ST6391 datasheet(15/68 Pages) STMICROELECTRONICS

15 / 68 page

D7-D5. These bits are not used.

PRPR4-PRPR0. These are the program ROM

banking bits and the value loaded selects the cor-

responding page to be addressed in the lower part

of 4K program address space as specified in Table

3. This register is undefined on reset.

Note. The number of bits implemented depends

on the size of the ROM of the device. Only the

lower part of address space has been bank-

switched because interrupt vectors and common

subroutines should be available all the time. The

reason of this structure is due to the fact that it is

not possible to jump from a dynamic page to an-

other, unless jumping back to the static page,

changing contents of PRPR, and, than, jumping to

a different dynamic page.

Care is required when handling the PRPR as it is

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

change the PRPR contents while executing inter-

rupts drivers, as the driver cannot save and than

restore its previous content. Anyway, this opera-

tion may be necessary if the sum of common rou-

tines and interrupt drivers will take more than 2K

bytes; in this case could be necessary to divide the

PRPR

Program ROM Page Register

(CAh, Write Only)

D7 D6 D5 D4 D3 D2 D1 D0

PRPR0

PRPR1

PRPR2

PRPR3

UNUSED

Figure 13. Program ROM Page Register

PRPR3

PRPR2

PRPR1

PRPR0

PC11

Memory

Page

XXX

Static

Page

(Page 1)

000

Page 0

000

Page 1

(Static

Page)

001

Page 2

001

Page 3

010

Page 4

010

Page 5

011

Page 6

011

Page 7

100

Page 8

100

Page 9

Table 3. ST639x Program ROM Page Register

Coding

MEMORY SPACES (Continued)

interrupt driver in a (minor) part in the static page

(start and end), and in the second (major) part in

one dynamic page. 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 loca-

tion, and each time the program writes the PRPR it

writes also the image register. The image register

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

tween the two instructions the PRPR is not af-

fected.

ST6391,92,93,95,97,99

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