DAS (Intel x86/64 assembly instruction)

모두의 코드
DAS (Intel x86/64 assembly instruction)

작성일 : 2020-09-01 이 글은 795 번 읽혔습니다.

DAS

Decimal Adjust AL after Subtraction

참고 사항

아래 표를 해석하는 방법은 x86-64 명령어 레퍼런스 읽는 법 글을 참조하시기 바랍니다.

Opcode	Instruction	Op/ En	64-Bit Mode	Compat/ Leg Mode	Description
2F	DAS	NP	Invalid	Valid	Decimal adjust AL after subtraction.

Instruction Operand Encoding

Op/En	Operand 1	Operand 2	Operand 3	Operand 4
NP	NA	NA	NA	NA

Description

Adjusts the result of the subtraction of two packed BCD values to create a packed BCD result. The AL register is the implied source and destination operand. The DAS instruction is only useful when it follows a SUB instruction that subtracts (binary subtraction) one 2-digit, packed BCD value from another and stores a byte result in the AL register. The DAS instruction then adjusts the contents of the AL register to contain the correct 2-digit, packed BCD result. If a decimal borrow is detected, the CF and AF flags are set accordingly.

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

Operation

IF 64-Bit Mode
    THEN
          #UD;
    ELSE
          old_AL <- AL;
          old_CF <- CF;
          CF <- 0;
          IF (((AL AND 0FH) > 9) or AF = 1)
            THEN
            AL <- AL - 6;
                      CF <- old_CF or (Borrow from AL <- AL - 6);
                      AF <- 1;
                ELSE
                      AF <- 0;
          FI;
          IF ((old_AL > 99H) or (old_CF = 1))
                 THEN
                      AL <- AL - 60H;
                      CF <- 1;
          FI;
FI;

Example

SUB AL, BL Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX

After: AL = EEH, BL = 47H, EFLAGS(0SZAPC) = 010111

DAA Before: AL = EEH, BL = 47H, EFLAGS(OSZAPC) = 010111

After: AL = 88H, BL = 47H, EFLAGS(0SZAPC) = X10111

Flags Affected

The CF and AF flags are set if the adjustment of the value results in a decimal borrow in either digit of the result (see the "Operation" section above). The SF, ZF, and PF flags are set according to the result. The OF flag is unde-fined.

Protected Mode Exceptions

#UD

If the LOCK prefix is used.

Real-Address Mode Exceptions

#UD

If the LOCK prefix is used.

Virtual-8086 Mode Exceptions

#UD

If the LOCK prefix is used.

Compatibility Mode Exceptions

#UD

If the LOCK prefix is used.

64-Bit Mode Exceptions

#UD

If in 64-bit mode.

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모두의 코드 DAS (Intel x86/64 assembly instruction)

DAS