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A+ Study Guide: Domain 1.0: Personal Computer Components: Memory


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Contents:
Introduction
Types of RAM
Physical Types
Important Terms
Installing RAM
Virtual Memory

Introduction:
RAM stands for "random access memory" and is volatile. When an application is launched, it is placed in RAM for faster access by the CPU. It is one of the main components that affects system performance. Over the years a variety of memory types have emerged including DIP, SIP, SIMM, DIMM and most recently RIMM.

Types of RAM:
  • Static RAM (SRAM) - SRAM doesn’t have to be constantly refreshed. Uses a lot of power. Used in old IBM XT machines and was limited to 256K per chip. This type of memory is no longer used and has been replaced by DRAM.
  • Dynamic RAM (DRAM) - DRAM uses capacitors instead of transistors and switches. Needs constant refreshing. This type of memory is still in use, however, has undergone upgrades such as the SDRAM and RDRAM varieties below.
  • Synchronous DRAM (SDRAM) - SDRAM is tied to the system clock which provides support for the faster bus speeds of modern computers. The speed of the memory that you install in a system must match or exceed the system speed in order to work. Installing RAM that is faster than the system speed will operate at the system speed. For example, if you put PC 133 RAM into a system that is running at 100 MHZ, the RAM would operate at the 100 MHz speed. You can sometimes mix speed ratings when installing multiple modules, however, it is not recommended because it can cause the system to lock up or not start at all.
  • Rambus DRAM (RDRAM) - RDRAM added support for even faster bus speeds and first surfaced around the time of the Pentium IV. RDRAM used RIMMS which required a heat spreader to be attached to the RIMM to deal with its increased heat levels. Unused slots on a RDRAM motherboard had to be terminated with a CRIMM in order to function. Due to the higher cost and the licensing fees that manufacturers had to pay to Rambus, RDRAM never took off.
  • Double Data Rate Synchronous DRAM (DDR SDRAM) - With the failure of RDRAM and increasing bus speeds, manufacturers still needed an upgrade to regular SDRAM and DDR SDRAM was born. Like RDRAM, DDR SDRAM increases performance by supporting 2 processes per clock cycle. DDR SDRAM utilizes 184 pin DIMMS for desktops and either 200 pin SO DIMMS or 172 pin micro-DIMMS for laptops. The naming convention for this type of memory is PCxxxx. It is calculated by taking the clock speed, doubling it (double data rate), and multiply it by 8 (the number of RAM chips on a stick). So if a module has a 200 MHz clock speed, the name would be PC3200. Like RDRAM, you must have 2 identical sticks of RAM installed as a pair. Unused slots do not need terminating though. Recently, DDR2 SDRAM was created. DDR2 clock doubles the input/output circuits on the chips, but does not actually increase the core speed of the RAM.
  • Windows RAM (WRAM) - Specifically designed to speed up graphical windows operations.
  • Video RAM (VRAM) - Uses a dual port access system to speed up video operations.
Physical Types:
Type Pins
SIMMS 30/72 pins
DIMMS 168/184/240 pins
micro-DIMMS 68/144/172 pins
SO DIMMS 72/144/200 pins
RIMMS 184 pins
Important Terms:
  • Double-sided RAM - Double-sided RAM is a type of memory which has its chips divided into two sides (called "ranks"), only one of which can be seen at a time by the computer. To use the second half of the storage available, the computer must switch to the second rank, and can no longer read or write to the first half until it switches back again. Single-sided RAM refers to a RAM expansion with a single "rank" of chips, which the computer can access all at once. The terms double-sided RAM and single-sided RAM have nothing to do with having physical chips on one or both sides, although that is a common misconception.
  • Parity RAM - RAM occasionally "misfires" and makes mistakes. For home users this isn't a big deal, but for mission critical applications it can be. Parity checking adds an extra bit to the data that the MCC uses for error detection. Parity RAM is unable to correct the errors and doesn't always catch them.
  • Error Correction Code RAM (ECC) - ECC RAM is a high-end type of memory that detects and corrects RAM errors. Due to the expense, this type of RAM is rare and only used in mission critical situations. The motherboard must support ECC in order for it to work.
Installing RAM:
Installing RAM is easy. The hardest part is making sure that you have the right kind and you should always check the motherboard manual for the specs. You should also try not to mix manufacturers, speeds, or capacity when buying multiple sticks or upgrading existing RAM. While it will most likely work, it is better not to do this in order to avoid problems.

Once you have the right kind, intalling the memory stick is as simple as placing it in the slot on the motherboard. Make sure that you handle it from the top and avoid touching the contacts. You may have to push with some force to get it in and to get the tabs on either end of the slot (see image) to lock onto the notches in the ends of the stick, but don't push so hard that you damage the memory or the motherboard. If a memory stick is not going in, make sure it is the right kind - they are keyed so that only the right type of RAM will fit in the slot. In the image to the right, you can see a notch in the middle of the slot - this is the key. Once your RAM is installed, you can boot the computer and watch for the RAM count during startup to make sure that it is being recognized properly. If you miss that, you can always go into the Device Manager in Windows to see how much RAM the system sees.

Virtual Memory:
Protected Mode became available with the 80286 and provided the ability to use Virtual Memory. Virtual Memory is the ability for the computer to use free hard drive space as extra memory. Excessive paging of the hard drive is usually a sign that the system needs more RAM.

Further Reference:
Memory Tutorials





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