Zilog Z8F0130 Manual do Utilizador descarregar pdf (Página 60)

eZ8

™

CPU Core

User Manual

UM012820-0810 Illegal Instruction Traps

Illegal Instruction Traps

The instruction set of the eZ8

™

CPU does not cover all possible

sequences of binary values. Binary values and sequences for which no

operation is defined are illegal instructions. When the eZ8 CPU fetches

one of these illegal instructions, it performs an Illegal Instruction Trap

operation.

The Illegal Instruction Trap functions similarly to a TRAP #%3

instruction (object code

F2h 03h). The Flags and Program Counter are

pushed on the stack. When the Program Counter detects an illegal

instruction it does not increment. The Program Counter value that is

pushed onto the stack points to the illegal instruction.

The most significant byte (MSB) of the Illegal Instruction Trap Vector is

stored at Program Memory address

0006h. The least significant byte

(LSB) of the Illegal Instruction Trap Vector is stored at Program Memory

address

0007h. The 16-bit Illegal Instruction Trap Vector replaces the

value in the Program Counter (PC). Program execution resumes from the

new value in the Program Counter.

An IRET instruction must not be performed following an Illegal

Instruction Trap service routine. Because the stack contains the

Program Counter value of the illegal instruction, the IRET

instruction returns the code execution to this illegal instruction.

Symbolic Operation of an Illegal Instruction Trap

SP  SP - 2

@SP  PC

SP  SP - 1

@SP  Flags

PC  Vector

Caution:

1 2 ... 55 56 57 58 59 60 61 62 63 64 65 ... 313 314

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