# Introduction to Secondary Memory

https://www.youtube.com/watch?v=I1Gz2jlBg-s

[00:06] Hello everyone and welcome back.
[00:10] In this session we will be properly introduced to the secondary memories.
[00:16] So without any further ado let's get to learning.
[00:21] Now let's reconsider the big picture that we observed previously in the session introduction to memory.
[00:28] There we learned that within the processor we have the registers but they were incapable of storing even a single instruction.
[00:37] So we opted for the main memory.
[00:41] Now although it resolved the capacity crisis, it wasn't fast enough to cope up with the processor.
[00:48] So we came up with the cache memory to store the frequently used data.
[00:54] In our course of learning we also have seen the various cache memory mapping techniques using which the main memory and the cache communicates.
[01:04] And lately we have also studied about.
[01:07] The architecture of primary memories in details.
[01:11] But the thing is none of this, neither the cachet nor the main memory, I'm of course talking about the RAM in here, neither of these can store data permanently.
[01:22] So in order to store data permanently, the computer uses secondary memory.
[01:30] Now if we talk about the type of data that are generally stored in the secondary memory, well starting from the operating system which actually manages the entire system for us.
[01:42] Then we have the other system softwares like device drivers, various application programs, images, audio files, videos, games, documents, you name it.
[01:52] We store almost everything in the secondary storage.
[01:58] Now I remember telling you this that the processor remains completely unaware about the existence of this secondary memory.
[02:06] To it, the RAM is the only.
[02:10] Physical memory and that's why its size is called the physical address space.
[02:15] On the other hand, the secondary memory is managed by the operating system and, using the virtual memory mapping technique, it communicates with the main memory through pages.
[02:27] Now, the concept of virtual memory mapping is a topic of the operating system course, so in this course we will mainly focus on the architectural specifications of the secondary memory storages.
[02:40] But before going into that, let me walk you through the various types of secondary memories.
[02:47] Now, we can classify the secondary memories in two different categories.
[02:52] The first category is removable auxiliary storages, whereas the second one is fixed auxiliary storages.
[03:02] Here, the first category gives us portability; that is, using these we can easily transfer data from one computer.
[03:12] To another.
[03:14] For an instance, suppose we have applied for an exam and the whole ticket has been provided to us through our email id.
[03:23] Now, in order to appear in the exam, we are to bring the physical copy of the whole ticket along with us.
[03:30] Now, if we don't own a printer, usually we would take the digital copy of the hall ticket in a usb flash drive and go to a nearby internet cafe.
[03:41] And from there, we would get it printed, wouldn't we?
[03:46] Now think about the usb drive.
[03:49] The sole purpose of it was to only store the hall tickets digital copy.
[03:55] Moreover, using it we actually could move the digital hall ticket from our machine to the cafes machine.
[04:03] So it gave us portability as well.
[04:08] Now let's learn about the different removable auxiliary storages one by one.
[04:14] Let us begin with the very first removable auxiliary storage that is the magnetic tapes.
[04:21] Now magnetic tape is a medium for magnetic storage made of thin magnetizable coating on a long narrow strip of plastic film.
[04:32] It was first developed in Germany in the year 1928.
[04:37] The technology was based on magnetic ware recording.
[04:40] Many of us may have seen old audio and video cassettes which used to be recorded and played back using the audio and video tape recorders.
[04:50] These are mostly obsolete nowadays.
[04:53] Now the devices that are used to store data on magnetic tapes are known as tape drives.
[05:01] Using these tape drives digital data can be both recorded on and read from the magnetic tapes.
[05:08] Earlier magnetic tapes were widely used in mainframe computers.
[05:13] Unlike audio and video cassettes digital.
[05:16] Magnetic tapes are not completely obsolete yet.
[05:20] These are still used in tape vaulting.
[05:24] Now tape vaulting is a process of backing up critical data in magnetic tapes.
[05:29] Big companies use this technology to backup their data off site.
[05:35] This data is mainly used for analytical processing which helps the companies to take big decisions.
[05:43] This is a very cost effective approach.
[05:46] We can store 148 gigabytes of data in per square inch of these magnetic tapes.
[05:54] Which amounts to 185 terabytes of space per cottage.
[06:00] This particular storage capacity was achieved by Sony Corporation in the year 2014.
[06:07] So this is all about the magnetic tapes.
[06:12] Let's observe the next type of removable auxiliary storage that is the floppy.
[06:19] Disks commercially debuted in the year 1960.
[06:23] These used to come in three different form factors.
[06:27] The first one was the 8 inch floppy disk.
[06:31] Thereafter the next type, the 5.25 inch variation, and finally the 3.5 inch floppy disks.
[06:39] In all of these, the 3.5 inch floppy disks became very popular and the storage capacity of the floppy disks used to range from 1.2 megabytes to 1.44 megabytes.
[06:55] Now, floppy is a type of disk storage composed of a thin flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disc.
[07:13] Let me illustrate this.
[07:15] Let's take this 3.5 inch disc and let's observe its various components.
[07:25] Now this is the shutter which was used to protect the surface of the disk once the floppy was removed from the floppy drive.
[07:35] Now these are the plastic housings which protect the disk inside.
[07:41] These are the polyester fabrics that reduce the friction of the rotating disc and also protect the disc from dust particles.
[07:51] Now on both the top and bottom plastic housings there is this lifter that presses the fabrics against the disc to trap the dust.
[08:02] And this one here is the spring for the shutter.
[08:06] Finally this is the magnetic disc.
[08:10] Now let's take a closer look at this one, shall we?
[08:15] Now on this disc this is called the hub which rotates the disc.
[08:21] And the rotation is necessary because the disc is subdivided into tracks.
[08:26] Then again, the tracks are subdivided into sectors.
[08:31] And a sector holds the amount of data that the floppy drive could read at a time.
[08:37] Floppies became very popular and dominated the market of removable auxiliary storages for more than two decades.
[08:47] It was so popular that as a legacy even today its symbol is used in different softwares as an icon for saving files.
[08:59] Another interesting fact, do you ever wonder why does the name of our computer local drives always begin with C?
[09:08] You must have observed this; the name of the partitions of our storage drives are like C, D, E and so on.
[09:17] Do you know why these are named like that?
[09:20] Let me tell you why: in earlier days we used to have two floppy drives, drive A.
[09:27] And drive B in the computers, and that's why the hard drives partitions used to get the name starting from C.
[09:33] Interesting, right?
[09:38] Now let's move on to the next one.
[09:41] Now we will observe the next type of removable secondary storage, that is the optical disks.
[09:48] So optical disk is an electronic data storage medium which data can be read from and written on using a low powered laser beam.
[10:00] There were three types of it.
[10:02] The first one, the CD or compact disc.
[10:06] Next was the digital versatile disc or DVD.
[10:10] And the last one was the BRD or Blu-ray discs.
[10:14] Now the CDs used to have the capacity of 700 megabytes.
[10:18] Later on when the DVDs were introduced, they could store 4.7 gigs of data.
[10:24] Coming to BRDs, single layer BRDs could store 25 to 33.4.
[10:30] Gigabytes of data on the other hand, quad layer BRDs could store even up to 128 gigabytes.
[10:39] For recording and reading the data, these used to rely on either a red or a blue laser beam.
[10:46] Now, all of these used to have the same diameter of 12 centimeters.
[10:52] And for data storage, these used to follow similar architectural specification as the magnetic disk of floppy.
[11:00] That is, the recording surfaces of these are also subdivided into tracks and sectors.
[11:06] So, that is all about the optical disks.
[11:09] Now let's move on to the next one.
[11:13] Now next in line is the memory cards.
[11:17] These are the data storage devices that use the concept of flash memory.
[11:24] Memory cards are commonly used in portable electronic devices such as the digital cameras, smartphones, tablets.
[11:32] Video game consoles and so on.
[11:35] Now coming to the flash memory, it was invented by Toshiba Incorporation in 1980s.
[11:40] And flash memory is based on the EEPROM or electronically erasable programmable ROM technology.
[11:50] Based on dimension, we can categorize memory cards in three different types.
[11:56] The first one is the SD card.
[11:59] Remember, SD stands for secure digital.
[12:02] Then comes the mini SD cards and finally the very popular micro SD cards.
[12:10] Apart from these classification, based on capacity we can further categorize memory cards into four different types.
[12:19] The first one is the standard SD, which can store up to 2 gigabytes.
[12:25] Next up is SDHC, which can store 2 gigabytes to 32 gigabytes.
[12:32] SDHC stands for secure digital host.
[12:35] Controller.
[12:36] The next type is SDXC, which can store 32 gigabytes to 2 terabytes of data.
[12:45] And here XC stands for extended capacity.
[12:50] Finally, the SDUC, the latest one, which can store 2 terabytes to 128 terabytes of data and has a bandwidth of 985 megabytes per second.
[13:04] Do remember here the UC stands for ultra capacity.
[13:09] So this is all about the memory cards.
[13:13] Let's now check out the next one.
[13:15] So the next one in line is the removable auxiliary storage, the flash drives.
[13:22] Now flash drive, also known as thumb drive, is actually a flash memory with an integrated universal serial bus or USB interface.
[13:33] Basically a flash memory chip is.
[13:35] Interfaced with the digital devices using a USB port.
[13:40] These appeared in the late 2000s and, due to their portability, these are still very popular now.
[13:47] Capacity wise, flash drives have wide ranges.
[13:52] Most popularly used flash drives are the ones which can store 8 gigabytes to 256 gigabytes of data.
[14:01] Nevertheless, there are some more variations which are less popular, especially due to their high cost, and these have the capacity of 512 gigabytes to 1 terabytes.
[14:13] The largest storage capacity flash drive can store up to 2 terabytes of data.
[14:19] Now flash drives are also popular because of their lifespan.
[14:24] Under normal circumstances, these can last for anywhere between 10 to 100 years.
[14:29] Flash drives are commonly used as supplementary backups, transferring computer files, and as a means for.
[14:37] Installing operating system.
[14:39] Also these are way more durable than floppy or optical discs.
[14:44] So that was all about removable auxiliary storages.
[14:47] From the next session onwards we will observe various fixed auxiliary storages in details.
[14:56] Alright people, that will be all for this session.
[14:58] I hope to see you in the next one.
[15:00] Thank you all for watching.
[15:04] [Applause]
[15:06] [Music]
[15:14] You.