This can be used to execute piece of code and make sure it is not interrupted by
selected interrupts.

Mechanism to invoke a software interrupt. This allows execution
to jump into privileged(interrupt handling) context.
Depending on priority assigned you could allow selective interruption.


Example - 
			// Global var - 
			uint32_t piVal = 3.1416;
			
			// Function we want to execute in SVC  - 
			void testSWI(void *args)
			{
				uint32_t pi= *(uint32_t *)args;
			}
			
			//	Suppose we want to execute testSWI function and we want to pass
			//	piVal as an argument to function and we want the function to 
			//	execute with interrupt priority level of 1. Then in our code 
			//	we call
			SWI_Req(1, testSWI, (void*)(&piVal));
			
			//	Any interrupt more or equal to 1 cannot prempt execution 
			//  of this function

reference - 
	The Definitive Guide to the ARM®Cortex-M3
	See section(using SVC with C function call) in chapter 'Exception prog'