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#844: What is Virtual Memory?, Worms, Viruses, and Trojans, Oh My!, The End is Nigh (For Your Optical Drive, at Least)


Hello all,

It was snowing when I left my house this morning…and I think those few flakes already surpass last year’s total in the Champlain Valley. According to the Farmer’s Almanac, it is going to be cold and snowy here this year. We’ll have to wait and see! I find it a bit disconcerting to think about having children who grow up barely remembering snow, but I certainly did not mind not shoveling a single scoop of snow last year.

Also on the wait-and-see list is the iPad mini. It will be the battle to see whether the nifty form factor and lower price outweigh the lack of Retina display and that seemingly every person in the known universe seems to own an iPad already.

In other news, we are seeing the effects of last week’s storm in the form of delayed parts deliveries. Most of our parts come from Pennsylvania and New Jersey, and after getting almost nothing all week, we received about fifty parts in two days from alternate locations around the country. When they speak of losses due to the storm, things like this certainly are included.

I can’t imagine the human hours and added expense just to make adjustments for this one small slice of the economy, much less any industry directly affected in the storm’s damage area. I am off to pitch in and start bailing — not water, but parts our customers have been waiting for.


  What is Virtual Memory?  

Virtual Memory is a system of memory management that divides up the contents of physical RAM, manages these contents in memory, and stores them in either physical RAM or in secondary storage, such as the computers hard disk.

When the System and applications’ thirst for memory address space outstrips the available physical RAM, or RAM fragmentation results in inadequate amounts of free RAM in consecutive blocks, portions of the RAM’s contents are managed by Virtual Memory system and are written to the hard drive. A portion of the contents of the physical RAM are divided into pages and page frames, which are small segments of memory sized between 1k to 4k in size.

A Page Table is developed to track these page frames and their location. When the data in RAM exceeds the capacity of the physical RAM, some of the pages are written to swap space on secondary storage — usually the computer’s internal hard drive. When the processor needs data stored in a page, it uses the Page Table to find the page. If it is found still residing on in the physical RAM, it is considered a page hit. If the page is not found in the physical memory of the machine, it is called a Page Fault. With a Page Fault, the system must retrieve the page from the swap space on the hard disk.

For increased machine performance, the system tries to predict what data is least likely to be needed. The pages the system considers as having the least value are called Victims. Victims are the pages that the system has determined are the least frequently used or oldest pages still in memory. The page frames are often moved to secondary storage as the system likely will not need to use them as soon as other contents of the RAM.

The issues related to Virtual Memory management are two-fold, though. There is overhead required to track the changes of the location of the pages and update the Page Table. The second issue results when the system has a Page Fault and has to access information written to the swap space on the hard disk. The time to access these pages frames is slower due to the fact it is in secondary storage. The system must make a call to the hard drive, the hard drive must find the page in the swap space, read it into the drives buffer, and then transport the frame across the system bus, into physical memory. Only after these steps have occurred can it then be accessed by the processor and the thread it is needed for.

The Virtual RAM swap space for a Mac is held within the directory /private/var/vm. There are often several different swap space directories and also the directory of the sleepimage. This directory, private/var/vm/sleepimage, is where the entire contents of the machines RAM is written, as an encrypted disk image, when the computer sleeps long enough to go into hibernation.

Editor’s note: Virtual RAM needs can be significant and your machine will use every bit of free disk it has for this function. My workstation at Small Dog currently has about 225GB of free space. According to Activity Monitor, every scrap of that free space is allocated to virtual RAM. The general rule used to be to keep 10% or so of your hard drive free for virtual RAM. For a 200GB drive, that means roughly 20GB. I’m not sure what the cutoff is at this point, but the bottom line is that as your drive gets closer to filling up, you will notice performance dropping to the point where the machine may barely work at all.

  Worms, Viruses, and Trojans, Oh My!  
  malware virus trojan

You have probably heard the terms “viruses, trojans and worms,” which are all under the umbrella term for malicious software called malware. These items are usually thought of as interchangeable or the same thing. Although they are similar in that they are created with malicious intent, if you understand the differences, you will better be able to better understand the threat so you can protect your Mac.

Like a virus that infects people, a computer virus seeks to replicate itself and spread to another host to repeat the process. Since they need execute and write permissions for this, viruses attach themselves to programs (sometimes legitimate) so that when you open the host program the virus is also activated. There are a couple of subclasses of viruses, but they have the same basic characteristics.

A worm is very similar to a virus, but it can travel between other computers or devices without interaction from any user. Also unlike a virus, a worm does not need to attach itself to an existing program. One of the most famous worms was Stuxnet. Stuxnet was created by a unknown source and its main target was a nuclear power facility in Iran. It could spread from a computer to a flash drive and then to a phone and back to a computer (and so on) until it found its target. Even though this is the most advanced worm to date, a common malicious computer worm can do the same traveling to infect machines.

The word trojan rises from the way this type of malware works. Similar to Greek mythology, the user is tricked into installing the trojan horse which is often disguised as friendly software. The effect of a trojan horse can vary, depending on the creator’s intention. The most recent well-known trojan for the Mac is “Flashback,” which disguised itself as an Adobe Flash Player update. Once installed, it would be able to collect user data and passwords and make them accessible to the creator of the trojan. It could also set up a BotNet which is a network of computers to attack other machines and services.

So what does this mean for us as Mac users? It has long been one of the points in the Windows/Mac debate that Macs are immune to viruses and malware. It would be more true to say that there are currently no virus threats to Macs. As far as other forms of malware, we have all seen that certainly any machine can be a victim. Certainly though, as Mac users we only deal with a small fraction of the headaches that Windows users have with this problem.

The bottom line is to both be careful and pragmatic — as time goes on, the number of threats is guaranteed to rise. Use a malware program — either one of the paid options, such as Norton or MacAfee, or one of the free utilities out there that performs well and seems to have minimal impact on system performance such as Sophos and Clam XAV.

  The End is Nigh (For Your Optical Drive, at Least)  

Apple recently updated their iMac line with new insanely thin models that now lack an optical drive just like their newer laptops. I think we’re now witnessing the beginning of the end of a very old technology. It’s about time, in my opinion.

Optical discs are easily damaged and do not store as much data as modern flash drives can. Most software manufacturers now offer download options for nearly everything, and we’re seeing a rise in savvy vendors making use of legal bit torrent sharing to speed the download of their software so you don’t spend all day getting your new programs.

Many of us still have programs that are on physical media that will no longer work on the newer operating systems released by Apple. However, what if you are planning on upgrading your computer but don’t want to lose access to those programs that still actually do work? If these applications won’t transfer with Migration Assistant, you have some obvious options: Let go of the older software, purchase an external optical drive, or use Disc Sharing to hijack the optical drive of another Mac load the software into your new machine.

Another lesser-known option is to convert your physical media to the the ubiquitous modern software delivery tool called a disk image which can be easily moved, mounted, and accessed on any machine.

Step 1) Insert the optical disc into your Mac.

Step 2) Go to your Utilities folder and open Disk Utility.

Step 3) You’ll see the disc listed on the left hand side of the window; select it and then click on the New Image button

Step 4) Select a location where you’d like the disk image to be saved. I recommend a place that would be included in your backup. The one downside to this process is that the image is as vulnerable as any other information stored on your hard drive. The eventual loss of optical drives from computers should stress the importance of doing backups on a regular basis.

Step 5) Wait until the image is created then eject your disc. Double click the image file created by Disk Utility to verify that it opens.

Step 6) Move your copy of Asteroids for OS 9 to your new Mountain Lion-equipped iMac and… (well, of course that’s not going to work, but you get the point.)

You can keep a big software library in a relatively small space this way, and no need for an optical drive for any of it.

On a side note, just to avoid emails saying I’m spelling disc inconsistently as disk, or vise versa, a disc refers to optical media such as a CD or DVD. Disk refers to magnetic media like your hard drive or a floppy disk (everyone remember those?). Some people refer to flash drives as disks as well, though they aren’t magnetic and the word chip would be more accurate.

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