ST72325 datasheet(24/196 Pages) STMICROELECTRONICS

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5 CENTRAL PROCESSING UNIT

5.1 INTRODUCTION

This CPU has a full 8-bit architecture and contains

six internal registers allowing efficient 8-bit data

manipulation.

5.2 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

■ Low power HALT and WAIT modes

■ Priority maskable hardware interrupts

■ Non-maskable software/hardware interrupts

5.3 CPU REGISTERS

The six CPU registers shown in Figure 10 are not

present in the memory mapping and are accessed

by specific instructions.

Accumulator (A)

The Accumulator is an 8-bit general purpose reg-

ister 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 fol-

lowing instruction refers to the Y register.)

The Y register is not affected by the interrupt auto-

matic 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).

Figure 10. CPU Registers

ACCUMULATOR

X INDEX REGISTER

Y INDEX REGISTER

STACK POINTER

CONDITION CODE REGISTER

PROGRAM COUNTER

1I1 H I0 N Z

RESET VALUE = RESET VECTOR @ FFFEh-FFFFh

PCH

PCL

RESET VALUE = STACK HIGHER ADDRESS

RESET VALUE = 1X

11X 1X X

RESET VALUE = XXh

X = Undefined Value

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