RAM

The CPU needs to temporarily store and retrieve large amounts of data during its operation. The amount of data that can be stored on the CPU itself (in CPU registers and CPU L1, L2, L3 caches) is very limited. So, the PC architecture uses RAM (Random Access Memory) modules installed on the motherboard to provide a high-speed data buffer for CPU.

Today, there are two types of RAM that are being used - DDR RAM (DDR SDRAM) and DDR2 RAM (DDR2 SDRAM). DDR RAM (Double Data Rate Synchronous Dynamic RAM) is the older standard and is getting replaced by DDR2 RAM.

SDRAM was the predecessor of DDR RAM and was used by older CPUs such as Intel Pentium-IIIs and early AMD Durons and Athlons. SDRAM stores data in memory cells, which are activated by a clock signal. Data in the memory cells is fetched through a path in the RAM module called the external data bus. DDR-RAM differed from SDRAM because it could fetch twice the amount of data as SDRAM during the same signal cycle - once during the rising phase and once during the falling phase of the signal curve. DDR2 RAM had a data bus that works twice as fast as the memory cells (their clock speed) because it could fetch data from two different memory cells during the same signal cycle. Because the data from different cells need to be synchronised, it had significant latency (delay) issues initially. When DDR2 RAM which operated at higher speeds came out, the latency issue became less of a problem.

RAM operations are managed by a component called memory controller. On AMD CPUs, the memory controller is integrated on the CPU. For Intel CPUs, the memory controller is part of the motherboard. The memory controller is connected to the memory banks by one or two channels. Memory modules can be organised in a single-channel configuration or dual-channel configuration. In a single-channel configuration, any combination of compatible memory modules can be used together but they will use a single channel to the memory controller. In a dual-channel configuration, two identical sets of memory modules are used to employ two separate channels to the memory controller, effectively doubling the memory bandwidth.

Memory modules with different clock speeds can be used together but they will function at the lowest supported speed. Also, SDRAM, DDR RAM and DDR2 RAM cannot be used together. It is always best to buy all memory banks together with same specifications. When you add an extra module as a later upgrade, ensure you buy modules with equivalent (or, if there is no choice, higher) ratings.

The data transfer rate for DDR RAM is twice the memory clock speed because it performs two data fetches per clock cycle. For DDR2 RAM, it is four times the memory clock speed because it fetches twice the data during the same clock cycle as DDR RAM. The peak bandwidth of DDR2 RAM is calculated by the width of the data channel between the memory (RAM) and memory controller. Today, memory channels are 64 bits wide. With dual-channel memory configuration, this width is effectively doubled to 128 bits

If you are considering purchasing a new computer/laptop then having the correct amount of RAM would save you the heartache of slow running operating system/programme and having to upgrade.

My recommendation* is:

Windows XP (with service packs) minimum RAM 512MB

Windows Vista (with service packs) minimum RAM 1GB

* for Microsoft advice please see their website