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| <title>DAS—Decimal Adjust AL after Subtraction </title></head> |
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| <h1>DAS—Decimal Adjust AL after Subtraction</h1> |
| <table> |
| <tr> |
| <th>Opcode</th> |
| <th>Instruction</th> |
| <th>Op/En</th> |
| <th>64-Bit Mode</th> |
| <th>Compat/Leg Mode</th> |
| <th>Description</th></tr> |
| <tr> |
| <td>2F</td> |
| <td>DAS</td> |
| <td>NP</td> |
| <td>Invalid</td> |
| <td>Valid</td> |
| <td>Decimal adjust AL after subtraction.</td></tr></table> |
| <h3>Instruction Operand Encoding</h3> |
| <table> |
| <tr> |
| <td>Op/En</td> |
| <td>Operand 1</td> |
| <td>Operand 2</td> |
| <td>Operand 3</td> |
| <td>Operand 4</td></tr> |
| <tr> |
| <td>NP</td> |
| <td>NA</td> |
| <td>NA</td> |
| <td>NA</td> |
| <td>NA</td></tr></table> |
| <h2>Description</h2> |
| <p>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.</p> |
| <p>This instruction executes as described above in compatibility mode and legacy mode. It is not valid in 64-bit mode.</p> |
| <h2>Operation</h2> |
| <pre>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;</pre> |
| <h2>Example</h2> |
| <p>SUB</p> |
| <p>AL, BL</p> |
| <p>Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX</p> |
| <p>After: AL = EEH, BL = 47H, EFLAGS(0SZAPC) = 010111</p> |
| <p>DAA</p> |
| <p>Before: AL = EEH, BL = 47H, EFLAGS(OSZAPC) = 010111</p> |
| <p>After: AL = 88H, BL = 47H, EFLAGS(0SZAPC) = X10111</p> |
| <h2>Flags Affected</h2> |
| <p>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.</p> |
| <h2>Protected Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If the LOCK prefix is used.</td></tr></table> |
| <h2>Real-Address Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If the LOCK prefix is used.</td></tr></table> |
| <h2>Virtual-8086 Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If the LOCK prefix is used.</td></tr></table> |
| <h2>Compatibility Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If the LOCK prefix is used.</td></tr></table> |
| <h2>64-Bit Mode Exceptions</h2> |
| <table class="exception-table"> |
| <tr> |
| <td>#UD</td> |
| <td>If in 64-bit mode.</td></tr></table></body></html> |