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DigiBoard Intelligent Serial Communications Boards
PC/Xi and PC/16e Issues
PC/Xi boards can be ordered with 128K, 256K or 512K of RAM. PC/Xi boards
with 128K of RAM can be configured for 64K operation by changing a jumper
setting on the board see page. PC/16e boards are only available with 64K of
RAM. Only the 64K and 128K configurations can be addressed in the region
below 1 megabyte.
The 128K PC/Xi board’s dual-ported memory can be addressed at 80000h or
C0000h (below 1 megabyte), or at one of several addresses between the 1
megabyte and 16 megabyte boundaries. 256K and 512K PC/Xi boards can only
be addressed above 1 megabyte. Most operating systems allow boards with
dual-ported memory to use addresses above the 1 megabyte boundary. These
addresses cannot, however, be used when installing the board in a computer with
16 megabytes or more of memory. Also, some cache controllers interfere with
the correct operation of peripherals with dual-ported memory addressed above 1
megabyte (they don’t affect peripherals addressed below 1 megabyte). If your
computer has memory caching, you may have to either disable the cache or
address the PC/Xi below 1 megabyte.
As mentioned above, 128K PC/Xi boards can only be installed in two address
ranges below the 1 megabyte boundary: 80000h- 9FFFFh or C0000h-DFFFFh.
There is always some sort of video memory between A0000h and BFFFFh, and
the system BIOS at F0000h makes the E0000h-FFFFFh range unusable for 128K
boards. Further, it will probably be necessary to sacrifice something in order to
install a 128K board at 80000h or C0000h. To install a board at 80000h, you
must have no more than 512K of system memory in the first megabyte. If you
have a 640K motherboard you must be able to disable the last 128K and make it
into a 512K motherboard. This usually isn’t as serious as it sounds in multi-user
systems with lots of extended memory, but may cause DOS application
programs to fail due to insufficient memory. To install a 128K board at C0000h,
you need to clear the C0000h-DFFFFh range of other devices which have
memory in that range. This includes (but is not limited to) plug-in EGA and
VGA video adapters, ESDI hard disk controllers, SCSI interfaces, many LAN
adapters, etc.
MS-DOS does not permit addressing above 1 megabyte. Therefore, PC/Xi
boards must be installed below 1 megabyte. This eliminates the 256K and 512K
versions, because they would overlap the entire video memory, the system
BIOS, or both. It is recommended that only 64K PC/Xi boards be used in DOS
systems.
64K PC/Xi boards (and PC/16e boards) offer more flexibility of addressing than
128K boards. Conflicts with VGA, ESDI and SCSI interfaces can usually be
avoided by installing the boards at D0000h (up to four boards may be installed
with the same memory address range), and A0000h-AFFFFh may be available in
monochrome systems.
