Freescale Semiconductor SC140 Return From Exception Instructions, Exception Vector Address Table

			Exception Processing
	Table 5-19. Exception Vector Address Table
Exception Address	Priority	Type	Description
Offset	(0 - highest)
0x00	0	TRAP	TRAP instruction
0x40	-	Reserved
0x80	1	ILLEGAL	ILLEGAL instruction, and illegal
			instruction set
0xC0	2	DEBUG	DEBUG exception from EOnCE
0x100	3	Overflow	DALU overflow
0x140	-	Reserved
0x180	5	Auto-NMI	NMI default vector
0x1C0	6	Auto-IR	Interrupt default vector
0x200-0xFC0	5	NMI	NMI
		External interrupts	External interrupts

5.8.2 Return From Exception Instructions

Return from exception should be done with dedicated Return from Exception instructions (RTE, RTED, - termed collectively “RTE-like” instructions). These instructions pop from the active stack two values: The return PC, form which execution resumes, and SR value. The SR value sets (among other things) the working mode of the core. The RTE-like instructions are used to automatically enable resuming the task that was interrupted (by restoring the next PC to be executed and the SR, including the working mode). It could also be manipulated by the RTOS to change the task that is restored.

For maximum efficiency on return from an exception, the SC140 instruction set provides a delayed return from the exception instruction. This takes five or six cycles to execute, but allows the usage of some of these cycles to execute instructions. Refer to Appendix A, “SC140 DSP Core Instruction Set,” for details on return-from-exception usage in the RTE and RTED instructions.

The RTE/D instructions do not affect the shadow return address of subroutines, see Section 5.5.5, “Fast Return from Subroutines.” In this way, the interrupts interfere less with the interrupted task, allowing it to continue and enjoy the reduced cycle count when performing an RTS, if applicable. However, if during the ISR, the task was switched, the RAS may hold content from the previous task that may corrupt the execution of the restored task. In such a case, the user should clear the RAS by performing, in the ISR that switches the tasks, a dummy jump to subroutine (JSR) and return (RTS). .

SC140 DSP Core Reference Manual 5-49