ST7293 datasheet(8/69 Pages) STMICROELECTRONICS

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ST7294

Figure 4. Organisation of Internal CPU Registers

VR0B1767

ACCUMULATOR:

RESET VALUES:

XXX

RESET VALUES:

X INDEX REGISTER:

Y INDEX REGISTER:

RESET VALUES:

XXX

RESET VALUES:

PROGRAM COUNTER:

000

RESET VECTOR (1FFEh:1FFFh)

STACK POINTER:

RESET VALUES:

000

111

CONDITION CODE REGISTER:

RESET VALUES:

111

X = Undefined

2 CENTRAL PROCESSING UNIT

2.1 INTRODUCTION

The CPU has a full 8-bit architecture. Six internal

registers allow efficient 8-bit data manipulations.

The CPU is able to execute 74 basic instructions.

It features 17 main addressing modes and can ad-

dress 6 internal registers. It is able to address

6671 bytes of memory and registers with its pro-

gram counter.

2.2 CPU REGISTERS

The 6 CPU registers are shown in the program-

ming model in Figure 4. Following an interrupt, the

registers are pushed onto the stack in the order

shown in Figure 5. 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 it, if needed, through the

POP and PUSH instructions.

Accumulator (A). The accumulator is an 8-bit

general purpose register used to hold operands

and the results of the arithmetic and logic calcula-

tions as well as data manipulations.

Index Registers (X and Y). These 8-bit registers

are used to create effective addresses or as tem-

porary storage area for data manipulations. The

cross-assembler generates a PRECEDE instruc-

tion (PRE) to indicate that the following instruction

refers to the Y register. The Y register is never au-

tomatically stacked. Interrupt routines must push

or pop it by using the POP and PUSH instructions.

Program Counter (PC). The program counter is a

16-bit register containing the address of the next

instruction to be executed by the CPU. In the

ST7294, only the 13 low order bits are used, bits

13, 14 and 15 are forced to ”0”.

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