Memory

Memory

The PC has two basic levels of memory:

l RAM (Random Access Memory): Holds the instructions and data in use by the operating system and software applications before and after they pass to the CPU. RAM is volatile; to hold its contents, it must have a steady power source. Should the power stream be broken, anything stored in RAM is erased. No power--no data; pretty straightforward.

l ROM (Read-Only Memory): Is nonvolatile and retains its contents through even the darkest power outage. For more information on ROM, RAM is by far the faster of the two types of memory. In fact, RAM is often used to shadow the BIOS ROM to improve its performance during the boot process. If you're asked what copying the contents of the BIOS ROM into RAM for faster access to the instructions by the CPU is called--the answer is shadowing.

Tip When volatile memory loses its power source, it also loses its contents. Most types of RAM, especially DRAM, is volatile and that's why when you turn off the PC's power (or trip over the power cord), everything in RAM is lost. Nonvolatile memory, like ROM and other types, do not lose their contents when their power source is lost.

Random Access Memory (RAM)

RAM is the primary memory of the PC and is generally installed directly on the motherboard in a variety of package types, which I deal with later. Many types of RAM exist, and the ones you'll need to know something about for the Core Hardware exam are:

l DRAM (Dynamic Random Access Memory)

l EDO RAM (Extended Data Output RAM)

l RIMM (Rambus Inline Memory Module)

l SRAM (Static RAM)

In addition to the types of RAM used in primary memory, you should also need to know about two of the types of RAM used on video cards:

l VRAM (Video RAM)

l WRAM (Windows Accelerator Card RAM)

You need a solid understanding of RAM, its packaging forms, and its technologies for the exam. Many questions assume you know what the types of RAM are, where they're installed, and their characteristics. For example, you may be asked which is the fastest of the RAM types (SRAM), or how DIMM memories are installed (in vertical sockets on the motherboard).

Dynamic RAM (DRAM)

DRAM (pronounced "dee-ram," but not in the same context as "dee-bears," "dee-blackhawks," or "deebulls") is the RAM everyone talks about. It's the most commonly used type of memory. It's not a complex circuit compared to some and, as a result, is not expensive. However, its design also requires that it be refreshed regularly or it loses its contents. This need for constant refreshing gives DRAM its dynamic tag.

Instant Answer DRAM must be refreshed every two milliseconds. A special refresh logic circuit reads and then rewrites the contents of each DRAM address, whether it's in use or not.

DRAM is also the slowest of the memories, clocking in with access speeds around 50 nanoseconds (ns) or higher (remember that higher here means slower). Older DRAM had an access speed of around 120 ns.

DRAM technologies

Remember DRAM comes in a variety of popular technologies. The following lists the characteristics of each of the DRAM technologies:

l Extended Data Out (EDO): This is the most common type of DRAM. It's common in most Pentium and later PCs, except those with memory buses over 75MHz.

l Fast Page Mode (FPM): This type of DRAM is occasionally called non-EDO RAM. It's generally compatible with motherboards with memory buses with speeds under 66MHz.

l Burst Extended Data Out (BEDO): This DRAM is EDO memory with pipelining technology added for faster access times. BEDO allows much higher bus speeds than EDO.

l Synchronous DRAM (SDRAM): Like its SRAM cousins, SDRAM is tied to the system clock and reads or writes memory in burst mode.

l Rambus DRAM (RDRAM): Rambus is a proprietary DRAM technology developed by Rambus, Inc. ( www.rambus.com ) that has memory speeds of up to 3.2Gbps. RDRAM comes on a module that is very similar to a DIMM, called a RIMM (Rambus Inline Memory Module).

l Synchronous Link DRAM (SLDRAM): This is an enhanced version of SDRAM memory that uses a multiplexed bus to transfer data to and from the chips rather than fixed pin settings.

SLDRAM has transfer rates as high as 3GBps range. Unlike RDRAM, this is an open technology.

l Video RAM (VRAM): This specialized DRAM is used on video cards and not for main memory.

VRAM applies dual porting, which means that it can be written to and read from at the same time.

This allows the processor and the refresh circuitry for the monitor to access VRAM at the same time. Another type of video RAM is Windows RAM (WRAM), also called Windows Accelerator RAM, which has essentially the same properties as VRAM.

You will definitely see the DRAM technologies on the A+ exams. Familiarize yourself with the general descriptions provided here, which should be enough for FPM, EDO, burst EDO, RDRAM, and SDRAM. VRAM and WRAM.

DRAM packages

DRAM memory comes in three package forms:

l DIP (Dual In-line Package)

l SIMM (Single Inline Memory Module)

l DIMM (Dual Inline Memory Module)