Texas Instruments TMS320 User Manual page 142

Software Interrupts
4-38
Note:
An important side effect of SWI_disable is that task preemption is also
disabled. This is because DSP/BIOS uses software interrupts internally to
manage semaphores and clock ticks.
To delete a dynamically created software interrupt, use SWI_delete.
The memory associated with swi is freed. SWI_delete can only be called from
the task level.