参数资料
型号: ST62T18CB6
厂商: STMICROELECTRONICS
元件分类: 微控制器/微处理器
英文描述: 8-BIT, OTPROM, 8 MHz, MICROCONTROLLER, PDIP20
封装: 0.300 INCH, PLASTIC, DIP-20
文件页数: 3/82页
文件大小: 937K
代理商: ST62T18CB6
Obsolete
Product(s)
- Obsolete
Product(s)
11/82
ST62T18C/E18C
MEMORY MAP (Cont’d)
1.3.5 Data Window Register (DWR)
The Data read-only memory window is located from
address 0040h to address 007Fh in Data space. It
allows direct reading of 64 consecutive bytes locat-
ed anywhere in program memory, between ad-
dress 0000h and 1FFFh (top memory address de-
pends on the specific device). All the program
memory can therefore be used to store either in-
structions or read-only data. Indeed, the window
can be moved in steps of 64 bytes along the pro-
gram memory by writing the appropriate code in the
Data Window Register (DWR).
The DWR can be addressed like any RAM location
in the Data Space, it is however a write-only regis-
ter and therefore cannot be accessed using single-
bit operations. This register is used to position the
64-byte read-only data window (from address 40h
to address 7Fh of the Data space) in program
memory in 64-byte steps. The effective address of
the byte to be read as data in program memory is
obtained by concatenating the 6 least significant
bits of the register address given in the instruction
(as least significant bits) and the content of the
DWR register (as most significant bits), as illustrat-
ed in Figure 5 below. For instance, when address-
ing location 0040h of the Data Space, with 00h
loaded in the DWR register, the physical location
addressed in program memory is 00h. The DWR
register is not cleared on reset, therefore it must
be written to prior to the first access to the Data
read-only memory window area.
Data Window Register (DWR)
Address: 0C9h
Write Only
Bits 7 = Not used.
Bit 6-0 = DWR6-DWR0:
Data read-only memory
Window Register Bits. These are the Data read-
only memory Window bits that correspond to the
upper bits of the data read-only memory space.
Caution:
This register is undefined on reset. Nei-
ther read nor single bit instructions may be used to
address this register.
Note: Care is required when handling the DWR
register as it is write only. For this reason, the
DWR contents should not be changed while exe-
cuting an interrupt service routine, as the service
routine cannot save and then restore the register’s
previous contents. If it is impossible to avoid writ-
ing to the DWR during the interrupt service routine,
an image of the register must be saved in a RAM
location, and each time the program writes to the
DWR, it must also write to the image register. The
image register must be written first so that, if an in-
terrupt occurs between the two instructions, the
DWR is not affected.
Figure 5. Data read-only memory Window Memory Addressing
70
-
DWR6 DWR5 DWR4 DWR3 DWR2 DWR1 DWR0
DATA ROM
WINDOW REGISTER
CONTENTS
DATA SPACE ADDRESS
40h-7Fh
IN INSTRUCTION
PROGRAM SPACE ADDRESS
7654
32
0
5
43210
5
43210
READ
1
6
7
8
9
10
11
01
VR01573A
12
1
0
DATA SPACE ADDRESS
59h
0
1
00
1
Example:
(DWR)
DWR=28h
11
0000
0
00
1
ROM
ADDRESS:A19h
11
13
0
1
11
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