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DigiBoard Intelligent Serial Communications Boards
Appendix—Memory Conflicts
Contention for Memory Addresses
Many intelligent peripheral devices require a block of vacant memory addresses
on the host computer’s bus for their own use. These memory addresses may be
used to access a dual-ported RAM on the peripheral device to facilitate the
transfer of large volumes of data between the host and the peripheral (memory-
to-memory transfers are much faster than I/O bus transfers), or they may point to
a ROM BIOS (a firmware program used by the host computer to control the
peripheral device). In either case, the host computer sees this memory as its
own, and can use the same high-speed memory-referencing commands that it
uses with its internal memory.
Figure 39 on the following page shows a map of the first megabyte of a host
system’s memory bus. The shaded areas represent the computer’s internal RAM
(nearly always 640K, but can often be reduced to 512K—there is also usually
more RAM above the 1 Meg boundary). The area between the RAM and the 1
Meg boundary is used for the host system’s BIOS, option ROMs and peripheral
memory. The memory map shows the locations typically used by some of the
most common peripheral devices.
The memory map on the following page shows typical memory usage. To be
certain of what areas are available in your system, you will need to consult the
documentation for your computer and peripheral devices.
