ST72561K9-Auto datasheet(36/324 Pages) STMICROELECTRONICS

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Central processing unit

ST72561-Auto

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Doc ID 12370 Rev 8

Central processing unit

4.1

Introduction

This CPU has a full 8-bit architecture and contains six internal registers allowing efficient

8-bit data manipulation.

4.2

Main features

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Enable executing 63 basic instructions

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Fast 8-bit by 8-bit multiply

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17 main addressing modes (with indirect addressing mode)

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Two 8-bit index registers

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16-bit stack pointer

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Low power HALT and WAIT modes

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Priority maskable hardware interrupts

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Non-maskable software/hardware interrupts

4.3

CPU registers

The six CPU registers shown in Figure 8 are not present in the memory mapping and are

accessed by specific instructions.

4.3.1

Accumulator (A)

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

of the arithmetic and logic calculations and to manipulate data.

4.3.2

Index registers (X and Y)

These 8-bit registers are used to create effective addresses or as temporary storage areas

for data manipulation. (The Cross-Assembler generates a precede instruction (PRE) to

indicate that the following instruction refers to the Y register.)

The Y register is not affected by the interrupt automatic procedures.

4.3.3

Program counter (PC)

The program counter is a 16-bit register containing the address of the next instruction to be

executed by the CPU. It is made of two 8-bit registers PCL (Program Counter Low which is

the LSB) and PCH (Program Counter High which is the MSB).

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