CHAPTER 7 INTERRUPTION/EXCEPTION PROCESSING FUNCTION
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7.4
Software Exception
A software exception is generated when the CPU executes the TRAP instruction, and can always be
acknowledged.
7.4.1 Operation
If a software exception occurs, the CPU performs the following processing, and transfers control to the handler
routine:
<1> Saves the restored PC to EIPC.
<2> Saves the current PSW to EIPSW.
<3> Writes an exception code to the lower 16 bits (EICC) of ECR (interrupt source).
<4> Sets the EP and ID bits of the PSW.
<5> Sets the handler address (00000040H or 00000050H) corresponding to the software exception to the PC,
and transfers control.
Figure 7-8 illustrates the processing of a software exception.
Figure 7-8. Software Exception Processing
TRAP instruction
EIPC
EIPSW
ECR.EICC
PSW.EP
PSW.ID
PC
Restored PC
PSW
Exception code
1
1
Handler address
CPU processing
Exception processing
Note
Note
TRAP instruction format: TRAP vector (the vector is a value from 0 to 1FH.)
The handler address is determined by the TRAP instruction’s operand (vector). If the vector is 0 to 0FH, it
becomes 00000040H, and if the vector is 10H to 1FH, it becomes 00000050H.