L9803 datasheet(15/126 Pages) STMICROELECTRONICS

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L9803

Central Processing Unit

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Central Processing Unit

2.1

Introduction

The CPU has a full 8-bit architecture. Six internal registers allow efficient 8-bit data

manipulation. The CPU is capable of executing 63 basic instructions and features 17 main

addressing modes.

2.2

CPU registers

The 6 CPU registers are shown in the programming model in Figure 3. Following an

interrupt, all registers except Y are pushed onto the stack in the order shown in Figure 4.

They are popped from stack in the reverse order.

The Y register is not affected by these automatic procedures. The interrupt routine must

therefore handle Y, if needed, through the PUSH and POP 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 as well as data

manipulations.

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.

Program Counter (PC). The program counter is a 16-bit register containing the address of

the next instruction to be executed by the CPU.

Figure 3.

Organization of Internal CPU Registers

ACCUMULATOR:

X INDEX REGISTER:

Y INDEX REGISTER:

STACK POINTER:

CONDITION CODE REGISTER:

PROGRAM COUNTER:

X = Undefined

RESET VALUE:

XXXXXX

RESET VALUE:

1 1 X1 XX

11H I

N Z

RESET VALUE = RESET VECTOR @ FFFEh-FFFFh

RESET VALUE:

XXXXXX

RESET VALUE:

XXXXXX

000000

RESET VALUE =0 0 0 0 0 0 0 1 0 1 1 1 1 1 1 1

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