One quarter is two eighths, so five eighths subtract two eighths is three eighths.
1 half is greater. This is because if you convert one half into eighths it = 4 eighths.
I highly recommend google.
How to subtract 2 from two eighths
Actually, you don't need JNZ. You simply subtract the low order halves, and then you subtract with borrow the high order halves. You can carry this to any arbitrary precision.
It is: 1.375-1.25 = 0.125 or 1/8
If you mean 8 and 7/12 subtract 2 and 3/8 then it is 6 and 5/24
3 eighths. three fourths is the same as six eighths, so subtract your 3/8 and you have 3/8 left