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<!doctype HTML public "-//W3O//DTD W3 HTML 3.0//EN"><HTML><HEAD><TITLE>8086 Assembler Tutorial for Beginners (Part 3)</TITLE><META name="description" content="Variables - 8086 Assembler Tutorial for Beginners"><META name="keywords" content="variables, 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 3)</B></FONT><BR><BR><FONT SIZE=+2><B>Variables</B></FONT><BR><BR>Variable is a memory location. For a programmer it is much easier to have somevalue be kept in a variable named "<B>var1</B>" then at the address 5A73:235B,especially when you have 10 or more variables.<BR><BR>Our compiler supports two types of variables: <B>BYTE</B> and <B>WORD</B>.<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=100%><TR><TD>Syntax for a variable declaration:<BR><BR><I><U>name</U></I> <B>DB</B> <I><U>value</U></I><BR><BR><I><U>name</U></I> <B>DW</B> <I><U>value</U></I><BR><BR><B>DB</B> - stays for <U>D</U>efine <U>B</U>yte.<BR><B>DW</B> - stays for <U>D</U>efine <U>W</U>ord.<BR><BR><I><U>name</U></I> - can be any letter or digit combination, though it should start with a letter. It's possible to declare unnamed variables by not specifying the name (this variable will have an address but no name).<BR><BR><I><U>value</U></I> - can be any numeric value in any supported numbering system (hexadecimal, binary, or decimal), or "<B>?</B>" symbol for variables that are not initialized.</TD></TR></TABLE><BR><BR>As you probably know from <I>part 2</I> of this tutorial,<B>MOV</B> instruction is used to copy values from source to destination.<BR>Let's see another example with <B>MOV</B> instruction:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">#MAKE_COM#ORG 100hMOV AL, var1MOV BX, var2RET ; stops the program.VAR1 DB 7var2 DW 1234h</FONT></PRE></TD></TR></TABLE><BR>Copy the above code to <I>Emu8086</I> source editor, and press <B>F5</B> key tocompile and load it in the emulator.You should get something like:<BR><BR><IMG SRC="tut3a.gif" WIDTH=371 HEIGHT=347><BR><BR>As you see this looks a lot like our example, except that variables are replacedwith actual memory locations. When compiler makes machine code, it automaticallyreplaces all variable names with their <B>offsets</B>. By default segment isloaded in <B>DS</B> register (when <B>COM</B> files is loaded the valueof <B>DS</B> register is set to the same value as <B>CS</B> register - code segment).<BR><BR>In memory list first row is an <B>offset</B>, second row is a<B>hexadecimal value</B>, third row is <B>decimal value</B>, and last row isan <B>ASCII</B> character value.<BR><BR>Compiler is not case sensitive, so "<B>VAR1</B>" and "<B>var1</B>" refer to the same variable.<BR><BR>The offset of <B>VAR1</B> is <B>0108h</B>, and full address is <B>0B56:0108</B>.<BR><BR>The offset of <B>var2</B> is <B>0109h</B>, and full address is <B>0B56:0109</B>,this variable is a <B>WORD</B> so it occupies <B>2 BYTES</B>.It is assumed that low byte is stored at lower address,so <B>34h</B> is located before <B>12h</B>.<BR><BR>You can see that there are some other instructions after the <B>RET</B>instruction,this happens because disassembler has no idea about where the data starts,it just processes the values in memory and it understands them asvalid 8086 instructions (we will learn them later).<BR>You can even write the same program using <B>DB</B> directive only:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">#MAKE_COM#ORG 100hDB 0A0hDB 08hDB 01hDB 8BhDB 1EhDB 09hDB 01hDB 0C3hDB 7DB 34hDB 12h</FONT></PRE></TD></TR></TABLE><BR>Copy the above code to <I>Emu8086</I> source editor, and press <B>F5</B> key tocompile and load it in the emulator. You should get the same disassembled code,and the same functionality!<BR><BR>As you may guess, the compiler just converts the program source tothe set of bytes, this set is called <B>machine code</B>, processorunderstands the <B>machine code</B> and executes it.<BR><BR><B>ORG 100h</B> is a compiler directive (it tells compiler how tohandle the source code). This directive is very important when youwork with variables. It tells compiler that the executable filewill be loaded at the <B>offset</B> of 100h (256 bytes), so compilershould calculate the correct address for all variables whenit replaces the variable names with their <B>offsets</B>.Directives are never converted to any real <B>machine code</B>.<BR>Why executable file is loaded at <B>offset</B> of <B>100h</B>? Operatingsystem keeps some data about the program in the first 256 bytes ofthe <B>CS</B> (code segment), such as command line parameters and etc.<BR>Though this is true for <B>COM</B> files only, <B>EXE</B> files are loadedat offset of <B>0000</B>, and generally use special segment for variables.Maybe we'll talk more about <B>EXE</B> files later.<BR><BR><HR><BR><FONT SIZE=+2><B>Arrays</B></FONT><BR><BR>Arrays can be seen as chains of variables. A text string is an example of abyte array, each character is presented as an ASCII code value (0..255).<BR><BR>Here are some array definition examples:<BR><BR><FONT FACE="Fixedsys">a DB 48h, 65h, 6Ch, 6Ch, 6Fh, 00h<BR>b DB 'Hello', 0</FONT><BR><BR><I>b</I> is an exact copy of the <I>a</I> array, when compiler seesa string inside quotes it automatically converts it to set of bytes. Thischart shows a part of the memory where these arrays are declared:<BR><BR><IMG SRC="array.gif" WIDTH=487 HEIGHT=109><BR><BR>You can access the value of any element in array using square brackets,for example:<BR><FONT FACE="Fixedsys">MOV AL, a[3]</FONT><BR><BR>You can also use any of the memory index registers <B>BX, SI, DI, BP</B>,for example:<BR><FONT FACE="Fixedsys">MOV SI, 3<BR>MOV AL, a[SI]<BR></FONT><BR><BR>If you need to declare a large array you can use <B>DUP</B> operator.<BR>The syntax for <B>DUP</B>:<BR><BR><FONT FACE="Fixedsys"><U>number</U> DUP ( <U>value(s)</U> )</FONT><BR><U>number</U> - number of duplicate to make (any constant value).<BR><U>value</U> - expression that DUP will duplicate.<BR><BR>for example:<BR><FONT FACE="Fixedsys">c DB 5 DUP(9)</FONT><BR>is an alternative way of declaring:<BR><FONT FACE="Fixedsys">c DB 9, 9, 9, 9, 9</FONT><BR><BR>one more example:<BR><FONT FACE="Fixedsys">d DB 5 DUP(1, 2)</FONT><BR>is an alternative way of declaring:<BR><FONT FACE="Fixedsys">d DB 1, 2, 1, 2, 1, 2, 1, 2, 1, 2</FONT><BR><BR>Of course, you can use <B>DW</B> instead of <B>DB</B> if it's required to keepvalues larger then 255, or smaller then -128. <B>DW</B> cannot be usedto declare strings!<BR><BR>The expansion of <B>DUP</B> operand should not be over 1020 characters!(the expansion of last example is 13 chars), if you need to declare huge arraydivide declaration it in two lines (you will get a single hugearray in the memory).<BR><BR><HR><BR><FONT SIZE=+2><B>Getting the Address of a Variable</B></FONT><BR><BR>There is <B>LEA</B> (Load Effective Address) instruction andalternative <B>OFFSET</B> operator. Both <B>OFFSET</B> and<B>LEA</B> can be used to get the offset address of the variable.<BR><B>LEA</B> is more powerful because it also allows you to get the address ofan indexed variables. Getting the address of the variable can be very usefulin some situations, for example when you need to pass parameters to a procedure.<BR><BR><HR><BR><B>Reminder:</B><BR><FONT SIZE=1>In order to tell the compiler about data type,<BR>these prefixes should be used:<BR><BR><B>BYTE PTR</B> - for byte.<BR><B>WORD PTR</B> - for word (two bytes).<BR><BR>For example:<BR><PRE><FONT FACE="Fixedsys">BYTE PTR [BX] ; byte access. orWORD PTR [BX] ; word access.</FONT></PRE><I>Emu8086</I> supports shorter prefixes as well:<BR><BR><B>b.</B> - for <B>BYTE PTR</B><BR><B>w.</B> - for <B>WORD PTR</B><BR><BR>sometimes compiler can calculate the data type automatically,but you may not and should not rely on that when one of theoperands is an immediate value.<BR><BR></FONT><HR><BR><BR>Here is first example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AL, VAR1 ; check value of VAR1 by moving it to AL.LEA BX, VAR1 ; get address of VAR1 in BX.MOV BYTE PTR [BX], 44h ; modify the contents of VAR1.MOV AL, VAR1 ; check value of VAR1 by moving it to AL.RETVAR1 DB 22hEND</FONT></PRE></TD></TR></TABLE><BR><BR>Here is another example, that uses <B>OFFSET</B> instead of <B>LEA</B>:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AL, VAR1 ; check value of VAR1 by moving it to AL.MOV BX, OFFSET VAR1 ; get address of VAR1 in BX.MOV BYTE PTR [BX], 44h ; modify the contents of VAR1.MOV AL, VAR1 ; check value of VAR1 by moving it to AL.RETVAR1 DB 22hEND</FONT></PRE></TD></TR></TABLE><BR>Both examples have the same functionality.<BR><BR>These lines:<BR><FONT FACE="Fixedsys">LEA BX, VAR1<BR>MOV BX, OFFSET VAR1</FONT><BR>are even compiled into the same machine code: <FONT FACE="Fixedsys">MOV BX, num</FONT><BR><I>num</I> is a 16 bit value of the variable offset.<BR><BR>Please note that only these registers can be usedinside square brackets (as memory pointers):<B>BX, SI, DI, BP</B>!<BR>(see previous part of the tutorial).<BR><BR><HR><BR><FONT SIZE=+2><B>Constants</B></FONT><BR><BR>Constants are just like variables, but they exist only until your programis compiled (assembled). After definition of a constant its value cannotbe changed. To define constants <B>EQU</B> directive is used:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys">name EQU < any expression ></FONT></BLOCKQUOTE><BR>For example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><FONT FACE="Fixedsys">k EQU 5<BR><BR>MOV AX, k</FONT></TD></TR></TABLE><BR><BR>The above example is functionally identical to code:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><FONT FACE="Fixedsys">MOV AX, 5</FONT></TD></TR></TABLE><BR><BR><HR><BR>You can view variables while your program executes by selecting "<B>Variables</B>"from the "<B>View</B>" menu of emulator.<BR><BR><IMG SRC="varview.gif" WIDTH=324 HEIGHT=221><BR><BR>To view arrays you should click on a variable and set <B>Elements</B> propertyto array size. In assembly language there are not strict data types, so any variablecan be presented as an array.<BR><BR>Variable can be viewed in any numbering system:<BR><UL><LI><B>HEX</B> - hexadecimal (base 16).<BR></LI><LI><B>BIN</B> - binary (base 2).<BR></LI><LI><B>OCT</B> - octal (base 8).<BR></LI><LI><B>SIGNED</B> - signed decimal (base 10).<BR></LI><LI><B>UNSIGNED</B> - unsigned decimal (base 10).<BR></LI><LI><B>CHAR</B> - ASCII char code (there are 256 symbols, some symbols are invisible).</LI></UL><BR>You can edit a variable's value when your program is running, simply double click it,or select it and click <B>Edit</B> button.<BR><BR>It is possible to enter numbers in any system, hexadecimal numbers should have"<B>h</B>" suffix, binary "<B>b</B>" suffix, octal "<B>o</B>" suffix, decimal numbersrequire no suffix. String can be entered this way:<BR><B>'hello world', 0</B><BR>(this string is zero terminated).<BR><BR>Arrays may be entered this way:<BR><B>1, 2, 3, 4, 5</B><BR>(the array can be array of bytes or words, it depends whether <B>BYTE</B> or <B>WORD</B> is selectedfor edited variable).<BR><BR>Expressions are automatically converted, for example:<BR>when this expression is entered:<BR><B>5 + 2</B><BR>it will be converted to <B>7</B> etc...<BR><BR><HR><BR><BR><BR><HR><CENTER><A HREF="asm_tutorial_02.html"><B> <<< Previous Part <<< </B></A> <A HREF="asm_tutorial_04.html"><B> >>> Next Part >>> </B></A></CENTER><HR><BR></FONT></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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