Intel 8080 - B4_Page_33

Chapter 6. Programming Techniques
Decimal addition is performed by letting each 8-bit byte represent two 4-bit decimal digits. The bytes are
summed
in the accumulator in standard fashion, and the DAA (decimal adjust accumulator) instruction is then
used to convert the 8-bit binary
result to the correct representation of 2 decimal digits. For multibyte strings,
you must perform the
decimal adjust before adding the next higher·order bytes. This is because you need the
carry
flag setting from the DAA instruction for adding the higher·order bytes.
To perform the
decimal addition:
the process works
as follows:
2985
+4936
7921
1. Clear the Carry and add the two lowest-order digits of each number (remember that each 2
decimal digits are represented by .one byte).
carry
85
= 10000101 B
36 = 00110110B
o
~Q]10111011B
Carry = 0 , Auxiliary Carry = 0
The accumulator now contains OBBH.
2. Perform a DAA operation. Since the rightmost four bits are greater than 9, a 6 is added to the
accumulator.
Accumulator
= 10111011 B
6
= 0110B
11000001
B
Since the
leftmost bits are greater than 9, a 6 is added to these bits, thus ~etting the carry flag.
Accumulator = 11000001 B
6
= 0110 B
Carry
flag == 1
/]00100001B
The accumulator now contains 21 H. Store these two digits.
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