Filter
Top Products
43
CHAPTER 4 RESET AND "EIT" PROCESSING
4.4.1 Undefined Instruction Exceptions
Undefined instruction exceptions are caused by attempts to execute instruction codes
that are not defined.
This section describes the operation, time requirements and uses of undefined-
instruction exceptions.
■ Overview of Undefined Instruction Exceptions
Undefined instruction exceptions are caused by attempts to execute instruction codes that are not defined.
■ Operations of Undefined Instruction Exceptions
The following operating sequence takes place when an undefined instruction exception occurs.
(1) The contents of the program status (PS) are saved to the system stack.
(2) The address of the instruction that caused the undefined-instruction exception is saved to the system
stack.
(3) The value of the system stack pointer (SSP) is reduced by 8.
(4) The value "0" is written to the "S" flag in the condition code register (CCR) in the "PS".
(5) The value "TBR + 3C4
H
" is stored in the program counter (PC).
■ Time to Start of Undefined Instruction Exception Processing
The time required to start exception processing is 7 cycles.
■ "PC" Values Saved for Undefined Instruction Exceptions
The address saved to the system stack as a "PC" value represents the instruction itself that caused the
undefined instruction exception. When a RETI instruction is executed, the contents of the system stack
should be rewritten with the exception processing routine so that execution will either resume from the
address of the next instruction after the instruction that caused the exception, or branch to the appropriate
processing routine.
■ How to Use Undefined Instruction Exceptions
The following programming steps must be set up to enable the use of undefined instruction exceptions.
(1) Enter values in the interrupt vector table (defined as data).
(2) Set up the "SSP" value.
(3) Set up "TBR" value.
■ Undefined Instructions Placed in Delay Slots
Undefined instructions placed in delay slots do not generate undefined instruction exceptions. In such
cases, undefined instructions have the same operation as "NOP" instructions.