Understanding Computer Memory - Part 4

Rambus Memory was used years ago in the old Pentium 4 days on specialty computers. It was also known “RIMM” (Rambus Inline Memory Module), or RDRAM. There were generally 2 types of RIMMS, a 16 bit version with a 184Pin connector, and a 32 bit version with 232Pins. A major problem with motherboards that used this type of RAM (which was very few actually) was that all RAM slots on the board must be filled! If you had 2 RIMM slots on the board, and you only needed one RIMM, then you had to install what was called a continuity module in the empty slot or slots, otherwise the computer would not boot and would not function at all. Honestly, you will be hard pressed to come across these types of PC’s today as they are long obsolete. Infact, Of the close to 30,000 hours of working with computers since 2006 I have yet to come across one. You’re more likely to win the powerball. I’ve won a $100.00 or more on scratchoffs more than 100 times, yet I’ve never even seen Rambus module ever. For the exam, just remember what a continuity module is and what it is used for. That’s all you need to know about it. Also, you wont be hearing about it on the new 800 series exams when they hit the market early next year.

Dual Channel Memory:
What is dual channel? Well, a lot of older PC’s had multiple Slots for RAM, but their memory controllers were only capable of addressing one module at a time (single channel). They would read and write from one module, and then the next, repeating this function over and over again. All computers today are dual channel since their memory controllers can address 2 or more modules at a single time. Some PC’s today have tri-channel memory controllers. Beware, most motherboards that support tri-channel memory, will only work in tri-channel mode if there are 3 of the 4 onboard slots filled with modules. These are usually color coded. The 3 black slots are for the tri-channel configuration, and the blue slot is for dual channel. If you have 4 slots and use all four, then you effectively using dual channel and only one set of 2 modules can be addressed at a time. If you use the 3 slots indicated for tri-channel, leaving the blue or last slot blank, then you are in tri-channel mode. Note: when installing RAM it is always best to install the same brand, make, model, speed, etc. Don’t replace one cartridge leaving to old ones to work in tandem with the new one, you will get all kinds of errors and system freezes. Also, many boards will default to single channel mode if the RAM modules are not the same speed!

Error Correcting Code and Parity RAM:
Parity is an older form of error detection used on RAM modules. It works like this, memory is always read and grouped in bytes (sets of 8 bits), RAM with Parity correction add an extra single bit of data to every 1 byte of data, creating a 9 bit string. The 8 bits are the actual data that’s being processed while the 9th bit is the Parity bit. Normally as data is read and written, a chunk of data will be the equivalent of 9 bits producing an odd valued number. If there’s a problem with memory, such as a memory leak, etc, the parity value may equal an even number instead, causing a “Parity error”, and the system then haults and shutsdown! This type of RAM was primarily used in computer systems and servers where there was just no room for error, such as banking, government, and other industries where data integrity must be kept in place no matter what. As we’ve stated, one bit (the parity bit) is added to each 8 bits, so the actual data bus of the RAM is different then compared to standard non-Parity RAM. For instance, a normal 64bit RAM module would be 64bits, while a Parity RAM module would actually be 72 bits. Remember that we added an extra single bit of data to each 8 bits (the Parity bit), so instead of transferring data 64 bits at a time, we are actually transferring 72 bits. 8 bits X 9 (the extra bit) = 72 instead of 64. The problem with this type of memory is that it provides “error detection”, but there’s no error correction.

Continue to page 5 (RAM)