forked from NRZCode/ia32-64
116 lines
4.7 KiB
HTML
116 lines
4.7 KiB
HTML
<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml" xmlns:svg="http://www.w3.org/2000/svg" xmlns:x86="http://www.felixcloutier.com/x86"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8"><link rel="stylesheet" type="text/css" href="style.css"></link><title>DAS
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— Decimal Adjust AL After Subtraction</title></head><body><header><nav><ul><li><a href='index.html'>Index</a></li><li>December 2023</li></ul></nav></header><h1>DAS
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— Decimal Adjust AL After Subtraction</h1>
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<table>
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<tr>
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<th>Opcode</th>
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<th>Instruction</th>
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<th>Op/En</th>
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<th>64-Bit Mode</th>
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<th>Compat/Leg Mode</th>
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<th>Description</th></tr>
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<tr>
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<td>2F</td>
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<td>DAS</td>
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<td>ZO</td>
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<td>Invalid</td>
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<td>Valid</td>
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<td>Decimal adjust AL after subtraction.</td></tr></table>
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<h2 id="instruction-operand-encoding">Instruction Operand Encoding<a class="anchor" href="#instruction-operand-encoding">
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¶
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</a></h2>
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<table>
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<tr>
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<th>Op/En</th>
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<th>Operand 1</th>
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<th>Operand 2</th>
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<th>Operand 3</th>
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<th>Operand 4</th></tr>
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<tr>
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<td>ZO</td>
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<td>N/A</td>
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<td>N/A</td>
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<td>N/A</td>
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<td>N/A</td></tr></table>
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<h2 id="description">Description<a class="anchor" href="#description">
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¶
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</a></h2>
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<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>
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<p>This instruction executes as described above in compatibility mode and legacy mode. It is not valid in 64-bit mode.</p>
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<h2 id="operation">Operation<a class="anchor" href="#operation">
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¶
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</a></h2>
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<pre>IF 64-Bit Mode
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THEN
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#UD;
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ELSE
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old_AL := AL;
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old_CF := CF;
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CF := 0;
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IF (((AL AND 0FH) > 9) or AF = 1)
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THEN
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AL:=AL -6;
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CF := old_CF or (Borrow from AL := AL − 6);
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AF := 1;
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ELSE
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AF := 0;
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FI;
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IF ((old_AL > 99H) or (old_CF = 1))
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THEN
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AL := AL − 60H;
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CF := 1;
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FI;
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FI;
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</pre>
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<h2 id="example">Example<a class="anchor" href="#example">
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¶
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</a></h2>
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<p>SUB AL, BL Before: AL = 35H, BL = 47H, EFLAGS(OSZAPC) = XXXXXX</p>
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<p>After: AL = EEH, BL = 47H, EFLAGS(0SZAPC) = 010111</p>
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<p>DAA Before: AL = EEH, BL = 47H, EFLAGS(OSZAPC) = 010111</p>
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<p>After: AL = 88H, BL = 47H, EFLAGS(0SZAPC) = X10111</p>
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<h2 id="flags-affected">Flags Affected<a class="anchor" href="#flags-affected">
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¶
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</a></h2>
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<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 undefined.</p>
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<h2 class="exceptions" id="protected-mode-exceptions">Protected Mode Exceptions<a class="anchor" href="#protected-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#UD</td>
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<td>If the LOCK prefix is used.</td></tr></table>
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<h2 class="exceptions" id="real-address-mode-exceptions">Real-Address Mode Exceptions<a class="anchor" href="#real-address-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#UD</td>
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<td>If the LOCK prefix is used.</td></tr></table>
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<h2 class="exceptions" id="virtual-8086-mode-exceptions">Virtual-8086 Mode Exceptions<a class="anchor" href="#virtual-8086-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#UD</td>
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<td>If the LOCK prefix is used.</td></tr></table>
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<h2 class="exceptions" id="compatibility-mode-exceptions">Compatibility Mode Exceptions<a class="anchor" href="#compatibility-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#UD</td>
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<td>If the LOCK prefix is used.</td></tr></table>
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<h2 class="exceptions" id="64-bit-mode-exceptions">64-Bit Mode Exceptions<a class="anchor" href="#64-bit-mode-exceptions">
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¶
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</a></h2>
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<table>
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<tr>
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<td>#UD</td>
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<td>If in 64-bit mode.</td></tr></table><footer><p>
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This UNOFFICIAL, mechanically-separated, non-verified reference is provided for convenience, but it may be
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inc<span style="opacity: 0.2">omp</span>lete or b<sub>r</sub>oke<sub>n</sub> in various obvious or non-obvious
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ways. Refer to <a href="https://software.intel.com/en-us/download/intel-64-and-ia-32-architectures-sdm-combined-volumes-1-2a-2b-2c-2d-3a-3b-3c-3d-and-4">Intel® 64 and IA-32 Architectures Software Developer’s Manual</a> for anything serious.
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</p></footer></body></html>
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