Shouldn't have pressed post so soon..

Here is how to do it, in unoptimized pseudocode.

Bitset (byte sfrval, bit newval)
{
MOV A,sfrval; the 5 upper bits are the byte adr
ANL A,#0F8
MOV R0,A

MOV A,sfrval; the 3 lower bits are the "bit"
ANL A,#07
MOV B,A
;... tired of writing assembly
R2 = 2 ^ B
if newval @R0 |= R2
else @R0 = @r0 & ~R2
; done


}