CST12.0MTWA datasheet(23/153 Pages) STMICROELECTRONICS

23 / 153 page

ST72334J/N, ST72314J/N, ST72124J

23/153

8 CENTRAL PROCESSING UNIT

8.1 INTRODUCTION

This CPU has a full 8-bit architecture and contains

six internal registers allowing efficient 8-bit data

manipulation.

8.2 MAIN FEATURES

63 basic instructions

Fast 8-bit by 8-bit multiply

17 main addressing modes

Two 8-bit index registers

16-bit stack pointer

Low power modes

Maskable hardware interrupts

Non-maskable software interrupt

8.3 CPU REGISTERS

The 6 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)

In indexed addressing modes, these 8-bit registers

are used to create either effective addresses or

temporary storage areas for data manipulation.

(The Cross-Assembler generates a precede in-

struction (PRE) to indicate that the following in-

struction refers to the Y register.)

The Y register is not affected by the interrupt auto-

matic procedures (not pushed to and popped from

the stack).

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

11 H I

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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