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Solution:
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The SUBLW instruction is the only instruction that will subtract a value from the W register and that subtracts the literal value that is included in the SUBLW instruction from the W register. For example, SUBLW 0x05 will subtract 5 from the value that was in the W register and place the result back in to the W register. However, a SUBWF will always subtract the contents of the W register from the contents of the register. Therefore, if the W register contains 5 and the register contains 2 then the result will be (2-5 =) minus 3 (253). It is not possible to do a 5-2 calculation without storing the W register into another register temporarily or by doing other esoteric things. For example, the following code will reverse the W register and the other register so that whatever was in the W register will now be in the other register and vice versa.
XORWF reg, 0
XORWF reg, 1
XORWF reg, 0
For example, if W contains 60 (00111100) and the register contains 170 (10101010).
XORWF reg, 0 ;00111100 (w) XOR 10101010 (reg) = 10010110, result to W
XORWF reg, 1 ;10010110 (w) XOR 10101010 (reg) = 00111100, result to reg
XORWF reg, 0 ;10010110 (w) XOR 00111100 (reg) = 10101010, result to W
Therefore, W now contains 10101010 (170) and the register contains 00111100 (60)
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