Access MIPS Co-processor condition flag directly

Question

I wish to access the value of the MIPS coprocessor 1 condition flag.

For example

c.eq.s $f4 $f6
move $t0 condflag1 #clearly illegal

I understand that the following is possible:

c.eq.s $f4 $f6
bc1f L_CondFalse
li $t0 0
j L_CondEnd
L_CondFalse : li $t0 1
L_CondEnd :

Any ideas?

Answer 1

You can avoid branching by using conditional move instruction (available since MIPS32).

li     $t0, 1          # move 1 into t0
c.eq.s $f4, $f6        # compare f4 and f6; set FCC[0] to the result
movt   $t0, $0, $fcc0  # move 0 into t0 if comparison was true

(not tested)

Another option is to move the FCCR into a GPR and read the bit 0.

cfc1   $t0, $25  # load FCCR into t0
andi   $t0, 1    # mask the bit 0 only

FCCR is not available before MIPS32, so you might have to use FCSR ($31) instead.