ST63156 datasheet(17/86 Pages) STMICROELECTRONICS

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DWR

Data ROM Window Register

(C9h, Write Only)

D7 D6 D5 D4 D3 D2 D1 D0

DWR0 = Data ROM Window 0

DWR1 = Data ROM Window 1

DWR2 = Data ROM Window 2

DWR3 = Data ROM Window 3

DWR4 = Data ROM Window 4

DWR5 = Data ROM Window 5

DWR6 = Data ROM Window 6

UNUSED

Figure 14. Data ROM Window Register

Data ROM Addressing. All the read-only data are

physically implemented in the ROM in which the

Program Space is also implemented. The ROM

therefore contains the program to be executed and

also the constants and the look-up tables needed

for the program. The locations of Data Space in

which the different constants and look-up tables

are addressed by the ST631xx Core can be con-

sidered as being a 64-byte window through which

it is possible to access to the read-only data stored

in the ROM. This window is located from address

40H to address 7Fh in the Data space and allows

the direct reading of the bytes from the address

000h to address 03Fh in the ROM. All the bytes of

the ROM can be used to store either instructions 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 Win-

dow register (DRWR, location C9h). The effective

address of the byte to be read as a data in the ROM

is obtained by the concatenation of the 6 less sig-

nificant 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 lo-

cation 40h of data space, and 0 is loaded in the

DRWR, the physical addressed location in ROM is

00h.

D7. This bit is not used.

DWR6-DWR0. These are the Data Rom Window bits

thatcorrespondto theupperbits of data ROM program

space. This register is undefined after reset.

This register is undefined on reset. Neither

read nor single bit instructions may be used to

address this register.

Note. Care is required when handling the DRWR

as it is write only. For this reason, it is not allowed

to change the DRWR contents while executing in-

terrupts drivers, as the driver cannot save and than

restore its previous content. If it is impossible to

avoid the writing of this register in interrupts driv-

ers, an image of this register must be saved in a

RAM location, and each time the program writes

the DRWR it writes also the image register. The im-

age register must be written first, so if an interrupt

occurs between the two instructions the DRWR

MEMORY SPACES (Continued)

DATA ROM

WINDOW REGISTER

CONTENTS

DATA SPACE ADDRESS

40h-7Fh

IN INSTRUCTION

PROGRAM SPACE ADDRESS

65432

543

210

543

210

READ

VR01573B

DATA SPACE ADDRESS

59h

Example:

(DWR)

DWR=28h

ROM

ADDRESS:A19h

Figure 15. Data ROM Window Memory Addressing

ST63140,142,126,156

13/82

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