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An address bus is a bus that is used to specify a physical address. When a processor or DMA-enabled device needs to read or write to a memory location, it specifies that memory location on the address bus (the value to be read or written is sent on the data bus). The width of the address bus determines the amount of memory a system can address.
For example, the Pentium Pro processor is a 32-bit machine, with 32-bit registers and instructions that manipulate 32-bit quantities, but the external address bus is 36 bits wide, giving a larger address space than 4 GB, and the external data bus is 64 bits wide, primarily in order to permit a more efficient prefetch of instructions and data. [7]
Very often, when referring to the word size of a modern computer, one is also describing the size of address space on that computer. For instance, a computer said to be "32-bit" also usually allows 32-bit memory addresses; a byte-addressable 32-bit computer can address 2 32 = 4,294,967,296 bytes of memory, or 4 gibibytes (GiB). This allows one ...
One of the major improvements the PCI Local Bus had over other I/O architectures was its configuration mechanism. In addition to the normal memory-mapped and I/O port spaces, each device function on the bus has a configuration space, which is 256 bytes long, addressable by knowing the eight-bit PCI bus, five-bit device, and three-bit function numbers for the device (commonly referred to as the ...
Bus width may refer to: Bus § Dimensions, the width of the road vehicle; Bus width, in computer architecture, the amount of data that can be accessed or transmitted ...
A n as in A16, A24, A32 refers to the width of the address On the VME bus, all transfers are DMA and every card is a master or slave. In most bus standards, there is a considerable amount of complexity added in order to support various transfer types and master/slave selection.
Only 24 pins were connected to the address bus, therefore limiting addressing to 16 MB, [d] but this was not a critical constraint at the time. Performance differences were due not only to differing data-bus widths, but also due to performance-enhancing cache memories often employed on boards using the original chip. This version can run 32-bit ...
The address bus is typically a double octet wide, due to practical and economical considerations. This implies a direct address space of 64 KB (65,536 bytes) on most 8-bit processors. Most home computers from the 8-bit era fully exploited the address space, such as the BBC Micro (Model B) with 32 KB of RAM plus 32 KB of ROM.