Continuing from the previous section: Below I have explained the main types of computer memory (such as virtual and graphics memory) so that you have a better idea of what they do and therefore know what specifications to look for when buying a new apple mac computer for example.
Memory is known by two types. ROM (Read Only Memory) which means hardware and software can only read the byte values stored inside rom memory and RAM (Random Access Memory) which means hardware and software can not only read the byte values stored inside ram memory but they can also, temporarily, change the byte values stored inside ram memory.
ROM memory is a microchip on a motherboard (green microchip board), such as a Modem motherboard, that contains the instructions (byte values) needed to make the hardware (Modem) work. This is why ROM memory cannot have its byte values changed. The instructions (byte values) are preprogrammed onto the microchip, so that when you first switch on the computer the instructions (byte values) are always the same.
RAM memory is a microchip that is permanently fitted onto a Card (small motherboard), such as a Graphics Card or Memory Card, or a single removable microchip that you attach to the main motherboard. The main motherboard is the big green microchip board inside the computer (base unit). RAM memory can have its byte values temporarily changed, and read, so that you can change Graphics settings (Graphics Card) or store data (Memory Card) for example.
Even though RAM memory is cleared when you switch off or restart the computer, because it is meant as a temporary storage place only, the data inside the memory is normally saved by an application (as a file) beforehand. For example: If you change the Modem (hardware) settings using a modem application it will save your changes as a settings file on the hard drive before the computer is switched off or restarted. When the computer is switched on again the modem uses its default (normal, preprogrammed) settings before looking at, and using, the settings from the saved settings file.
The modem uses its default settings first for two reasons. One is so that it knows it is setup correctly (by using its normal, manufacturer's, preprogrammed settings) and two is in case the settings file is corrupt, damaged or missing. If it reads a settings file incorrectly (i.e. because it is corrupt) you may get problems with the modem or it may revert back to its default settings.
When OS X finally runs out memory, or when it cannot fit something really big inside memory as one piece, it starts using a thing called Virtual Memory, which is Hard Drive space that is used as memory. For example:
Imagine you have just typed out a 10,000 line document using Microsoft Word 2011 and then you run out memory. Instead of OS X telling you "You cannot type anything else - No more Memory available" it will just let you carry on typing as normal. This is because the rest of your typing will be stored on the hard drive as a file, which you have no access to. As far as you are concerned you are just typing a document and so do not have to worry about virtual memory. However, you should be aware that over time this wears out the hard drive. So always make sure you have plenty of memory.
Graphics Memory is a microchip on the motherboard of a Graphics Card. It is the memory that stores your Imagery (Desktop Picture, Game Scenes, Photographs, Icons, Text and so on). Its main job is to store Imagery inside its graphics memory and then display that imagery, on the monitor screen, when an application asks it to. For example:
If you are editing a photograph your photo editor application would of allocated some of the graphics memory as file memory (for the original photograph) and some of the graphics memory as edit memory (for editing purposes); In the same way the file memory and edit memory is allocated for a Microsoft Word 2011 document.
The graphics card will not only display the photograph (file memory) on the monitor screen, but it will also update (refresh) the monitor screen instantly every time you edit the photograph. In the same way that Microsoft Word 2011 updates instantly any changes you make to a document. Graphics memory is also in charge of storing other imagery. For example:
If you have one game open, a photo application open and the internet open, it is graphics memory storing the imagery of those three applications. The more applications open that use graphics memory the more tasks (jobs) you are giving the graphics card. Meaning: If you give it too much to do and/or it runs out of graphics memory, it will not be able to update the monitor screen quick enough, which means editing and game playing for example will not be instant. The monitor screen will update (draw the imagery) but it will be so slow that it would not be worth editing a photograph or playing a game.
A normal sign of this is when the monitor screen is updated (drawn) line by line as opposed to instantly. So get a good Graphics Card with plenty of Graphics Memory because the one built into the computer might not be good enough for your needs.
Memory Size is measured in multiples of 8 MegaBytes (i.e. 256MB, 512MB and 1024MB), which means you could never have 274MB or 519MB for example, and Memory Speed is measured in MegaHertz (Mhz).
Computers made before the year 2000 normally use memory known as SDRAM, which is now very difficult to find because it is no longer manufactured. A SDRAM memory chip can either be 100Mhz, 133Mhz or 166Mhz and is named after its speed: PC100, PC133 or PC166.
Computers made between the years 2000 to 2005 (approximately) normally use memory known as DDR, which is still being sold at the present time. A DDR memory chip normally comes with one of the following specifications: PC1600 (200 Mhz), PC2100 (266 Mhz), PC2400 (300 Mhz), PC2700 (333 Mhz), PC3200 (400 Mhz) or PC4000 (500 Mhz).
Computers made between the years 2005 to 2008 (approximately) normally use memory known as DDR 2. A DDR 2 memory chip normally comes with one of the following specifications: PC2-3200 (400 Mhz), PC2-4200 (533 Mhz), PC2-5300 (667 Mhz) or PC2-6400 (800 Mhz).
At the time of writing (2018) the standard memory type is now DDR3. It comes in specifications such as PC3-8500 (1066 Mhz), PC3-10600 (1333 Mhz) and PC3-12800 (1600 Mhz). The 13" MacBook Pro (Mid 2012 / £999 model) for example uses DDR3 Memory - PC3-12800 (1600 Mhz) whereas other MacBook Pros use DDR3 - PC3-10600 (1333 Mhz). So as you can see: Memory capacity has got bigger and faster over the years, and cheaper too!
Fig 1.0 The newer DDR3 Memory Stick for laptops - Size: 4GB (GigaBytes). Speed: 1600Mhz.
NOTE: Because newer/better memory is manufactured every so often, the memory specifications above are only here to give you an idea of the kind of memory specifications available. I have not mentioned every single memory specification out there in other words. The time overlaps above (i.e. in 2005) mean that newer memory was created in the same year.
If you are wondering how much memory your apple mac computer should have inside it (installed) in order to function properly and reasonably well, it should generally be double the amount of whatever memory your computer came with. So if it came with 4GB installed then try and upgrade it to 8GB, if it can be upgraded to 8GB of course. Remember, applications are becoming bigger and more memory hungry these days and will continue to be in the future.
Another thing to remember is that apple mac laptops for example only have 2 Memory Slots built-in to them. So if you have a MacBook Pro Laptop computer with 4GB of memory installed it usually means Apple have fitted a 2GB Memory Stick into each memory slot when ideally they should of fitted a 4GB memory stick into the first memory slot and then left the second memory slot empty. That way it would work out cheaper for you just to buy an extra 4GB memory stick to fit inside the second, empty, memory slot. As it stands with this scenario, it means you would have to take out both 2GB memory sticks in order to replace them with two 4GB memory sticks; which is costly and means you end up with two perfectly good but redundant 2GB memory sticks.
If you are going to upgrade your computer's memory you are better off shopping online for the memory sticks simply because Apple charge a ridiculous, "rip-off", price for their memory upgrades, that in my eyes can never be justified. You can find much cheaper memory at Crucial for example. To find out what type of memory sticks are inside your apple mac computer, and therefore what type of memory sticks to buy and upgrade with, I recommend using Crucial's Crucial System Scanner.