AAS

ASCII Adjust AL After Subtraction

참고 사항

Instruction Operand Encoding

Description

Adjusts the result of the subtraction of two unpacked BCD values to create a unpacked BCD result. The AL register is the implied source and destination operand for this instruction. The AAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one unpacked BCD value from another and stores a byte result in the AL register. The AAA instruction then adjusts the contents of the AL register to contain the correct 1-digit unpacked BCD result.

If the subtraction produced a decimal carry, the AH register decrements by 1, and the CF and AF flags are set. If no decimal carry occurred, the CF and AF flags are cleared, and the AH register is unchanged. In either case, the AL register is left with its top four bits set to 0.

This instruction executes as described in compatibility mode and legacy mode. It is not valid in 64-bit mode.

Operation

IF 64-bit modeTHEN#UD;ELSEIF ((AL AND 0FH) > 9) or (AF = 1)
                THEN
                      AX <- AX - 6;
                      AH <- AH - 1;
                      AF <- 1;
                      CF <- 1;
                      AL <- AL AND 0FH;
                ELSE
                      CF <- 0;
                      AF <- 0;
                      AL <- AL AND 0FH;
          FI;
FI;

Flags Affected

The AF and CF flags are set to 1 if there is a decimal borrow; otherwise, they are cleared to 0. The OF, SF, ZF, and PF flags are undefined.

Protected Mode Exceptions

#UD

• If the LOCK prefix is used.

Real-Address Mode Exceptions

Same exceptions as protected mode.

Virtual-8086 Mode Exceptions

Same exceptions as protected mode.

Compatibility Mode Exceptions

Same exceptions as protected mode.

64-Bit Mode Exceptions

#UD

• If in 64-bit mode.

첫 댓글을 달아주세요!

강좌에 관련 없이 궁금한 내용은 여기를 사용해주세요

다음으로 로그인

또는 직접 입력하세요 (댓글 수정시 비밀번호가 필요합니다)

댓글 달기

댓글을 불러오는 중입니다..