The CPU and motherboard architecture (chipset) dictates a particular computer's physical memory capacity and the types and forms of memory that can be installed. Over the years, three primary changes have occurred in computer memory—it has gradually become faster, wider, and larger in capacity. The CPU and the memory controller circuitry dictate the speed, width, and maximum amount supported. The memory controller in a modern PC resides in either the processor or the motherboard chipset. Even though a system might physically support a given maximum amount of memory, the type of software you run may dictate how much memory can actually be used.
We've already discussed memory types, speeds, and widths. Modern memory modules are 64 bits wide, and depending on the memory controller design, they are accessed in single-, dual-, or tri-channel mode. In single-channel mode, the memory is read and written 64 bits at a time, whereas in dual- or tri-channel mode, the memory bus width increases to 128 bits or 192 bits, respectively. With the exception of the ill-fated RDRAM memory type, memory is one of the few components in the PC to remain massively parallel. Most other parts of the PC have transitioned to serial interface designs.
Maximum physical memory capacity is dictated by several factors. The first is the amount addressable by the processor itself, which is based on the number of physical address lines in the chip. The original PC processors (8086/8088) had 20 address lines, which resulted in those chips being able to recognize up to 1MB (2 to the 20th power bytes) of RAM. The 286/386SX increased memory addressing capability to 24 lines, making them capable of addressing 16MB (2 to the 24th power bytes). Modern x86 processors have from 32 to 36 address lines, resulting in from 4GB to 64GB of addressable RAM. Modern x86-64 (64-bit) processors have 40 address lines, resulting in a maximum of 1TB (1 terabyte) of supported physical RAM.
See "Processor Specifications," p. 37 (Chapter 3).
The operating mode of the processor may place further limits on memory addressability. For example, when the processor is operating in backward-compatible real mode, only 1MB of memory is supported.
See "Processor Modes," p. 45 (Chapter 3).
Note that even though modern 64-bit processors can address up to 1TB, modern motherboards and/or chipsets generally limit the maximum amount of RAM to 8GB, 16GB, or 24GB. The type of software also has an effect. The 32-bit versions of Windows XP, Vista, and Windows 7 limit memory support to 4GB, whereas the 64-bit versions limit support to 8GB, 16GB, or 192GB, depending on the edition.