How come a 32 bit kernel can run a 64 bit binary?

Question

On my OS X box, the kernel is a 32 bit binary and yet it can run a 64 bit binary. How does this work?

cristi:~ diciu$ file ./a.out
./a.out: Mach-O 64-bit executable x         


        

Answer 1

An ELF32 file can contain 64bit instructions and run in 64 bit mode. Only thing it is having is that organization of header and symbols are in 32bit format. Symbols table offsets are 32 bits. Symbol table entries are 32 bit wide etc. A file which contain both 64 bit code and 32 bit code can expose itself as 32 bit ELF file wheres it uses 64 bit registors for its internal calculations. mach_kernel is one such executable. Advantage it get is that 32 bit driver ELFs can linked to it. If it take care of passing pointers which are located below 4GBs to other linked ELF binaries it will work fine.