IBCS Syllabus 3.2.6

3.2.6 Outline the characteristics of secondary memory and define
sequential and direct access.

Teaching Note:
Secondary memory should refer to flash memory, disks, CDs and
DVDs and tape. Students must know the type of access of the above
secondary memory media. They should also be able to give an
application of each type and justify its use for this application.

Sample Question 1:

Define the term direct access. [2 marks]

Sample Question 2:

...Sequential file access is used to update individual product prices when they change.

(b) State one disadvantage of using sequential access in this way. [1 mark]

JSR Notes:

The first part of this assessment statement is quite broad indeed. And the textbook does a quite good job covering all of the various secondary storage techniques. Use it as your prime source, and use this for clarifications. But on the off chance that the change in focus was missed: we had just been talking about primary memory, and now we are talking about secondary memory. Primary memory is basically RAM and cache; it’s the fast memory that is held temporarily. Secondary memory, otherwise know as secondary storage, or storage memory, is just that: memory used for storing data long-term.

Tapes
So, the first clarification that must be made is that the textbook is a little out of date regarding the latest in storage techniques; it acknowledges this fact. The main point in this regard to note is that tapes are still very much in the world of the textbook. And though they definitely are still used frequently today for storage, they don’t have to be, with the price and capacity of hard drives what it is. But it is convenient that the textbook is able to focus on tapes, since tapes are the one secondary storage device that allow focus on sequential access. And sequential access is a bit deal in this assessment statement.

Sequential Access Vs. Direct Access
The first thing to draw to your attention here is that the second part of this assessment statement is actually quite straight-forward: “define sequential and direct access”. Certainly it’s good to go beyond just a definition, but for what it’s worth, the definitions are required knowledge, so here you go:
Sequential access: Access to data on a storage device that is based on the order of the data, and any particular data record cannot be accessed except with reference to the data records that came before it. Tapes are the most common device that use sequential access.
Direct access: Access to data on a storage device that is direct; it does not require reference to the other data records that came before it to be accessed. It is also called random access. It is the kind of access used on most storage devices now-a-days, including hard drives.

Now, here’s a little more background on the difference between sequential and direct access. Like I say, it’s covered pretty well in the textbook, but it wouldn’t hurt to be reiterated. Of the secondary storage devices mentioned, only tapes use sequential access. And this is out of necessity. The way they are physically structured, there is no choice but to “fast-forward” or “re-wind” past other material to get to the data that you require. Not so with all the other secondary storage; one way or another, they are able to more-or-less directly pick out what they are looking for. In the case of a disk drive, there are times involved rotating the read/write arm, and waiting for the desired data to spin around to where it is, but you’re talking about milliseconds, and you certainly don’t have to go past all the other data on the disk to get to where you want. So this is as direct as can be, and is very different from the sequential access of a tape. ***Look at the textbook for details on the differences and advantages/disadvantages, page 147, 148; it’s good enough that I don’t have to clarify anything else here.***

Serial Vs. Sequential
It also wouldn’t hurt at this point to clear up the difference between “serial” and “sequential”. As the name implies, sequential files should actually be sequences, i.e. ordered somehow – usually alphanumerically by some key field of the records (more on this in topic 7). But serial just implies one after the other, though not necessarily in any particular order. In the olden days, distinguishing between these two types of storage was a big deal. But for now the main take-away point is that tapes are able to be most efficiently searched if they indeed do keep things ordered, i.e. that they are indeed sequential files. Meantime, though, and importantly, all direct access storage techniques are not required to be ordered, since one way or another (more in Topic 7 about how) the read/write head, etc. is able to find them directly.

Two important points to keep in mind
And again, it’s important to keep in mind that sequential access existed out of necessity, and now, given the choice, you’ll almost always go with direct access techniques. But another point not to be missed is that in situations where the given data is always read in a certain way anyway, sequential access is actually faster, and so to be preferred.

The teaching note
The first teaching note says you should be aware of which kind of access each of the listed storage media use, and the answer is direct access, except for tape. And I’ll not bother putting here a response to the teaching note that says you should be able to give an application of all the different storage media, but do stop to consider for a moment at least one advantage, and one disadvantage of each of:
Tape
CD/DVD
Hard Drives
Flash
Can you?? If not, look at the Mr Rayworth handout/link “How Computers Work and What Makes Them Faster”. Perhaps the only one to add here now is that Flash is very fast, but still relatively expensive compared to other storage media.

The last thing to mention is that the technical details of tracks, sectors, and so on are more Topic 7 that Topic 3. But for HL students, remember that there are some good details back here, when you get to Topic 7.