Converting from lower case to upper case

Question

I am trying to convert from lower case to upper case. I know it can easily be done by,

SUB AL, 20H

But I am have been given another solution which is,

AND AL, 0DFH

Please help me understand this. Thanks

Answer 1

Look at the bit patterns:

• A (0x41): 0100 0001
• a (0x61): 0110 0001
• M (0x4d): 0100 1101
• m (0x6d): 0110 1101
• Z (0x5a): 0101 1010
• z (0x7a): 0111 1010

Lower case ASCII is upper case ASCII + 0x20 (0010 0000) - i.e. the same bit pattern with the sixth bit set.

0xdf is 1101 1111 in binary. AND:ing AL with that will set the sixth bit to zero but preserve the other bit values.

Answer 2

SUB AL, 20His subtracting 0x20 from AL

AND AL,0DFHwill clear bit 5 (make it '0') . Because 0x20 is 00100000 ,subtracting it from a value its the same as just clear bit 5.