STM8S datasheet(23/467 Pages) STMICROELECTRONICS

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Central processing unit (CPU)

1.1

CPU introduction

The CPU has an 8-bit architecture. Six internal registers allow efficient data manipulations.

The CPU is able to execute 80 basic instructions. It features 20 addressing modes and can

address six internal registers. For the complete description of the instruction set, refer to the

STM8 microcontroller family programming manual (PM0044).

1.2

CPU registers

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

interrupt, the registers are pushed onto the stack in the order shown in Figure 2. They are

popped from stack in the reverse order. The interrupt routine must therefore handle it, if

needed, through the POP and PUSH instructions.

1.2.1

Description of CPU registers

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 are 16-bit registers used to create effective addresses. They may also be used as a

temporary storage area for data manipulations and have an inherent use for some

instructions (multiplication/division). In most cases, the cross assembler generates a

PRECODE instruction (PRE) to indicate that the following instruction refers to the Y register.

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.

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