83C795 Datasheet, PDF (60/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

HOST INTERFACE SECTION
83C795
B ecaus e of the 16-bit memory mechanis m
employed in IS A bus es , do not allow the memory
window to overlap the DF F F F H to E 0000H
boundary when us ing 16-bit memory. If the
boundary cros s es , hos t memory acces s es to
portions above E 0000H are made into 8-bit cycles .
Note
Use caution when overlapping the
ROM and RAM windows if the RAM is
16-bit wide and both windows are en-
abled. With 16-bit access enabled, the
M16CS is asserted for all accesses
within the same 128K address block as
the RAM base window address. Should
the access actually be intended to the
ROM, the host falsely expects the ROM
to return 16-bit data. THIS CAN
CRASH THE OPERATING SYSTEM.
To avoid such problems on 16-bit
boards, copy ROM code to system
RAM and disable the ROM window.
Alternatively, map the ROM into other
128K address block.
Program control enables buffer memory decoding
through a regis ter in the host interface section.
When connectedtoa16-bit bus , this comparis on is
qualified by ME MR , ME MW, and the inverse of
AE N. When connected to an 8-bit bus , the
qualification is by S ME MR , S ME MW, and the
invers e of AE N. T he buffer memory window s ize is
program-s electable as 8K, 16K, 32K, 64K bytes or
dis abled. T hebuffer bas eaddres s can bes et toany
8K boundary from C0000H through E E 000H.
B y s etting a bit in the R AM Addres s R egis ter
(R AR .HR AM), the decoded buffer range can be
changed to the range F C0000h - F E E 000h. (T his
range is not pos sible when us ing Plug and Play.)
When connected to a PC/XT bus having no LA
lines , it is required that the BPR .M16E N be kept a
zero (inactive).
T he IPL R OM decoding is enabled by program
control through theBIOregister in thehos t interface
s ection. Decoder qualification is by S ME MR andan
inverted AE N. R OM window s iz e is program
s electable from 8K, 16K, 32K bytes or dis abled.
T heR OMwindowplacementin hos t memory s pace
FIGURE 6-2. OVERLAPPING ADDRESS STRUCTURE
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