Protecting the index registers in interrupt programs

The compiler’s job is to ensure that index registers that are used in interrupt programs or in functions or function blocks called from interrupt programs are not overwritten.

The example below shows the concept of the code generation for PLCs (index register in interrupt: program code, programmer’s comments on the right) to ensure this behavior:

PLCs with index register banks (FP2SH and FP10SH)

INT0
ST R9010
F411_CHGB K1	Index register bank set to 1

ST R9010
F0_MV	DT458, I5	Actual program code
F0_MV	K5, I5DT455

ST R9010
F412_POPB		Index register bank reset

PLCs without index register banks

INT0
ST R9010
F0_MV	I5, DT461	Protecting the current content

ST R9010
F0_MV	DT458, I5	Actual program code
F0_MV	K5, I5DT455
ST R9010F0_MV DT461, I5	Rewriting the current content
IRET