You can get that down to 5 instructions. You could either use a typecast, or
for portability, use a union definition. For that matter you could even
typecast *mptr as a pointer to the union and extract the data from the
string however you choose. But it still takes 5 instructions, unless you're
pulling the data into another buffer, in which case you're down to 4.
-> -----Original Message-----
-> From: Richard Hodges [mailto:email@example.com]
-> Sent: Thursday, March 07, 2002 11:57 AM
-> To: Ralf Baechle
-> Cc: Linux MIPS
-> Subject: Re: Questions?
-> On Thu, 7 Mar 2002, Ralf Baechle wrote:
-> > The MIPS ABI only covers big endian systems - every "real"
-> MIPS UNIX
-> > system is big endian. Everything else is a GNU extension.
-> There is
-> > hardly any reason to choose a particular byteorder as
-> usually endianess
-> > swapping takes so little CPU time that it isn't even
-> meassurable but so
-> > I'm told there are exceptions.
-> To me, byte swapping on MIPS actually seems rather
-> expensive. The code
-> for htonl (linux/byteorder/swab.h) ends up something like this:
-> srl $5,$4,8
-> andi $5,$5,0xff00
-> srl $2,$4,24
-> andi $3,$4,0xff00
-> or $2,$2,$5
-> sll $3,$3,8
-> or $2,$2,$3
-> sll $4,$4,24
-> This may not be an issue if it is only needed a few times
-> per packet, but
-> my system must byte-swap (LE to BE) about 500KB (or 4mb) per second.
-> Actually, I save a bit of work by combining the byte
-> swapping with the
-> memory move, just after copy_from_user, and looks something like:
-> unsigned char a, b, c, d, *mptr;
-> a = mptr;
-> b = mptr;
-> c = mptr;
-> d = mptr;
-> mptr = d;
-> mptr = c;
-> mptr = b;
-> mptr = a;
-> This method works, but it is still 8 instructions per word.
-> Yuck! Does
-> anyone know of a _decent_ way to handle this on MIPS?
-> All the best,