ST6369 datasheet(13/71 Pages) STMICROELECTRONICS

13 / 71 page

MEMORY SPACES

The MCUs operate in three different memory

spaces: Stack Space, Program Space and Data

Space. A description of these spaces is shown in

Figure 9.

Stack Space

The stack space consists of six 12 bit registers that

are used for stacking subroutine and interrupt return

addressesplus the current program counterregister.

Program Space

The program space is physically implemented in

the ROM and includes all the instructions that are

to be executed, as well as the data required for the

immediate addressing mode instructions, the re-

served test area and user vectors. It is addressed

thanks to the 12-bit Program Counter register (PC

dress up to 4K bytes of Program Space. Neverthe-

less, the Program Space can be extended by

the addition of 2-Kbyte ROM banks as it is shown

in Figure 11 in which a 8K bytes memory is de-

scribed. These banks are addressed by pointing to

the 000H-7FFH locations of the Program Space

thanks to the Program Counter, and by writing the

appropriate code in the Program ROM Page Reg-

ister (PRPR) located at the CAH address of the

Data Space. Because interrupts and common sub-

PROGRAM SPACE

VR001568

I N TERRUPT &

R ESET VEC TOR S

AC C UM U L ATOR

WR EGIST ER

RA M

D ATA ROM

WI NDOW

R AM / EEPR OM

BAN KI N G AR EA

DATA SPACE

DATA RAM

BA NK S EL ECT

DA TA R OM

W I ND OW SELE CT

VR EGI STER

YR EGI STER

XR EGI STER

0-63

00 00h

07FFh

08 00h

0FF0h

0FFFh

000h

03Fh

040h

070h

080h

081h

082h

083h

084h

0FFh

0C0h

ROM

STACK LEVE L 1

STACK LEVE L 2

STACK LEVE L 3

STACK LEVE L 4

STACK LEVE L 5

STACK LEVE L 6

PROGRAM COUNTER

STACK SPACE

Figure 9. Memory Addressing Description Diagram

routines should be available all the time only the

lower 2K byte of the 4K program space are bank

switched while the upper 2K byte can be seen as

static space. Table 3 gives the different codes that

allows the selection of the corresponding banks.

Note that, from the memory point of view, the Page

1 and the Static Page represent the same physical

memory: it is only a different way of addressing the

same location.

Program

counter

space

0000H

1FFFH

0FFFH

Static Page

Page 1

0800H

07FFH

Page 0

Page 1

Static Page

Page 2

Page 3

0000H

Figure 10. 8K Bytes Program Space Address-

ing Description

ST6369

9/67

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