TYPES OF RAM

There are two types of random access memory, which are as follows:

• Static RAM (SRAM): This RAM retains the data as long as power is provided to the memory chip. It need not be 'refreshed' (pulse of current through all the memory cells) periodically. It is very fast but much more expensive than DRAM. SRAM is often used as cache memory due to its high speed.
• Dynamic RAM (DRAM): This RAM, unlike SRAM, must be continually refreshed to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. DRAM is used for most system memory because it is cheap and small 

Maximum memory supported by operating system

Memory upgrades for Windows 8 Editions
Windows 8 has a minimum memory requirement of 1 GB RAM (32-bit) or 2 GB RAM (64-bit). Information about the maximum amount of memory is not available at this time.
Memory upgrades for Windows 7 Editions

Windows 7 has a minimum memory requirement of 1 GB RAM (32-bit) or 2 GB RAM (64-bit). The maximum amount of memory depends on the edition used:

Edition of Windows 7	Maximum addressable memory
Starter (32-bit or 64-bit)	2 GB
Any 32-bit version of 7 (except Starter)	4 GB (approximately 3.3 GB available for use)
Home Basic 64-bit	8 GB
Home Premium 64-bit	16 GB
Enterprise 64-bit	192 GB
Business 64-bit	192 GB
Ultimate 64-bit	192 GB

Memory upgrades for Windows Vista Editions

Windows Vista comes in a variety of editions. All versions of Vista have a minimum memory requirement of 512 MB (1 GB to take advantage of certain premium features, such as Aero graphics). The maximum amount of memory depends on the edition used:

Edition of Windows Vista	Maximum addressable memory
Starter (32-bit)	1 GB
Any 32-bit version of Vista	4 GB (approximately 3.3 GB available for use)
Home Basic 64-bit	8 GB
Home Premium 64-bit	16 GB
Business 64-bit	128 GB
Ultimate 64-bit	128 GB

Memory upgrades for Windows XP

Windows XP comes in 32-bit or 64-bit version. The maximum amount of memory depends on the version used:

Version of Windows XP	Maximum addressable memory
32-bit	4 GB (approximately 3.3 GB available for use)
64-bit	128 GB