SVC is a privileged instruction in ARM architecture that generates a software interrupt for requesting system services.

Key Characteristics:

• Purpose: Creates a controlled pathway from unprivileged to privileged mode
• Instruction: SVC #immediate (where immediate is typically a service number)
• Exception Type: Exception number 11 in ARM Cortex-M processors

How it Works:

1. User code executes the SVC instruction with a parameter
2. Processor automatically:
   • Saves context on stack
   • Switches to Handler mode (privileged)
   • Jumps to the SVC handler function

Common Uses:

• Operating Systems: Implementing system calls
• RTOS: Task switching and privilege management
• Security: Creating boundaries between privileged and unprivileged code
• Resource Access: Controlled access to protected hardware resources

Creating SVC Services in arm gcc (cubeide)

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The stack is a region of memory used for temporary storage during program execution. It operates in a "last in, first out" (LIFO) manner. The stack is used to store function parameters, local variables, return addresses, and to save CPU registers during function calls and interrupts. Each function call pushes data onto the stack, and data is popped off when the function returns.

The stack grows downward in memory

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ARM GCC calling convention (AAPCS, ARM Architecture Procedure Call Standard):

Argument Passing

• First four arguments: Passed in registers r0, r1, r2, r3.
• Additional arguments: Passed on the stack (right to left).
• Floating-point arguments: Passed in s0–s15 (if FPU is enabled and used).

Return Values

• Integer/pointer return: In r0 (and r1 for 64-bit values).
• Floating-point return: In s0 (or d0 for double).

Register Usage

• Caller-saved (volatile): r0–r3, r12 (can be changed by called function).
• Callee-saved (non-volatile): r4–r11, sp, lr (must be preserved by called function).

Stack Alignment

• Stack pointer (sp): Must be 8-byte aligned at function entry.

Function Prologue/Epilogue

• Prologue: Callee saves non-volatile registers it uses, sets up stack frame.
• Epilogue: Callee restores non-volatile registers, stack pointer, and returns.

Special Registers

• r12: Intra-procedure-call scratch register (used by linker, not preserved).
• lr: Link register (return address).
• sp: Stack pointer.

Example

int foo(int a, int b, int c, int d, int e);
// a → r0, b → r1, c → r2, d → r3, e → stack

References:

• ARM Procedure Call Standard (AAPCS)
• ARM GCC Wiki

Let me know if you want details for a specific ARM variant or ABI!

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