📄 asm_tutorial_07.html
字号:
<!doctype HTML public "-//W3O//DTD W3 HTML 3.0//EN"><HTML><HEAD><TITLE>8086 Assembler Tutorial for Beginners (Part 7)</TITLE><META name="description" content="Program Flow Control - Tutorial for Beginners"><META name="keywords" content="program flow control, 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 7)</B></FONT><BR><BR><FONT SIZE=+2><B>Program Flow Control</B></FONT><BR><BR>Controlling the program flow is a very important thing, this is whereyour program can make decisions according to certain conditions.<BR><BR><UL><LI><B>Unconditional Jumps</B><BR><BR> The basic instruction that transfers control to another point in the program is <B>JMP</B>. <BR><BR> The basic syntax of <B>JMP</B> instruction: <BLOCKQUOTE> <FONT FACE="Fixedsys"> JMP <U>label</U> </FONT> </BLOCKQUOTE> To declare a <I>label</I> in your program, just type its name and add "<B>:</B>" to the end, label can be any character combination but it cannot start with a number, for example here are 3 legal label definitions: <BLOCKQUOTE> <FONT FACE="Fixedsys"> label1:<BR> label2:<BR> a: </FONT> </BLOCKQUOTE> Label can be declared on a separate line or before any other instruction, for example: <BLOCKQUOTE> <FONT FACE="Fixedsys"> x1:<BR> MOV AX, 1<BR><BR> x2: MOV AX, 2<BR> </FONT> </BLOCKQUOTE> Here is an example of <B>JMP</B> instruction:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AX, 5 ; set AX to 5.MOV BX, 2 ; set BX to 2.JMP calc ; go to 'calc'.back: JMP stop ; go to 'stop'.calc:ADD AX, BX ; add BX to AX.JMP back ; go 'back'.stop:RET ; return to operating system.END ; directive to stop the compiler.</FONT></PRE></TD></TR></TABLE><BR>Of course there is an easier way to calculate the some of two numbers,but it's still a good example of <B>JMP</B> instruction.<BR>As you can see from this example <B>JMP</B> is able to transfer control bothforward and backward.It can jump anywhere in current code segment (65,535 bytes).<BR><BR><BR></LI><LI><B>Short Conditional Jumps</B><BR><BR> Unlike <B>JMP</B> instruction that does an unconditional jump, there are instructions that do a conditional jumps (jump only when some conditions are in act). These instructions are divided in three groups, first group just test single flag, second compares numbers as signed, and third compares numbers as unsigned. <BR><BR> <B>Jump instructions that test single flag</B> <BR><BR> <FONT FACE="Fixedsys"> <TABLE BORDER=1 CELLPADDING=7> <TR> <TD BGCOLOR=yellow>Instruction</TD> <TD BGCOLOR=yellow>Description</TD> <TD BGCOLOR=yellow>Condition</TD> <TD BGCOLOR=yellow>Opposite Instruction</TD> </TR> <TR> <TD>JZ , JE</TD> <TD>Jump if Zero (Equal).</TD> <TD> ZF = 1</TD> <TD>JNZ, JNE</TD> </TR> <TR> <TD><NOBR>JC , JB, JNAE</NOBR></TD> <TD>Jump if Carry (Below, Not Above Equal).</TD> <TD> CF = 1</TD> <TD>JNC, JNB, JAE</TD> </TR> <TR> <TD>JS</TD> <TD>Jump if Sign.</TD> <TD> SF = 1</TD> <TD>JNS</TD> </TR> <TR> <TD>JO</TD> <TD>Jump if Overflow.</TD> <TD> OF = 1</TD> <TD>JNO</TD> </TR> <TR> <TD>JPE, JP</TD> <TD>Jump if Parity Even.</TD> <TD> PF = 1</TD> <TD>JPO</TD> </TR> <TR><TD></TD></TR> <TR> <TD>JNZ , JNE</TD> <TD>Jump if Not Zero (Not Equal).</TD> <TD> ZF = 0</TD> <TD>JZ, JE</TD> </TR> <TR> <TD><NOBR>JNC , JNB, JAE</NOBR></TD> <TD>Jump if Not Carry (Not Below, Above Equal).</TD> <TD> CF = 0</TD> <TD>JC, JB, JNAE</TD> </TR> <TR> <TD>JNS</TD> <TD>Jump if Not Sign.</TD> <TD> SF = 0</TD> <TD>JS</TD> </TR> <TR> <TD>JNO</TD> <TD>Jump if Not Overflow.</TD> <TD> OF = 0</TD> <TD>JO</TD> </TR> <TR> <TD>JPO, JNP</TD> <TD>Jump if Parity Odd (No Parity).</TD> <TD> PF = 0</TD> <TD>JPE, JP</TD> </TR> </TABLE> </FONT> <BR><BR> As you can see there are some instructions that do that same thing, that's correct, they even are assembled into the same machine code, so it's good to remember that when you compile <B>JE</B> instruction - you will get it disassembled as: <B>JZ</B>.<BR> Different names are used to make programs easier to understand and code.<BR><BR><BR> <B>Jump instructions for signed numbers</B> <BR><BR> <FONT FACE="Fixedsys"> <TABLE BORDER=1 CELLPADDING=7> <TR> <TD BGCOLOR=yellow>Instruction</TD> <TD BGCOLOR=yellow>Description</TD> <TD BGCOLOR=yellow>Condition</TD> <TD BGCOLOR=yellow>Opposite Instruction</TD> </TR> <TR> <TD>JE , JZ</TD> <TD>Jump if Equal (<FONT COLOR=red>=</FONT>).<BR> Jump if Zero.</TD> <TD ALIGN=Center>ZF = 1</TD> <TD>JNE, JNZ</TD> </TR> <TR> <TD>JNE , JNZ</TD> <TD>Jump if Not Equal (<FONT COLOR=red><></FONT>).<BR> Jump if Not Zero.</TD> <TD ALIGN=Center>ZF = 0</TD> <TD>JE, JZ</TD> </TR> <TR> <TD>JG , JNLE</TD> <TD>Jump if Greater (<FONT COLOR=red>></FONT>).<BR> Jump if Not Less or Equal <NOBR>(<FONT COLOR=red>not <=</FONT>).</NOBR></TD> <TD ALIGN=Center>ZF = 0<BR>and<BR>SF = OF</TD> <TD>JNG, JLE</TD> </TR> <TR> <TD>JL , JNGE</TD> <TD>Jump if Less (<FONT COLOR=red><</FONT>).<BR> Jump if Not Greater or Equal <NOBR>(<FONT COLOR=red>not >=</FONT>).</NOBR></TD> <TD ALIGN=Center>SF <> OF</TD> <TD>JNL, JGE</TD> </TR> <TR> <TD>JGE , JNL</TD> <TD><NOBR>Jump if Greater or Equal (<FONT COLOR=red>>=</FONT>).</NOBR><BR> Jump if Not Less (<FONT COLOR=red>not <</FONT>).</TD> <TD ALIGN=Center>SF = OF</TD> <TD>JNGE, JL</TD> </TR> <TR> <TD>JLE , JNG</TD> <TD><NOBR>Jump if Less or Equal (<FONT COLOR=red><=</FONT>).</NOBR><BR> Jump if Not Greater <NOBR>(<FONT COLOR=red>not ></FONT>).</NOBR></TD> <TD ALIGN=Center>ZF = 1<BR>or<BR>SF <> OF</TD> <TD>JNLE, JG</TD> </TR> </TABLE> </FONT> <BR><BR> <FONT FACE="Fixedsys"><></FONT> - sign means not equal.<BR><BR><BR> <B>Jump instructions for unsigned numbers</B> <BR><BR> <FONT FACE="Fixedsys"> <TABLE BORDER=1 CELLPADDING=7> <TR> <TD BGCOLOR=yellow>Instruction</TD> <TD BGCOLOR=yellow>Description</TD> <TD BGCOLOR=yellow>Condition</TD> <TD BGCOLOR=yellow>Opposite Instruction</TD> </TR> <TR> <TD>JE , JZ</TD> <TD>Jump if Equal (<FONT COLOR=red>=</FONT>).<BR> Jump if Zero.</TD> <TD ALIGN=Center>ZF = 1</TD> <TD>JNE, JNZ</TD> </TR> <TR> <TD>JNE , JNZ</TD> <TD>Jump if Not Equal (<FONT COLOR=red><></FONT>).<BR> Jump if Not Zero.</TD> <TD ALIGN=Center>ZF = 0</TD> <TD>JE, JZ</TD> </TR> <TR> <TD>JA , JNBE</TD> <TD>Jump if Above (<FONT COLOR=red>></FONT>).<BR> Jump if Not Below or Equal <NOBR>(<FONT COLOR=red>not <=</FONT>).</NOBR></TD> <TD ALIGN=Center>CF = 0<BR>and<BR>ZF = 0</TD> <TD>JNA, JBE</TD> </TR> <TR> <TD><NOBR>JB , JNAE, JC</NOBR></TD> <TD>Jump if Below (<FONT COLOR=red><</FONT>).<BR> Jump if Not Above or Equal <NOBR>(<FONT COLOR=red>not >=</FONT>).</NOBR><BR> Jump if Carry.</TD> <TD ALIGN=Center>CF = 1</TD> <TD>JNB, JAE, JNC</TD> </TR> <TR> <TD><NOBR>JAE , JNB, JNC</NOBR></TD> <TD>Jump if Above or Equal (<FONT COLOR=red>>=</FONT>).<BR> Jump if Not Below <NOBR>(<FONT COLOR=red>not <</FONT>).</NOBR><BR> Jump if Not Carry.</TD> <TD ALIGN=Center>CF = 0</TD> <TD>JNAE, JB</TD> </TR> <TR> <TD><NOBR>JBE , JNA</NOBR></TD> <TD>Jump if Below or Equal (<FONT COLOR=red><=</FONT>).<BR> Jump if Not Above <NOBR>(<FONT COLOR=red>not ></FONT>).</NOBR></TD> <TD ALIGN=Center>CF = 1<BR>or<BR>ZF = 1</TD> <TD>JNBE, JA</TD> </TR> </TABLE> </FONT> <BR><BR> Generally, when it is required to compare numeric values <B>CMP</B> instruction is used (it does the same as <B>SUB</B> (subtract) instruction, but does not keep the result, just affects the flags).<BR><BR> The logic is very simple, for example:<BR> it's required to compare <FONT FACE="Fixedsys">5</FONT> and <FONT FACE="Fixedsys">2</FONT>,<BR> <FONT FACE="Fixedsys">5 - 2 = 3</FONT><BR> the result is not zero (Zero Flag is set to <FONT FACE="Fixedsys">0</FONT>). <BR><BR> Another example:<BR> it's required to compare <FONT FACE="Fixedsys">7</FONT> and <FONT FACE="Fixedsys">7</FONT>,<BR> <FONT FACE="Fixedsys">7 - 7 = 0</FONT><BR> the result is zero! (Zero Flag is set to <FONT FACE="Fixedsys">1</FONT> and <B>JZ</B> or <B>JE</B> will do the jump). <BR><BR> Here is an example of <B>CMP</B> instruction and conditional jump:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">include emu8086.incORG 100hMOV AL, 25 ; set AL to 25.MOV BL, 10 ; set BL to 10.CMP AL, BL ; compare AL - BL.JE equal ; jump if AL = BL (ZF = 1).PUTC 'N' ; if it gets here, then AL <> BL,JMP stop ; so print 'N', and jump to stop.equal: ; if gets here,PUTC 'Y' ; then AL = BL, so print 'Y'.stop:RET ; gets here no matter what.END</FONT></PRE></TD></TR></TABLE><BR><BR>Try the above example with different numbers for <B>AL</B> and <B>BL</B>,open flags by clicking on [<B>FLAGS</B>] button, use [<B>Single Step</B>]and see what happens, don't forget to recompile and reload after every change(use <B>F5</B> shortcut).<BR><BR><HR><BR>All conditional jumps have one big limitation, unlike <B>JMP</B> instructionthey can only jump <B>127</B> bytes forward and <B>128</B> bytes backward (note that mostinstructions are assembled into 3 or more bytes).<BR><BR>We can easily avoid this limitation using a cute trick:<BR><BR> <UL> <LI>Get a opposite conditional jump instruction from the table above, make it jump to <I>label_x</I>. <BR><BR></LI> <LI>Use <B>JMP</B> instruction to jump to desired location. <BR><BR></LI> <LI>Define <I>label_x:</I> just after the <B>JMP</B> instruction. <BR><BR></LI> </UL> <I>label_x:</I> - can be any valid label name. <BR><BR> Here is an example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">include emu8086.incORG 100hMOV AL, 25 ; set AL to 25.MOV BL, 10 ; set BL to 10.CMP AL, BL ; compare AL - BL.<FONT COLOR=green>JNE not_equal ; jump if AL <> BL (ZF = 0).JMP equalnot_equal:</FONT>; let's assume that here we; have a code that is assembled; to more then 127 bytes...PUTC 'N' ; if it gets here, then AL <> BL,JMP stop ; so print 'N', and jump to stop.equal: ; if gets here,PUTC 'Y' ; then AL = BL, so print 'Y'.stop:RET ; gets here no matter what.END</FONT></PRE></TD></TR></TABLE></UL><BR><BR><HR><BR>Another, yet rarely used method is providing an immediate value insteadof a label. When immediate value starts with a '$' character relative jumpis performed, otherwise compiler calculates instruction that jumps directlyto given offset. For example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100h; unconditional jump forward:; skip over next 2 bytes,JMP $2a DB 3 ; 1 byte.b DB 4 ; 1 byte.; JCC jump back 7 bytes:; (JMP takes 2 bytes itself)MOV BL,9DEC BL ; 2 bytes.CMP BL, 0 ; 3 bytes.JNE $-7RETEND</FONT></PRE></TD></TR></TABLE><BR><BR><BR><HR><CENTER><A HREF="asm_tutorial_06.html"><B> <<< Previous Part <<< </B></A> <A HREF="asm_tutorial_08.html"><B> >>> Next Part >>> </B></A></CENTER><HR><BR></FONT></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -