutual understanding.
In an early stage, that was the true purpose of the OS. Slowly they evolved into what they are today: a true interface between hardware, software, and user, without which no computer can operate.
But enough with flattering the HDD, for it shows its worth by demonstrating its permanent importance.
In this chapter we will analyze the electromechanical functioning and the physical data storing form in the HDD. The logic behind its organization and accessing depends on the OS, to which it uses File Systems, which will be discussed shortly after.
The HDD registers the information in the form of magnetic fields, like an ordinary cassette tape. Only contrary to the cassette recorder, in which the access and the recording are made sequentially, in the HDD, both the reading and recording are random, i. e. any data at any point can be accessed anywhere, as long as it’s location is supplied, similarly to what happened with RAM Memory, which we analyzed earlier.
In the HDD, these accessing methods are completely different, as we will see.
The HDD registers the data as magnetic fields. In that case, its capacity varies proportionally to the density of magnetic fields it can support. This is called “Areal Density”, once converted to our well known bits, which define its logic state 1 or 0 by the tension level they carry.
History check: the first HDD ever used had the brutal density of 2.000 b/in2. Nowadays, HDDs have densities that near the 400 Gb/in2, i. e., 400.000.000.000 b/in2. And always growing (b means bit, the information unit, and B means Byte). HDD density is always referred to in inches.
All this growth is connected to great developments in the hardware mean, the performing of magnetic registry, the construction of the read-and-write heads, and the design of elaborate HDD controllers.
We can say that the HDD is one of the wonders of technology execution, for its required precision in an electromechanical operation at a nanometer measuring level.
The HDD Components
A Hard Disk Drive is composed by 3 fundamental elements:
- The platters
- The read/write heads
- The controller
In the following description, we’ll use figure 1, referencing the annotations made in the pictures with (x).
The platters (9) constitute the support element for the magnetic recording which consists of the stored information on the HDD. So, they should have specific characteristics for creating and modifying those electromagnetic fields.
They are made of a magnetically neutral material, particularly aluminum, and more recently, in some cases, glass.
That material is polished until it’s perfectly flat, attaining a mirrored like look.
That flatness is required due to the tiny distance the heads hover over the platters (nowadays measured at nanometer level) and to the fact that they can never touch the platter’s surface when they are in motion.
Over the platters and on both surfaces, a thin layer of magnetic material called thin magnetic film is placed. By “thin” we mean thickness in the range of 10-20 nanometers (as a comparison term, an ordinary sheet of paper is in the range of 100.000 nanometers).
This layer is in turn coated by another thin layer of resistant film, like carbon, which is supposed to protect the magnetic film from possible little accidents with a head. The emplacement of these materials is made possible by a similar method described for integrated circuits, in high temperature ovens and through vapor deposits. The platters are thus magnetizable on both faces.
In this specific case, documented with photos, the HDD in question has 4 platters.
The platters are placed parallel to each other over a base connected concentrically with the axis of the “spin motor”.
The distance between platters is guaranteed by spacer rings (19). There are also spacer parts (20), between which the disks rotate, and its width’s sum guarantees that the lid of the box is fixated at an adequate distance from the whole set.
On top of the set it’s placed a ring (13) which is screwed to the axis of the spin motor and applies pressure, fixating the set of 4 disks relatively to the motor.
The platters spin at speeds that vary from 5.400 to 15.000 rotations per minute. The most up to date speed is 7.200 RPM. Remember the vinyl Long Play that turned at 33 RPM? It’s only 228 times that. It’s 120 rotations per second, i. e., the same point on a platter goes under the read/write heads 120 times per second, once in every 8.33 milliseconds, and those heads hover those platters a few tens of nanometers apart, without touching them.
The Logic Board
In the two pictures identifying the logic board (both in Figure 1), we can see that on the left one it is in its natural position, and on the right one turned to the right out of position. The elements in touch with it are duly identified, whether they’re in the HDD box connected to it or out of it.
And what is the logic board for?
To convert addresses into positions for the heads over the platters and to convert binary code into magnetic fields. To establish dialogue between the OS and the HDD.
We will approach in more detail the functionalities of the logic board later on, when we address the HDD geometry and its logic.
On the logic board, besides other elements, we stand out two of particular importance:
- The chip that controls all the information that inputs or outputs from the HDD (7), data and addresses.
- The cache memory (8), which retains the information that tends to be repeated, avoiding like so, slow access to the HDD.
We can also see highlighted, the connections between the actuator (5) and the spin motor (6).
The Read/Write Heads
The read write heads due to its importance and to the length of its description, will be part of the next article.