PM0044 datasheet(13/162 Pages) STMICROELECTRONICS

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PM0044

STM8 core description

Doc ID 13590 Rev 3

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STM8 core description

3.1

Introduction

The CPU has a full 8-bit architecture, with 16-bit operations on index registers (for address

computation). Six internal registers allow efficient 8-bit data manipulation. The CPU is able

to execute 80 basic instructions. It features 20 addressing modes and can address 6 internal

registers and 16 Mbytes of memory/peripheral registers.

3.2

CPU registers

The 6 CPU registers are shown in the programming model in Figure 1. Following an

interrupt, the register context is saved. The context is saved by pushing registers onto the

stack in the order shown in Figure 2. They are popped from the stack in the reverse order.

Accumulator (A)

The accumulator is an 8-bit general purpose register used to hold operands and the results

of the arithmetic and logic calculations as well as data manipulations.

Index registers (X and Y)

These 16-bit registers are used to create effective addresses or as temporary storage area

for data manipulations. In most of the cases, the cross assembler generates a PRECODE

instruction (PRE) to indicate that the following instruction refers to the Y register. Both X and

Y are automatically saved on interrupt routine branch.

Program Counter (PC)

The program counter is a 24-bit register used to store the address of the next instruction to

be executed by the CPU. It is automatically refreshed after each processed instruction. As a

result, the STM8 core can access up to 16-Mbytes of memory.

Figure 1.

Programming model

A ACCUMULATOR

SP STACK POINTER

X INDEX

Y INDEX

PC PROGRAM COUNTER

PCH

PCL

CC CODE CONDITION

- I1 H I0 N Z C

PCE

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