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<!doctype HTML public "-//W3O//DTD W3 HTML 3.0//EN"><HTML><HEAD><TITLE>8086 Assembler Tutorial for Beginners (Part 6)</TITLE><META name="description" content="Arithmetic and Logic Instructions"><META name="keywords" content="arithmetic, logic, instructions, 8086, tutorial, programming, assembler tutorial, tutorial for begginers"><META name="robots" content="nofollow"></HEAD><BODY bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#007099" alink="#FF0000"><TABLE WIDTH=80%><TR><TD><FONT FACE="Verdana" SIZE=3><FONT SIZE=+1><B>8086 Assembler Tutorial for Beginners (Part 6)</B></FONT><BR><BR><FONT SIZE=+2><B>Arithmetic and Logic Instructions</B></FONT><BR><BR>Most Arithmetic and Logic Instructions affect the processor status register(or <B>Flags</B>)<BR><BR><IMG SRC="flags.gif"><BR><BR>As you may see there are 16 bits in this register, each bit is called a <B>flag</B>and can take a value of <B>1</B> or <B>0</B>.<BR><BR><UL><LI><B>Carry Flag (CF)</B> - this flag is set to <B>1</B> when there isan <B>unsigned overflow</B>.For example when you add bytes <B>255 + 1</B> (result is not in range 0...255).When there is no overflow this flag is set to <B>0</B>.<BR><BR></LI><LI><B>Zero Flag (ZF)</B> - set to <B>1</B> when result is <B>zero</B>.For none zero result this flag is set to <B>0</B>.<BR><BR></LI><LI><B>Sign Flag (SF)</B> - set to <B>1</B> when result is <B>negative</B>.When result is <B>positive</B> it is set to <B>0</B>. Actually this flagtake the value of the most significant bit.<BR><BR></LI><LI><B>Overflow Flag (OF)</B> - set to <B>1</B> when there is a <B>signed overflow</B>.For example, when you add bytes <B>100 + 50</B> (result is not in range -128...127).<BR><BR></LI><LI><B>Parity Flag (PF)</B> - this flag is set to <B>1</B> when there is even number of one bits in result, and to <B>0</B> when there is odd number of one bits. Even if result is a word only 8 low bits are analyzed!<BR><BR></LI><LI><B>Auxiliary Flag (AF)</B> - set to <B>1</B> when there is an <B>unsigned overflow</B> for low nibble (4 bits).<BR><BR></LI><LI><B>Interrupt enable Flag (IF)</B> - when this flag is set to <B>1</B> CPU reacts tointerrupts from external devices.<BR><BR></LI><LI><B>Direction Flag (DF)</B> - this flag is used by some instructions to processdata chains, when this flag is set to <B>0</B> - the processing is done forward,when this flag is set to <B>1</B> the processing is done backward.<BR><BR></LI></UL><HR><BR>There are 3 groups of instructions.<BR><BR><HR><BR>First group: <B>ADD</B>, <B>SUB</B>,<B>CMP</B>, <B>AND</B>, <B>TEST</B>, <B>OR</B>, <B>XOR</B><BR><BR>These types of operands are supported:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys"> REG, memory<BR> memory, REG<BR> REG, REG<BR> memory, immediate<BR> REG, immediate</FONT></BLOCKQUOTE><B>REG</B>:<FONT SIZE=-1> AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.</FONT><BR><BR><B>memory</B>: [BX], [BX+SI+7], variable, etc...<BR><BR><B>immediate</B>: 5, -24, 3Fh, 10001101b, etc...<BR><BR>After operation between operands, result is always stored in first operand.<B>CMP</B> and <B>TEST</B> instructions affect flags only and do not store aresult (these instruction are used to make decisions during program execution).<BR><BR>These instructions affect these flags only:<BR> <B>CF</B>, <B>ZF</B>,<B>SF</B>, <B>OF</B>, <B>PF</B>, <B>AF</B>.<BR><BR><UL><LI><B>ADD</B> - add second operand to first.<BR><BR></LI><LI><B>SUB</B> - Subtract second operand to first.<BR><BR></LI><LI><B>CMP</B> - Subtract second operand from first <B>for flags only</B>.<BR><BR></LI><LI><B>AND</B> - Logical AND between all bits of two operands. These rules apply:<BR> <BLOCKQUOTE> <FONT FACE="Fixedsys"> 1 AND 1 = 1<BR> 1 AND 0 = 0<BR> 0 AND 1 = 0<BR> 0 AND 0 = 0<BR> </FONT> </BLOCKQUOTE> As you see we get <B>1</B> only when both bits are <B>1</B>.<BR><BR></LI><LI><B>TEST</B> - The same as <B>AND</B> but <B>for flags only</B>.<BR><BR></LI><LI><B>OR</B> - Logical OR between all bits of two operands. These rules apply:<BR> <BLOCKQUOTE> <FONT FACE="Fixedsys"> 1 OR 1 = 1<BR> 1 OR 0 = 1<BR> 0 OR 1 = 1<BR> 0 OR 0 = 0<BR> </FONT> </BLOCKQUOTE> As you see we get <B>1</B> every time when at least one of the bits is <B>1</B>.<BR><BR></LI><LI><B>XOR</B> - Logical XOR (exclusive OR) between all bits of two operands. These rules apply:<BR> <BLOCKQUOTE> <FONT FACE="Fixedsys"> 1 XOR 1 = 0<BR> 1 XOR 0 = 1<BR> 0 XOR 1 = 1<BR> 0 XOR 0 = 0<BR> </FONT> </BLOCKQUOTE> As you see we get <B>1</B> every time when bits are different from each other.<BR><BR></LI></UL><HR><BR>Second group: <B>MUL</B>, <B>IMUL</B>, <B>DIV</B>, <B>IDIV</B><BR><BR>These types of operands are supported:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys"> REG<BR> memory<BR></FONT></BLOCKQUOTE><B>REG</B>:<FONT SIZE=-1> AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.</FONT><BR><BR><B>memory</B>: [BX], [BX+SI+7], variable, etc...<BR><BR><B>MUL</B> and <B>IMUL</B> instructions affect these flags only:<BR> <B>CF</B>, <B>OF</B><BR>When result is over operand size these flags are set to <B>1</B>, when resultfits in operand size these flags are set to <B>0</B>.<BR><BR>For <B>DIV</B> and <B>IDIV</B> flags are undefined.<BR><BR><UL><LI><B>MUL</B> - Unsigned multiply: <BLOCKQUOTE> when operand is a <B>byte</B>:<BR> <FONT FACE="Fixedsys">AX = AL * operand</FONT>. </BLOCKQUOTE> <BLOCKQUOTE> when operand is a <B>word</B>:<BR> <FONT FACE="Fixedsys">(DX AX) = AX * operand</FONT>. </BLOCKQUOTE></LI><LI><B>IMUL</B> - Signed multiply: <BLOCKQUOTE> when operand is a <B>byte</B>:<BR> <FONT FACE="Fixedsys">AX = AL * operand</FONT>. </BLOCKQUOTE> <BLOCKQUOTE> when operand is a <B>word</B>:<BR> <FONT FACE="Fixedsys">(DX AX) = AX * operand</FONT>. </BLOCKQUOTE></LI><LI><B>DIV</B> - Unsigned divide: <BLOCKQUOTE> when operand is a <B>byte</B>:<BR> <FONT FACE="Fixedsys">AL = AX / operand<BR> AH = remainder (modulus). </FONT>. </BLOCKQUOTE> <BLOCKQUOTE> when operand is a <B>word</B>:<BR> <FONT FACE="Fixedsys">AX = (DX AX) / operand<BR> DX = remainder (modulus). </FONT>. </BLOCKQUOTE></LI><LI><B>IDIV</B> - Signed divide: <BLOCKQUOTE> when operand is a <B>byte</B>:<BR> <FONT FACE="Fixedsys">AL = AX / operand<BR> AH = remainder (modulus). </FONT>. </BLOCKQUOTE> <BLOCKQUOTE> when operand is a <B>word</B>:<BR> <FONT FACE="Fixedsys">AX = (DX AX) / operand<BR> DX = remainder (modulus). </FONT>. </BLOCKQUOTE></LI></UL><HR><BR>Third group: <B>INC</B>, <B>DEC</B>, <B>NOT</B>, <B>NEG</B><BR><BR>These types of operands are supported:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys"> REG<BR> memory<BR></FONT></BLOCKQUOTE><B>REG</B>:<FONT SIZE=-1> AX, BX, CX, DX, AH, AL, BL, BH, CH, CL, DH, DL, DI, SI, BP, SP.</FONT><BR><BR><B>memory</B>: [BX], [BX+SI+7], variable, etc...<BR><BR><B>INC</B>, <B>DEC</B> instructions affect these flags only:<BR> <B>ZF</B>, <B>SF</B>, <B>OF</B>, <B>PF</B>, <B>AF</B>.<BR><BR><B>NOT</B> instruction does not affect any flags!<BR><BR><B>NEG</B> instruction affects these flags only:<BR> <B>CF</B>, <B>ZF</B>, <B>SF</B>, <B>OF</B>, <B>PF</B>, <B>AF</B>.<BR><BR><UL><LI><B>NOT</B> - Reverse each bit of operand.<BR><BR></LI><LI><B>NEG</B> - Make operand negative (two's complement). Actually it reverses each bit of operand and then adds 1 to it. For example 5 will become -5, and -2 will become 2.</LI></UL><HR><BR><BR><BR><HR><CENTER><A HREF="asm_tutorial_05.html"><B> <<< Previous Part <<< </B></A> <A HREF="asm_tutorial_07.html"><B> >>> Next Part >>> </B></A></CENTER><HR><BR></FONT></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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