A software interrupt (SWI) is a synchronous exception generated by the execution of a particular instruction. Applications use software interrupts to request services from a protected system, such as an operating system, which can perform the services only while in a supervisor mode. Some ARM documentation uses the term Supervisor Calls (SVC) instead of "software interrupt".
A C/C++ application can invoke a software interrupt by associating a software interrupt number with a function name through use of the SWI_ALIAS pragma and then calling the software interrupt as if it were a function. For information, see Section 5.11.29.
Since a call to the software interrupt function represents an invocation of the software interrupt, passing and returning data to and from a software interrupt is specified as normal function parameter passing with the following restriction:
All arguments passed to a software interrupt must reside in the four argument registers (R0-R3). No arguments can be passed by way of a software stack. Thus, only four arguments can be passed unless:
For Cortex-M architectures, C SWI handlers cannot return values. Values may be returned by SWI handlers on other architectures.
The C/C++ compiler also treats the register usage of a called software interrupt the same as a called function. It assumes that all save-on-entry registers () are preserved by the software interrupt and that save-on-call registers (the remainder of the registers) can be altered by the software interrupt.