5.17 Determining the SVC to be called

When an SVC handler is entered, it must establish which SVC is being called.

For the ARM SVC instruction, this information can be stored in bits 0-23 of the instruction itself, as shown in the following figure, or passed in an integer register, usually one of R0-R3.

Figure 5-3 ARM SVC instruction
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The top-level SVC handler can load the SVC instruction relative to the LR. Do this in assembly language, C/C++ inline, or embedded assembler.

The handler must first load the SVC instruction that caused the exception into a register. At this point, the SVC LR holds the address of the instruction that follows the SVC instruction, so the SVC is loaded into the register, in this case R0, using:

    LDR R0, [lr,#-4]

The handler can then examine the comment field bits, to determine the required operation. The SVC number is extracted by clearing the top eight bits of the opcode:

    BIC R0, R0, #0xFF000000

The following example shows how you can put these instructions together to form a top-level SVC handler, for exceptions that occur in ARM state only.

Top-level SVC handler

    AREA TopLevelSVC, CODE, READONLY ; Name this block of code.
    EXPORT     SVC_Handler
    PUSH       {R0-R12,lr}           ; Store registers.
    LDR        R0,[lr,#-4]           ; Calculate address of SVC
                                     ; instruction and load it 
                                     ; into R0.
    BIC        R0,R0,#0xFF000000     ; Mask off top 8 bits of
                                     ; instruction to give SVC number.
    ; Use value in R0 to determine which SVC routine to execute.
    LDM         sp!, {R0-R12,pc}^    ; Restore registers and return.
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