ST6391 datasheet(18/68 Pages) STMICROELECTRONICS

18 / 68 page

MEMORY SPACES (Continued)

Data ROM Addressing. All the read-only data are

physically implemented in the ROM in which the Pro-

gram Space is also implemented. The ROM therefore

containsthe program to be executedandalso the con-

stants and the look-up tables neededfor the program.

The locationsof Data Space in which the differentcon-

stants and look-up tables are addressed by the

ST639x Core can be considered 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 lo-

cated from the 40h address to the 7Fh address in the

Data space and allows 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 instructions or read-only data. Indeed,the win-

dow 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 ef-

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

ROM is obtained by the concatenation 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 ROM is 00h.

Note. The data ROM window cannot address win-

dows above the 16k byte range.

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

DWR7 = Data ROM Window 7

Figure 16. Data ROM Window Register

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 17. Data ROM Window Memory Addressing

DWR7-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. 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 inter-

rupts drivers, as the driver cannot save and than re-

store its previous content. If it is impossible to avoid

the writing of this register in interrupts drivers, an im-

age 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 image register

must be written first, so if an interrupt occurs be-

tween the two instructions the DRWR register is not

affected.

ST6391,92,93,95,97,99

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