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MEMORY SPACES
(Continued)
Data ROMAddressing.
All the read-onlydata are
physically implemented in the ROM in which the
Program Space is also implemented. The ROM
therefore contains theprogramto be executedand
also the constants and thelook-up tablesneeded
for the program. The locations of Data Space in
which the different constants and look-up tables
are addressedby the ST6369 Core can be consid-
ered asbeing a 64-bytewindow through whichit is
possible to access to the read-only data stored in
the ROM.This window is located from the 40Had-
dress tothe 7FHaddressin theData space andal-
lows the direct reading of the bytes from the 000H
address to the 03FH address in the ROM. All the
bytes of the ROM can be used to store either in-
structions or read-only data. Indeed, the window
can be moved by step of 64 bytes along the ROM
in writing the appropriate code in the Write-only
Data ROM Window register (DRWR, location
C9H). The effectiveaddress of the byte to be read
as adataintheROM isobtainedbytheconcatena-
tion of the 6 less significant bits of the address in
the Data Space (as less significant bits) and the
content of the DRWR (as most significant bits). So
when addressing location 40H of data space, and
0 is loaded in the DRWR, the physical addressed
location in ROMis 00H.
DWR6-DWR0.
These are the Data Rom Window
bits that correspond to the upper bits of data ROM
program space.This register is undefined after re-
set.
Note.
CareisrequiredwhenhandlingtheDRWR as
it is write only. For this reason, it is not allowed to
change the DRWR contents while executing inter-
ruptsdrivers, asthe drivercannot saveand thanre-
store its previous content.If it is impossibleto avoid
thewriting ofthisregisterin interruptsdrivers, anim-
ageofthisregistermustbesavedinaRAMlocation,
and each time the program writes the DRWR it
writes also the image register. The image register
must be written first, so if an interrupt occurs be-
tweenthe two instructionsthe DRWR register is not
affected.
DWR
Data ROMWindow Register
(C9H, Write Only)
D7 D6 D5 D4 D3 D2 D1 D0
DWR0 = Data ROMWindow 0
DWR1 = Data ROMWindow 1
DWR2 = Data ROMWindow 2
DWR3 = Data ROMWindow 3
DWR4 = Data ROMWindow 4
DWR5 = Data ROMWindow 5
DWR6 = Data ROMWindow 6
UNUSED
Figure 14. Data ROM Window Register
DATA ROM
WINDOW REGISTER
CONTENTS
(DWR)
DATA SPACE ADDRESS
40h-7Fh
IN INSTRUCTION
PROGRAM SPACE ADDRESS
READ
6
5
4
3
2
0
5
4
3
2
1
0
5
4
3
2
1
0
1
6
7
8
9
10
11
0
1
VR01573B
12
1
0
DATA SPACE ADDRESS
59h
0
0
0
0
0
1
0
0
1
1
1
Example:
DWR=28h
1
1
0
0
0
0
0
0
0
0
1
ROM
ADDRESS:A19h
1
1
13
0
1
7
0
0
Figure15. Data ROM Window Memory Addressing
ST6369
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