How Good Is Your Memory

In our human body analogy, the memory would correspond to the memory sections of the brain.  That means that the CPU and the memory share the same tasks, and our absolutely vital to the proper running of a computer.  With bad memory, information is forgotten or corrupted.  Thankfully, we know more about computer memory systems then the human brain.

Memory inside a computer breaks down into 2 forms; Read-Only Memory (ROM) and Random Access Memory. (RAM)  There are various differences for these two forms of memory, and in reality the two forms are as far apart as they could be.

Read-Only Memory (ROM)

Read-Only memory is a non-volatile form of memory.  That means that it keeps it's memory even after electrical charges stop flowing through it.  This makes ROM more of a permanent storage medium then memory, but it is still referred to as memory by most technicians.  It's primary role is to store BIOS programming on the motherboard, video card, network interface card, and other equipment.

Although it's very name states that this information is read-only, there are ways to alter ROM.  The first ROM chips did not have this ability, and were called PROMs. (Programmable Read-Only Memory)  Once they were in place on a motherboard, they performed their functions without change.  This was great for computer systems that always stayed the same, but eventually a way to alter a ROM chip's code.

EPROMs (Erasable Programmable Read-Only Memory) was designed to allow changes to the ROM chips through use of ultra-violet light.  A window on the top of an EPROM chip allowed you to reprogram the chip using a chip burner.  This system worked, but because outside equipment was required not many people used this system.

EEPROMs (Electrically Erasable Programmable Read-Only Memory) allows technicians to flash an electric charge through the ROM chip to reprogram it's code.  Most current BIOS's have EEPROMs, and can be flashed while still connected to the motherboard.  This means that a simple software utility can re-write an entire BIOS. 

Random Access Memory (RAM)

As opposed to ROM, Random Access Memory is a volatile form of memory.  This means that when you turn off the electrical signals to this memory, all information contained within it is lost.  There are many different forms of RAM, including DRAM, SRAM, VRAM, WRAM, and more.  The important memory types to remember are Dynamic RAM (DRAM), Static RAM (SRAM), Synchronous Dynamic RAM (SDRAM), and Double Data Rate SDRAM. (DDR SDRAM) Below are all the common types or RAM and their specifications;

There are also many different forms of video-specific RAM.  They include VRAM (Video RAM), WRAM (Windows RAM), SGRAM (Synchronous Graphics RAM), and MDRAM. (Multibank Dynamic RAM)  Knowing what these RAM types are is not as important as knowing where they are used.  Just know the acronyms for video RAM and try to study more on other memory systems.

More About DRAM

DRAM was the standard RAM type used on many older systems.  It is a volatile memory, meaning that it erases itself when it isn't refreshed.  Most DRAM requires refreshing once every two milliseconds.  A special circuit reads and re-writes the contents of RAM every 2 milliseconds, whether the RAM is used or un-used.  It is the slowest of the memory forms, having access times only as good as 50ns.

DRAM comes in many formats.  Dual In-Line Package (DIP), Thin, Small Outline Package (TSOP), and Small Outline J-Lead (SOJ) are the major formats.  

DIP memory required a chip to be pressed into socket on the motherboard, and were known to suffer a process called Chip Creep.  This meant the chips could work their way out over time.  

TSOP and SOJ are formats of what is more commonly known as SIMMs (Single In-Line Memory Module) and DIMMs.  (Dual In-Line Memory Module.  Essentially, several TSOP or SOJ chips are soldered onto a special board that is then installed on the motherboard.  This is a much more secure connection, and is used on almost all new computer systems.

One of the biggest problems with memory systems in computers is knowing which forms are compatible with which.  Non-EDO motherboards will have a fit with EDO memory, and FPM DRAM SIMMs and EDO SIMMs don't work together when placed in the same bank.  If you're having problems determining which RAM works on which motherboard, the motherboard manual is your best friend.  Basically, if the motherboard doesn't say it's compatible, it isn't.  

All newer boards use SDRAM DIMMs, but Pentium boards in general have used EDO SIMMs, SDRAM SIMMs, and SDRAM DIMMs.  Remember that SIMMs have 30 or 72 pins, or connectors on the bottom, and DIMMS have 168 pins.  Most SIMMs must be installed in pairs, where DIMMs can be installed singly.  Finally, some older Pentium boards will allow you to add video RAM through an SOJ socket.  Read the motherboard manual before attempting to install this type of RAM.  

Remember that DIMMs use gold leads, as do 30 pin SIMMs.  Most 72 pin SIMMs use ton leads, but their are exceptions.  DIMMs come in 5 volt and 3.3 volt formats, so check your voltage settings before installing the RAM.  Finally, mixing different forms of RAM is never recommended, as it can cause oxidization and eventual failure of the memory chip.

Some Static About SRAM

Static Random Access Memory is exactly what it states, a static form of RAM.  Therefore, it doesn't have the refresh problems that DRAM does.  As long as electricity continues to flow, it holds it's data.  It also has incredibly fast access times, in the range of 15 to 20 nanoseconds.  Unfortunately, SRAM is larger then DRAM, and is therefore more costly and harder to use.  This makes SRAM a bad choice for Memory, but great for Cache.

SRAM is either synchronous (Tied to clock speed) or asynchronous. (Independent of clock timing)  It comes in either DIP or COAST (Cache on a Stick) formats.  Most processors currently have SRAM as a cache in several layers built into them, but because of it's size and cost it is not used on other parts of the system.

Cache memory differs from regular memory in that it involves a guess.  If you add 2+2 in your head, you probably come up with 4 right away.  That would probably be caching, because you didn't add 2+2 but pulled the answer from memory.  If you add 168+272, you have to process the answer, and therefore would not have the answer cached.

Cache memory in a computer works almost the same way.  Instead of giving the answer, it pre-selects the question, or program and/or data, and feeds it to the CPU.  It works in the following sequence;

1. The Cache controller guesses what application thread or data the CPU will need next and writes it from DRAM into the cache SRAM.  (The cache controller is generally right about 90% of the time)

2. The CPU asks for the information, as if it's in cache it can access this information 3 to 5 times as fast from cache then from memory.

Level 1 , level 2 and level 3 cache are new forms of cache that are built into systems.  In a system with only 1 level of cache, the cache in on the motherboard.  For 2 level cache systems, level 1 cache is on the processor and level 2 is on the motherboard.  Finally, level 3 cache has cache built into the chip, on an area near the chip, and on the motherboard.

Other Formats Of RAM

There is one more method of installing RAM into a computer.  It involves portable computers, and the use of PCMCIA slots (Personal Computer Memory Card Industry Association slot)  This allows adding of memory (and other peripherals) into a portable computer without physically opening the computer.  PCMCIA slots come in 3 standard sizes; Type I, which is 3.3" and used primarily for RAM, Type II, which is up to 5.5" and used for modems, and Type III, which is 10.5" and used for external disk drives.

Installing SIMMs and DIMMs

SIMMs and DIMMs have a simple commonality; their connections are all along the bottom of their board.  Installing these boards involves placing the board in a special socket and snapping it into place.  Once you have determined the right type of memory to install, follow the instructions below;  (As always, make sure you are grounded before touching any part inside the computer)

SIMMs - Place the SIMM so that it's resting away from the clasps at a 45 degree angle.  There is a shorter end and a longer end on the sides of the board - Make sure you match up the shorter side to the side with the clasps closer to the edge.  Press the board down so that it makes a good connection and you'll see the clasps tighten around when the memory is secured

DIMMs - DIMMs are installed at a 90 degree angle to the board.  Some SIMMs are installed at angles, but DIMMs are always straight up-and-down.  Make sure that the plastic clasps on the sides of the DIMM slot are pushed out, and line up the board with the socket.  Push down straight, and when the memory is in place the plastic clasps will push into the gaps on the board.

In Conclusion...

You'll see more information about memory management than about memory types.  It is still important to know the differences, as older computers may use proprietary RAM, and knowing the difference is vital.