Appendix
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Memory Conflicts
71
Conflicts Between 8-Bit and 16-Bit Memory Devices
The ISA (Industry-Standard Architecture) bus, used in many 80286 (AT
compatible), 80386, 80486 and Pentium based computers, supports 16-bit
memory and peripherals with 16-bit dual-ported memory or ROM. By default,
however, memory devices are considered to be 8-bit devices. 16-bit devices
must declare themselves by raising a flag, so that the host knows whether it is
referencing a byte (8 bits) or a word (16 bits) of memory.
Memory addressing on the ISA bus consists of 2 phases: First the unlatched
address bits (bits A17 to A23) are placed on the bus. Then the latched bits (A00
to A16) are placed on the bus. A 16-bit device must declare itself after the
unlatched bits go on the bus, but before the latched bits go out. Since the
unlatched bits can only resolve 128K blocks of memory, the presence of any 16-
bit device in a 128K segment causes the entire segment to be treated as 16-bit
memory. Thus if an 8-bit device resides in the same 128K block as a 16-bit
device, only even-numbered bytes will be accessible, and the device will fail.
PC/Xi and PC/16e boards are 16-bit devices however, their dual-ported memory
can be turned on or off by writing a 1 or 0 (respectively) to a bit in the I/O port.
This is so multiple boards can use the same memory address space without
conflicting with each other. No more than one board is turned on at any given
time. If all boards are turned off, they become invisible to the computer, and
memory accesses to the 128K block in which they reside will not result in the
16-bit memory flag being raised. In this way, an 8-bit VGA card can reside at
C0000h, a COM/Xi board can be at C8000h and a PC/Xi board can be at
D0000h with no ensuing conflict. (This is generally only an issue when the
board is to be installed below 1 megabyte, where there tends to be crowding of
peripherals due to the limited space available.)
As a rule, DigiBoard’s device drivers for PC/Xi and PC/16e boards always leave
the boards’ memory turned off after any operation. This minimizes the
likelihood of a conflict with an 8-bit device. It does not, however, guarantee
that no conflict will ensue. The only sure way to eliminate conflicts of this type
is to avoid mixing 8-bit and 16-bit devices in the same 128K region.
