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Exception Processing

SC140 DSP Core Reference Manual 5-49

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). .

Table 5-19. Exception Vector Address Table

Exception Address

Offset

Priority

(0 - highest)

Type Description

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

5NMI NMI

External interrupts External interrupts

1 2 ... 224 225 226 227 228 229 230 231 232 233 234 ... 759 760

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