ST9291 datasheet(18/20 Pages) STMICROELECTRONICS

18 / 20 page

1.2.1 INTRODUCTION

The memory of the ST9291 is functionally divided

into two areas, the Register File and Memory. The

Memory may optionallybe divided into two spaces,

Program Memory for Program code and Data

Memory for Data.

The memory spaces are selected by the execution

of the SDM and SPM instructions (Set Data Mem-

ory and Set Program Memory, respectively). There

is no need to use either of these instructions again

until the memory area required is to be changed.

1.2.1.1 Program Space

The Program memory space of the ST9291 con-

sists of 48K bytes of on-chip ROM (addressed from

0 to BFFF) and 640 bytes of on-chip RAM (ad-

dressed from FD80h to FFFFh); refer to the mem-

ory map tables and drawing on the following page

for the memory mapping for other ROM sizes. The

first 256 memory locations from address 0 to

00FFh (hexadecimal) hold the Reset Vector, the

Top-Level (Pseudo Non-Maskable) interrupt, the

Divide by Zero Trap vector and, optionally, the

interrupt vector table for use with the on-chip

peripherals and the external interrupt sources.

Each vector is contained in two consecutive byte

locations, the high order address held in the lower

(even) byte, the low order address held in the up-

per (odd) byte, forming the address which is

loaded into the Program Counter when selected

by the interrupt vector provided by the interrupt

source. This should point to the relevant Interrupt

Service routine provided by the User for immedi-

ate response to the interrupt.

1.2.1.2 Data Space

The ST9291 addresses the 640 bytes of on-chip

RAM memory from addresses FD80h to FFFFh in

both Program and Data Space. On-chip general

purpose Registers may be used as additional RAM

memory for minimum chip count systems.

The Data Space is selected by the execution of

the SDM instruction. All subsequ ent memory ref-

erences will access the Data Space. When a

separate Data Space is not required, data may

stored in RAM or ROM memory within the Pro-

gram Space.

ST9291

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