Search code examples
cassemblyinline-assemblypowerpc

Decoding this assembly inline code snippet on PowerPc

I have this below code snippet from kernel source for PowerPc

#define SPRN_IVOR32     0x210   /* Interrupt Vector Offset Register 32 */

unsigned long ivor[3];
ivor[0] = mfspr(SPRN_IVOR32);

#define __stringify_1(x)        #x
#define __stringify(x)          __stringify_1(x)

#define mfspr(rn)       ({unsigned long rval; \
                    asm volatile("mfspr %0," __stringify(rn) \
                            : "=r" (rval)); rval; })

Also, it this above exercise is about emulating MSR register's bits in PowerPc?

Can anyone help me on what exactly we are doing here?

Solution

  • The mfspr macro generates an asm instruction mfspr which reads the given special purpose register into a register chosen by the compiler, which then gets assigned to rval hence becomes the return value of the expression.

    As the comment says, SPRN_IVOR32 is the Interrupt Vector Offset Register 32, whose contents are thus fetched into ivor[0].