System Bus

What is an address bus?
The address bus is the set of wire traces that is used to identify which address in memory the CPU is accessing.
The number of wire traces in the address bus limits the maximum amount of RAM which the CPU can address.
CPU Address Bus Sizes
CPU Address Bus Size
8086 20 bit
8088 20 bit
80286 24 bit
80386SX 24 bit
80386DX 32 bit
80486SX 32 bit
80486DX 32 bit
Pentium I 32 bit
K6 32 bit
Duron 32 bit
Athlon 32 bit
Athlon XP 32 bit
Celeron 36 bit
Pentium Pro 36 bit
Pentium II 36 bit
Pentium III 36 bit
Pentium 4 36 bit
Athlon 40 bit
Athlon-64 40 bit
Athlon-64 FX 40 bit
Opteron 40 bit
Itanium 44 bit
Itanium 2 44 bit
Address Bus Memory Capacity Chart
Address Bus Size Maximum RAM
20 bits 1MB
24 bits 16MB
32 bits 4GB
36 bits 64GB
40 bits 1TB
44 bits 16TB


What is DMA?
DMA is Direct Memory Access.
DMA is a system that allows memory to be controlled without passing through the CPU.
Under most situations, the CPU handles the movement of memory for security and stability reasons. However, DMA can be enabled for certain operations, such as reading from a CD-ROM, so that the data read from the CD is not passed through the CPU but is instead pulled off of the CD and put directly into a memory location for use.
Although DMA is not always used, it has its uses. When it is enabled, the process of DMA overrides the CPU for handling the memory. This helps to solve some errors that occur for items when they are transferred through the CPU. Such errors include audio and video that is being read off of a CD-ROM. Occasionally, errors occur which can cause audio to be scratchy or video to be distorted. Under situations such as these, DMA can be a useful system since it works around these errors by bypassing the CPU.