Start with grade in AL
Clear Zero Flag
Subtract 50 from AL ; Fail
If Zero Flag is set, Jump to Fail
Subtract 20 from AL ; 60%+
If Zero Flag is set, Jump to "D"
Subtract 10 from AL; 70%+
If Zero Flag is set Jump to "C"
Subtract 10 from AL; 80%+
If Zero Flag is set, Jump to "B"
Subtract 10from AL; 90%+
If Zero flag is set, Jump to "A"
Something along those lines should work, once you get the subtraction correct, though you may want to do bounds checking in the "D" throgh "A" area to subtract the rest of 100 to ensure the answer is Zero in the event a number greater than 100 was input, then display an out of range error.