ST72E121 datasheet(12/92 Pages) STMICROELECTRONICS

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ST72E121 ST72T121

2 CENTRAL PROCESSING UNIT

2.1 INTRODUCTION

This CPU has a full 8-bit architecture and contains

six internal registers allowing efficient 8-bit data

manipulation.

2.2 MAIN FEATURES

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

8 MHz CPU internal frequency

Low power modes

Maskable hardware interrupts

Non-maskable software interrupt

2.3 CPU REGISTERS

The 6 CPU registers shown in Figure 6 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 6. CPU Registers

ACCUMULATOR

X INDEX REGISTE R

Y INDEX REGISTE R

STACK POINTER

CONDITIO N CODE REGIST ER

PROGRAM COUNTER

11H 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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