Why is defined PPC_ISYNC defined as "sync;isync"

[Sean P Curry]

Can anyone tell me why the macro PPC_ISYNC (located in
libc/sysdeps/powerpc/powerpc[32/64]/dl-machine.c) is defined as: asm
volatile("sync; isync"...) ?? (why is the "sync" instruction present
before the "isync")  

Thanks in advance,
   Sean