Omron FQM1-MMA21, FQM1-CM001, FQM1-MMP21 Summary of the Condition Flags, Using the Condition Flags

I/O Memory Appendix B

The Condition Flags cannot be force-set and force-reset except for the Carry Flag, which can be manipulated with the STC(040) and CLC(041) instructions.

Summary of the Condition Flags

The following table summarizes the functions of the Condition Flags, although the functions of these flags will vary slightly from instruction to instruction. Refer to the description of the instruction for complete details on the operation of the Condition Flags for a particular instruction.

Name	CX-Programmer	Function
	symbol
Error Flag	P_ER	Turned ON when the operand data in an instruction is incorrect (an instruction
		processing error) to indicate that an instruction ended because of an error.
Access Error Flag	P_AER	Turned ON when an Illegal Access Error occurs. The Illegal Access Error indi-
		cates that an instruction attempted to access an area of memory that should not
		be accessed.
Carry Flag	P_CY	Turned ON when there is a carry in the result of an arithmetic operation or a “1” is
		shifted to the Carry Flag by a Data Shift instruction.
		The Carry Flag is part of the result of some Data Shift and Math instructions.
Greater Than Flag	P_GT	Turned ON when the first operand of a Comparison Instruction is greater than the
		second or a value exceeds a specified range.
Equals Flag	P_EQ	Turned ON when the two operands of a Comparison Instruction are equal or the
		result of a calculation is 0.
Less Than Flag	P_LT	Turned ON when the first operand of a Comparison Instruction is less than the
		second or a value is below a specified range.
Negative Flag	P_N	Turned ON when the most significant bit (sign bit) of a result is ON.
Overflow Flag	P_OF	Turned ON when the result of calculation overflows the capacity of the result
		word(s).
Underflow Flag	P_UF	Turned ON when the result of calculation underflows the capacity of the result
		word(s).
Greater Than or	P_GE	Turned ON when the first operand of a Comparison Instruction is greater than or
Equals Flag		equal to the second.
Not Equal Flag	P_NE	Turned ON when the two operands of a Comparison Instruction are not equal.
Less Than or	P_LE	Turned ON when the first operand of a Comparison Instruction is less than or
Equals Flag		equal to the second.
Always ON Flag	P_On	Always ON. (Always 1.)
Always OFF Flag	P_Off	Always OFF. (Always 0.)

Using the Condition Flags

The Condition Flags are shared by all of the instructions, so their status may change often in a single cycle. Be sure to read the Condition Flags immediately after the execution of instruction, preferably in a branch from the same input condition.

Instruction A

The result from instruction A is

reflected in the Equals Flag.

Condition Flag Example: =

Instruction B

Instruction

LD

Instruction A

AND

Instruction B

Operand

=

Note (1) Since the Condition Flags are shared by all of the instructions, program operation can be changed from its expected course by interruption of a single task. Be sure to consider the effects of Condition Flags when writing the program. Refer to Condition Flags on page 281 for details.

(2)The Condition Flags are cleared when the program switches tasks, so the status of a Condition Flag cannot be passed to another task.

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