ST72344S4 datasheet(36/247 Pages) STMICROELECTRONICS

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Central processing unit

ST72344xx, ST2345xx

36/247

Doc ID 12321 Rev 6

Central processing unit

6.1

Introduction

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

bit data manipulation.

6.2

Main features

●

Enable executing 63 basic instructions

●

Fast 8-bit by 8-bit multiply

●

17 main addressing modes (with indirect addressing mode)

●

Two 8-bit index registers

●

16-bit stack pointer

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Low power Halt and Wait modes

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Priority maskable hardware interrupts

●

Non-maskable software/hardware interrupts

6.3

CPU registers

The six CPU registers shown in Figure 11 are not present in the memory mapping and are

accessed by specific instructions.

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.

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.

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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