Just for fun, here is yet one more variation on Peter's scheme, using only S/360 instructions. In the comments, YY and ZZ represent the two bit pairs of interest, lower case x represents bits or bytes of unknown value. Bit strings are enclosed in square brackets. Bytes are separated by underscore characters.
XR R14,R14 R14 = 00_00_00_00 IC R14,BYTE R14 = 00_00_00_[YYZZxxxx] SRDL R14,6(0) R14 = 00_00_00_[000000YY] * R15 = [ZZxxxxxx]_xx_xx_xx SRL R15,30(0) R15 = 00_00_00_[000000ZZ] LA R14,X'F1'(,R14) * Scale x'00' through x'03' LA R15,X'F1'(,R15) * to x'F1' through x'F4'
