A+ Certification/Exam Objectives/Hardware/Basics/Random Access Memory
Random Access Memory (RAM) is categorized into the following types:
Types of RAM
SRAM -- Static Random Access Memory. From the book bag analogy SRAM would mostly relate to a small notepad. One of the major features of SRAM is that it never has to be refreshed so data never changes; also SRAM is faster than DRAM. The major drawback however is price, although chip prices have continued to fall, SRAM is still more expensive than DRAM. The most common area SRAM is implemented in the CPU cache where memory doesn’t need to be refreshed often and speed is a factor, and where small quantities are economical. Classic Pentiums and 486 CPUs had SRAM on the motherboard. No memory addresses are assigned in this memory. The 'S' in SDRAM does not mean Static, but Synchronous. SDRAM stands for Synchronous Dynamic RAM.
DRAM -- Dynamic Random Access Memory. In the book bag analogy, DRAM would be scratch paper. DRAM is the primary memory used in computers today. The advantage of DRAM is that it is very dense, more and more memory is being crammed into chips every day, and getting cheaper at the same time. The main disadvantage is that it is slower, and has to be refreshed often. Today the most common found ram is SDRAM, Synchronous Dynamic Random Access Memory. The now more common variant of SDRAM is DDR which allows two times the data to be moved in a single cycle, and has dual channel ability. A less common, and now rarely used type of DRAM is RDRAM created by Rambus. Although faster than DDR, the industry decided to use DDR due to licensing issues and overall cost.
Video RAM memory is not used as much today as it once was due to the fact that most PC comes with integrated graphics that uses system ram to store display information. Most high-end video graphics cards contain VRAM. VRAM deals with only the display of the bitmap graphics. It can be read and manipulated at the same time, making it faster than ordinary RAM. Because of these enhancements, they are extremely expensive and thus most integrated graphics do not come with VRAM.
RAM Form Factors
RAM comes in many forms; the certification test has questions on legacy and current ram form factors. The following list of form factors have been used in the past:
|FPM (Fast Page Mode)||A "row" of DRam can be kept open by multiple reads or writes, making bursts of data faster.|
|EDO (Extended Data Out)||Begins data output on falling edge of /CAS.|
|SDRAM (Synchronous Dynamic RAM)|
|RDRAM (Rambus Dynamic RAM)||This type also requires a RAMBUS Continuity-RIMM, a terminator or dummy.|
|DDR (Double Data Rate SDRAM)||100 MHz - 200 MHz (PC-1600 -> PC3200)|
|DDR2||100 MHz - 266 MHz (PC2-3200 -> PC2-8600)|
|DDR3||100 MHz - 200MHz (PC-6400 -> PC3-12800)|
Graphics card memory is permanently attached to the graphics card.
- Real Mode is the main mode that was used in early PCs, it is still used today, during booting on a normal 32-bit pc. When windows starts, protected mode kicks in, and real mode is turned off. Real mode was introduced with the 16-bit 286 processor, in real mode, 1 Megabyte of memory could be addressed at most.
- Protected Mode is another mode that was introduced with the 286, allowing ***, protected mode mostly turned on when the operating system is loaded on the system. Protected mode when introduced was only exploited on windows 3.0 and up, however, other OSes also use protected mode. The main function of this mode is to give the ability to break the 1 MB barrier, and allow more memory to be addressed by the computer. Protected mode has changed sinced its initial introduction on the 286, on the 386 for instance, there was 'standard' and 'enhanced', standard allowed up to 16 bits to be addressed, while enhanced allowed 32 bits to be addressed.