A PC's third and principal level of system memory is referred to as main memory, or Random Access Memory (RAM). It is an impermanent source of data, but is the main memory area accessed by the hard disk. It acts, so to speak, as a staging post between the hard disk and the processor. The more data it is possible to have available in the RAM the faster the PC will run.
Main memory is attached to the processor via its address and data buses. Each bus consists of a number of electrical circuits or bits. The width of the address bus dictates how many different memory locations can be accessed and the width of the data bus how much information is stored at each location. Every time a bit is added to the width of the address bus, the address range doubles. In 1985, Intel's 386 processor had a 32-bit address bus, enabling it to access up to 4GB of memory. The Pentium processor - introduced in 1993 - increased the data bus width to 64-bits, enabling it to access 8 bytes of data at a time.
Each transaction between the CPU and memory is called a bus cycle. The number of data bits a CPU is able to transfer during a single bus cycle affects a computer's performance and dictates what type of memory the computer requires. By the late 1990s, most desktop computers were using 168-pin DIMMs, which supported 64-bit data paths.
Main memory is built up using DRAM chips, short for Dynamic RAM.