83C795 Datasheet, PDF (61/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

83C795
HOST INTERFACE SECTION
is programmableon any 8K boundary fromC0000H
to E E 000H. Where R OM and R AM decoding
overlaps , R OM takes precedence. F igure 6-2
s hows how the overlapping addres s s tructure
works .
You can relocate R AM and R OM bas e addres s es
in tandemabove1M(100000H) with theapplication
of an external cas caded addres s decoder. T his is
illustrated in F igure 6-3.
Note
Relocation of the memory windows in
this way is not supported when Plug
and Play is enabled.
6.3.1 Memory Address Generation
R OMs and buffer memories larger than their
programmed windows can be scrolled (paged) by
us ing programmable addres s modifiers (adders )
that lie between the host addres s and the R OM or
buffer memory. T hese addres s modifiers can add
independently toeither addres s any of thefollowing
values and expos e different parts of the targeted
memory within the window:
0000H
2000H
4000H
6000H
8000H
A000H
C000H
E000H
T helower addres s lines (S A12-0) fromthe hos t are
multiplexedwith theDMAaddress lines togenerate
part of the memory addres s . T wo potential memory
addres s es are generated by s ubtracting the
res pective R AM and R OM bas e addres s bits
(17-13) fr om S A17-S A13, then adding the
R P15-R P13 field of the CR R egister for the R AM
FIGURE 6-3. EXTERNAL CASCADED ADDRESS DECODER
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