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PC Memory

RAM (Random Acces Memory):

RAM is volatile memory and does not retain data without power.
RAM contains any active application, including the operating system.

ROM (Read-Only Memory):

ROM is a form of non-volatile memory.
Contains both POST and SETUP.

The data or programming code stored on a ROM (read-only memory) chip cannot be changed (the exception being Flash ROM which can be reprogrammed under certain conditions). The data and programming code stored on a RAM (random-access memory) chip can be changed.

Logical Type of Memory -

Convention or Base Memory

First mb of memory

In MS-DOS referred to first 640k, with the remaining 384 reserved for special uses

Upper Memory or UMA

Top 384K of first MB of memory

Split into 128kb segments for ROM-BIOS, Adapter ROM, and Video ROM

UMB - Upper memory blocks are unused portions of UMA

EMM386 is used to load drivers into UMB

Entended Memory or XMS

Area that extends from the first MB to the top address limit of the CPU

EMS - Expanded Memory Specification - limited to 32mb(now obsolete)

Types of RAM chips
SIP	Single inline package
DIP	Dual inline package
SIMM	Single inline memory module One 72-pin SIMM is required to make a bank in a 386 or 486, two 72-pin SIMM's are required to make a bank in a Pentium or Pentium Pro. A 30-pin SIMM can have as few as two or as many as nine individual DRAM chips. Regardless of the number of DRAM chips, a 30-pin SIMM is 8 bits wide (one byte).
DIMM	Dual inline memory module is similar to a SIMM but contains more memory. A DIMM has 168 pins on the module whereas a SIMM has only 30 or 72 pins

Classes of RAM
DRAM - Dynamic RAM -	Most commonly used, based on concept of a transitor and capacitor used to store one bit of data as an electrical charge. (A charged capacitor is a 1 and discharged one is 0. Like a battery, the capacitor holds a charge and then discharges it.) It cannot hold its data without being constantly refreshed.
SRAM - Static RAM -	Used in cache memory, is preloaded with frequently accessed RAM values and serves as a buffer between the DRAM and the CPU. It can hold its data as long as the power is on.
EDO - Enhanced Data out -	Can simultaneously read new data while discaeding the old
SDRAM - Synchronous DRAM -	similar to EDO, designed to support CPU speeds over 100 mhz
VRAM - Video RAM -	Specifically designed for video display adapters

Cache Memory	- improves performance by storing frequently used data
L1 cache	- internal CPU cache
L2 cache	- on the motherboard between the CPU and main memory banks, speeds up computer processing because SRAM, used by cache memory, is faster than DRAM, used for normal memory.
L3 cache	- software cache of hard drive reads and writes

NVRAM (Non-Volatile Memory):

Can maintain data without the use of power.

The ROM contains the code installed by the manufacturer. The System BIOS is a ROM chip used by the PC during the start up routine to check out the system. Maintains its data with the use of a battery for periods when the machine is powered down.

HMA (or high memory area) is the first 64K of extended memory.

Conventional memory is the first 640K of memory.

Upper memory is the memory between 640K and 1024K. Used to load DOS drivers to allow applications more conventional memory.

Expanded memory is addressed in pages of 16K.

The suspend mode of portables often causes problems with the expanded memory manager.

256 KB or 512 KB of cache is used on a typical system board.

The speed of SIMM memory is measured in nanoseconds (ns) (1 ns = 1 billionth of a second)

EDO (extended data output) memory is faster than other memory because it allows the memory controller to eliminate the 10-ns delay that it would normally wait before it issues the next memory address.

Pipelined burst memory uses more clock cycles per transfer than does burst memory. All types of burst memory are faster than normal memory because they send a burst of data without sending all the addresses of the data, only the first address.

Extended memory is memory that uses memory addresses greater than 1024 KB.

Expanded memory is memory that is not directly assigned system memory addresses, but rather uses a small window of addresses in upper memory that is shared by all of expanded memory.

Shadow RAM speeds up computer performance because programs that are permanently stored in ROM are temporarily copied to RAM, which provides faster access than ROM.

Virtual memory is space on a hard drive that is used as though it is memory. Virtual memory is slower than RAM because accessing a hard drive (which requires a mechanical process) is slower than accessing RAM (which involves no mechanical activity).

LOADHIGH causes the device driver or TSR that is being loaded from AUTOEXEC.BAT to be loaded into upper memory addresses, thus freeing up some of conventional memory.

DEVICEHIGH causes the device driver that is being loaded from CONFIG.SYS to be loaded into upper memory addresses, thus freeing up some of conventional memory.

MEMMAKER configures the OS to make the best use of upper and conventional memory by inserting commands and parameters to commands in the AUTOEXEC.BAT and CONFIG.SYS files.

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Please Read -DISCLAIMER: Technology changes very rapidly. The information presented here was believed to be accurate at the time it was gathered. No claim is made that this information is up to date, or that it represents the current technology used today.

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