Which of the following flags remaining undefined after the execution of instruction AAD (ASCII adjust after division) in the 8086 microprocessor ?
Carry flag
AAD leaves the carry flag undefined — option 1 — while it does set the sign, zero and parity flags meaningfully.
| Flag | After AAD |
|---|---|
| CF — carry | Undefined |
| SF — sign | Set from the result |
| ZF — zero | Set from the result |
| PF — parity | Set from the result |
| AF, OF | Undefined |
What AAD actually does. Despite its name, it runs before a division, not after. Unpacked BCD keeps one decimal digit per byte, so a two-digit number sits as a tens digit in AH and a units digit in AL. Ordinary DIV works on a binary value, so the two digits must first be combined:
\(AL\leftarrow\left(AH\times10\right)+AL,\qquad AH\leftarrow0\)
After that the pair is an ordinary binary number in AL and DIV can be used normally.
Why the carry is meaningless here. The largest value AAD can produce is
\(9\times10+9=99\)
which fits comfortably in a byte, so no carry out of bit 7 can ever occur in a legitimate use. The instruction's internal multiply-and-add simply leaves CF holding whatever the microcode happened to produce, and Intel documents it as undefined rather than guaranteeing a value. Testing it after AAD is therefore a programming error, however consistently one particular processor might behave.
Why SF, ZF and PF are defined. They describe the byte now in AL, which is a genuine result: SF is its bit 7, ZF records whether it is zero, PF its even parity. A program can legitimately act on those — checking ZF to detect a zero dividend, for instance.
The companion instructions follow the same pattern and are worth knowing together: AAA and AAS adjust after addition and subtraction, AAM adjusts after multiplication by splitting AL back into two unpacked digits, and DAA and DAS do the equivalent job for packed BCD, where two digits share one byte.
Hence, the flag left undefined is the carry flag.
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