Hmm, assembly or C? Your original message had examples in both. In C the
large part is the definitions in the union.
If you define
struct le_bitfield {
unsigned long low:8;
unsigned long lmid:8;
unsigned long hmid:8;
unsigned long high:8;
};
struct be_bitfield {
unsigned long high:8;
unsigned long hmid:8;
unsigned long lmid:8;
unsigned long low:8;
};
union byte_swap {
unsigned long original;
struct le_bitfield le;
struct be_bitfield be;
}swapper;
Now you can use your previous char pointer
swapper.original = *((unsigned long *)mptr);
mptr[0] = swapper.le.low;
mptr[1] = swapper.le.lmid;
mptr[2] = swapper.le.hmid;
mptr[3] = swapper.le.high;
5 instructions in C.
To swap the other way, just use swapper.be.xxxx. If this still translates to
8 assembly instructions (or worse), oh well... just trying to help. ;)
-> -----Original Message-----
-> From: Richard Hodges [mailto:rh@matriplex.com]
-> Sent: Thursday, March 07, 2002 3:00 PM
-> To: Siders, Keith
-> Cc: Linux MIPS
-> Subject: RE: Questions?
->
->
-> On Thu, 7 Mar 2002, Siders, Keith wrote:
->
-> > 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.
->
-> Which 5 instructions are those? For htonl from memory, the
-> only sequence
-> I can think of is the "obvious" one of lbu/shift (7
-> instructions). And
-> for swapping BE-LE in memory (an important thing for me), I
-> do not see any
-> method better than the "obvious" one I mentioned earlier:
->
-> 90c50000 lbu a1,0(a2)
-> 90c40001 lbu a0,1(a2)
-> 90c30002 lbu v1,2(a2)
-> 90c20003 lbu v0,3(a2)
-> a0c20000 sb v0,0(a2)
-> a0c30001 sb v1,1(a2)
-> a0c40002 sb a0,2(a2)
-> a0c50003 sb a1,3(a2)
->
-> Thanks,
->
-> -Richard
->
-> > -> From: Richard Hodges [mailto:rh@matriplex.com]
->
-> > -> 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
->
->
