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Memory
There are several different types of memory as discussed below:
ROM - ROM stands for "read only memory" and is non-volatile. This means that the information is stored even when the power is turned off to the computer. An example of this would be the computer's BIOS settings that are retained even when the computer is off. Recent advancements in EEPROM technologies have produced Flash ROM chips that can be updated from a disk or over the internet.
RAM - RAM stands for "random access memory" and is volatile. Over the years a variety of memory types have emerged including DIP, SIP, SIMM, DIMM and most recently RIMM.
| Type |
Pins |
| SIMMS |
30/72 pins |
| DIMMS |
168/184 pins |
| micro-DIMMS |
68/144/172 pins |
| SO DIMMS |
72/144/200 pins |
| RIMMS |
184 pins |
- 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.
- Dynamic RAM (DRAM) - DRAM uses capacitors instead of transistors and switches. Needs constant refresh.
- Extended Data Output RAM(EDO RAM) - Has a cache on the chip and is 10-15% faster than DRAM. EDO DRAM doesn't need to be refreshed as often as regular DRAM. Like DRAM, it is now obselete.
- Synchronous DRAM (SDRAM) - SDRAM is tied to the system clock which provides support for the faster bus speeds of modern computers.
- Rambus DRAM (RDRAM) - RDRAM adds support for even faster bus speeds and first surfaced around the time of the Pentium IV. RDRAM uses RIMMS which require a heat spreader to be attached to the RIMM to deal with its increased heat levels. Unused slots on a RDRAM motherboard must be terminated with a CRIMM in order to function.
- Windows RAM (WRAM) - Specific to speed up graphical windows operations.
- Video RAM (VRAM) - Uses a dual port access system to speed up video operations.
Parity checking adds an extra bit to the data for error detection.
The speed of the memory that you install in your system must be at least as fast as the speed that the system is running on. For example, you can put PC133 RAM into a system that is running at 100 MHZ, although you will not see an addition increase in performance over PC100 RAM. You can sometimes mix speed ratings when installing multiple modules, however, it is not recommended. This can cause the system to lock up or not start at all.
High Memory Area(HMA) is the first 64K of extended memory.
Conventional memory is the first 640K of memory.
Upper memory is between 640K and 1024K. Used to load DOS drivers to allow applications more conventional memory.
Extended memory is memory above 1024K.
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.
Further Reference:
http://www.techtutorials.com/Hardware/Memory/
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