An internal channel from the
CPU (Central Processing
memory across which the addresses of data (not the data) are transmitted. The
number of lines (wires) in the address bus determines the amount of memory that
can be directly addressed as each line carries one bit of the address.
example, a 20-line address bus represents the binary number 1,048,576 and
reaches that number of memory bytes (the size of the address bus in the
IBM PC in 1981). A computer with a 32-bit address bus can directly address
4GB of physical memory, while one with 36 bits can address 64GB.
Few people nowadays remember that the IBM PC was
not the first "personal computer" and that MS-DOS was not the first industry
standard operating system. In fact, MS-DOS was but an imperfect copy of the
operating system that really has a claim to that title.
The first generation of personal computers (or microcomputers, as they were
known then) used chips like the Intel 8008, 8080, Zilog Z80, MOS Technology 6502
and Motorola 6800. While some early microcomputers (for example, the Apple II)
used proprietary operating systems, hundreds of different manufacturers licensed
a product called CP/M (as in Control Program / Monitor) made by a company
called Digital Research. Long before the IBM PC and its clones / compatibles,
the CP/M architecture provided for industry standard software that was portable
across hundreds of different brands and models. This was DRI founder Gary
Kildall's main contribution to the software industry. Microsoft simply followed
in DRI's footsteps.