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<!doctype HTML public "-//W3O//DTD W3 HTML 3.0//EN"><HTML><HEAD><TITLE>8086 Assembler Tutorial for Beginners (Part 8)</TITLE><META name="description" content="Procedures - Tutorial for Beginners"><META name="keywords" content="procedures, 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 8)</B></FONT><BR><BR><FONT SIZE=+2><B>Procedures</B></FONT><BR><BR>Procedure is a part of code that can be called from yourprogram in order to make some specific task. Proceduresmake program more structural and easier to understand.Generally procedure returns to the same point from where it wascalled.<BR><BR> The syntax for procedure declaration: <BLOCKQUOTE> <FONT FACE="Fixedsys"> <U>name</U> PROC<BR><BR> ; here goes the code<BR> ; of the procedure ...<BR><BR> RET<BR> <U>name</U> ENDP </FONT> </BLOCKQUOTE><BR><U>name</U> - is the procedure name, the same name should be in the top and the bottom, this is used to check correct closing of procedures.<BR><BR>Probably, you already know that <B>RET</B> instruction is used to returnto operating system. The same instruction is used to return from procedure(actually operating system sees your program as a special procedure).<BR><BR><B>PROC</B> and <B>ENDP</B> are compiler directives, so they are not assembledinto any real machine code. Compiler just remembers the address of procedure.<BR><BR><B>CALL</B> instruction is used to call a procedure.<BR><BR>Here is an example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hCALL m1MOV AX, 2RET ; return to operating system.m1 PROCMOV BX, 5RET ; return to caller.m1 ENDPEND</FONT></PRE></TD></TR></TABLE><BR><BR>The above example calls procedure <B>m1</B>, does <B>MOV BX, 5</B>, and returnsto the next instruction after <B>CALL</B>: <B>MOV AX, 2</B>.<BR><BR>There are several ways to pass parameters to procedure, the easiest way topass parameters is by using registers, here is another example of a procedurethat receives two parameters in <B>AL</B> and <B>BL</B> registers, multipliesthese parameters and returns the result in <B>AX</B> register:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AL, 1MOV BL, 2CALL m2CALL m2CALL m2CALL m2RET ; return to operating system.m2 PROCMUL BL ; AX = AL * BL.RET ; return to caller.m2 ENDPEND</FONT></PRE></TD></TR></TABLE><BR><BR>In the above example value of <B>AL</B> register is update every timethe procedure is called, <B>BL</B> register stays unchanged, so thisalgorithm calculates <B>2</B> in power of <B>4</B>,<BR>so final result in <B>AX</B> register is <B>16</B> (or 10h).<BR><BR><HR><BR>Here goes another example,<BR> that uses a procedureto print a <I>Hello World!</I> message:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hLEA SI, msg ; load address of msg to SI.CALL print_meRET ; return to operating system.; ==========================================================; this procedure prints a string, the string should be null; terminated (have zero in the end),; the string address should be in SI register:print_me PROCnext_char: CMP b.[SI], 0 ; check for zero to stop JE stop ; MOV AL, [SI] ; next get ASCII char. MOV AH, 0Eh ; teletype function number. INT 10h ; using interrupt to print a char in AL. ADD SI, 1 ; advance index of string array. JMP next_char ; go back, and type another char.stop:RET ; return to caller.print_me ENDP; ==========================================================msg DB 'Hello World!', 0 ; null terminated string.END</FONT></PRE></TD></TR></TABLE><BR><BR>"<B>b.</B>" - prefix before [SI] means that we need to compare bytes,not words. When you need to compare words add "<B>w.</B>" prefix instead.When one of the compared operands is a register it's not required becausecompiler knows the size of each register.<BR><BR><BR><HR><CENTER><A HREF="asm_tutorial_07.html"><B> <<< Previous Part <<< </B></A> <A HREF="asm_tutorial_09.html"><B> >>> Next Part >>> </B></A></CENTER><HR><BR></FONT></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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