Page 200 - ARM 64 Bit Assembly Language

188 Chapter 7

Example 11. Ten’s complement subtraction.

Suppose we wish to calculate 384 10 − 56 10 using the complements method. We first pad
the numbers, so that they have leading zeros (the sign digit), and are the same length,
resulting in 0384 10 − 0056 10 From Chapter 1, the ten’s complement of 0056 10 is 9944 10 .
Adding them together gives us 0384 10 + 9944 10 = 10328 10 . Since we are using four
digits for the operation, we expect a four digit result. After discarding the leading ‘1’,
we have 328, which is the correct result. Note that if the carry into the most significant
column does not match the final carry out, then overflow has occurred. Both methods of
subtraction are shown below:

7 14 1 1
3 8 4 0384
=
− 56 +9944
328 10328

Example 12. Ten’s complement subtraction with a negative result.
Suppose we want to calculate 284 − 481. First, we pad with leading zeros (sign digits), to get
0284 − 0481. Then, we take the tens complement of 0481, which is 9519. Adding the ten’s
complement of the subtrahend to the menuend gives 0284 + 9519 = 9803. The First digit is
greater than 4, indicating that the result is negative. Therefore, we take the ten’s complement
of the result to get −0197.
Example 13. Signed addition and subtraction in decimal and binary.

023 00010111

+ 015 = + 00001111
038 00100110

023 023 00010111
− 015 = + 985 = + 11110001
008 1008 100001000

− 23 977 11101001
+ 15 = + 015 = + 00001111
− 8 992 11111000