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<!doctype HTML public "-//W3O//DTD W3 HTML 3.0//EN"><HTML><HEAD><TITLE>8086 Assembler Tutorial for Beginners (Part 9)</TITLE><META name="description" content="The Stack - Tutorial for Beginners"><META name="keywords" content="stack, 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 9)</B></FONT><BR><BR><FONT SIZE=+2><B>The Stack</B></FONT><BR><BR>Stack is an area of memory for keeping temporary data.Stack is used by <B>CALL</B> instruction to keep return address for procedure,<B>RET</B> instruction gets this value from the stack and returnsto that offset. Quite the same thing happens when <B>INT</B> instruction callsan interrupt, it stores in stack flag register, code segment and offset. <B>IRET</B>instruction is used to return from interrupt call.<BR><BR>We can also use the stack to keep any other data,<BR>there are two instructions that work with the stack:<BR><BR><B>PUSH</B> - stores 16 bit value in the stack.<BR><BR><B>POP</B> - gets 16 bit value from the stack.<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=100%><TR><TD>Syntax for <B>PUSH</B> instruction:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys"> PUSH REG<BR> PUSH SREG<BR> PUSH memory<BR> PUSH immediate<BR></FONT></BLOCKQUOTE><B>REG</B>: AX, BX, CX, DX, DI, SI, BP, SP.<BR><BR><B>SREG</B>: DS, ES, SS, CS.<BR><BR><B>memory</B>: [BX], [BX+SI+7], 16 bit variable, etc...<BR><BR><B>immediate</B>: 5, -24, 3Fh, 10001101b, etc...<BR><BR></TD></TR></TABLE><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=100%><TR><TD>Syntax for <B>POP</B> instruction:<BR><BR><BLOCKQUOTE><FONT FACE="Fixedsys"> POP REG<BR> POP SREG<BR> POP memory<BR></FONT></BLOCKQUOTE><B>REG</B>: AX, BX, CX, DX, DI, SI, BP, SP.<BR><BR><B>SREG</B>: DS, ES, SS, (except CS).<BR><BR><B>memory</B>: [BX], [BX+SI+7], 16 bit variable, etc...<BR><BR></TD></TR></TABLE><BR><BR>Notes:<UL><LI><B>PUSH</B> and <B>POP</B> work with 16 bit values only!<BR><BR></LI><LI>Note: <B>PUSH immediate</B> works only on 80186 CPU and later!</LI></UL><BR><BR>The stack uses <B>LIFO</B> (Last In First Out) algorithm,<BR>this means that if we push these values one by one into the stack:<BR><B>1, 2, 3, 4, 5</B><BR>the first value that we will get on pop will be <B>5</B>,then <B>4</B>, <B>3</B>, <B>2</B>, and only then <B>1</B>.<BR><BR><IMG SRC="stack.gif" WIDTH=224 HEIGHT=253><BR><BR>It is very important to do equal number of <B>PUSH</B>s and <B>POP</B>s,otherwise the stack maybe corrupted and it will be impossible to returnto operating system.As you already know we use <B>RET</B> instruction to return to operatingsystem, so when program starts there is a return address in stack (generallyit's 0000h).<BR><BR><B>PUSH</B> and <B>POP</B> instruction are especially useful because we don'thave too much registers to operate with, so here is a trick:<UL><LI>Store original value of the register in stack (using <B>PUSH</B>).<BR><BR></LI><LI>Use the register for any purpose.<BR><BR></LI><LI>Restore the original value of the register from stack (using <B>POP</B>).</LI></UL><BR>Here is an example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AX, 1234hPUSH AX ; store value of AX in stack.MOV AX, 5678h ; modify the AX value.POP AX ; restore the original value of AX.RETEND</FONT></PRE></TD></TR></TABLE><BR><BR>Another use of the stack is for exchanging the values,<BR>here is an example:<BR><BR><TABLE BORDER=1 CELLPADDING=10 WIDTH=50%><TR><TD><PRE><FONT FACE="Fixedsys">ORG 100hMOV AX, 1212h ; store 1212h in AX.MOV BX, 3434h ; store 3434h in BXPUSH AX ; store value of AX in stack.PUSH BX ; store value of BX in stack.POP AX ; set AX to original value of BX.POP BX ; set BX to original value of AX.RETEND</FONT></PRE></TD></TR></TABLE><BR>The exchange happens because stack uses <B>LIFO</B> (Last In First Out) algorithm,so when we push <B>1212h</B> and then <B>3434h</B>, on pop we will first get <B>3434h</B>and only after it <B>1212h</B>.<BR><BR><HR><BR>The stack memory area is set by <B>SS</B> (Stack Segment) register, and <B>SP</B> (StackPointer) register. Generally operating system sets values of these registers onprogram start.<BR><BR>"<B>PUSH <I>source</I></B>" instruction does the following:<BR><UL><LI>Subtract <B>2</B> from <B>SP</B> register.<BR><BR></LI><LI>Write the value of <B><I>source</I></B> to the address <B>SS:SP</B>.</LI></UL><BR>"<B>POP <I>destination</I></B>" instruction does the following:<BR><UL><LI>Write the value at the address <B>SS:SP</B> to <B><I>destination</I></B>.<BR><BR></LI><LI>Add <B>2</B> to <B>SP</B> register.</LI></UL><BR>The current address pointed by <B>SS:SP</B> is called <B>the top of the stack</B>.<BR><BR>For <B>COM</B> files stack segment is generally the code segment, andstack pointer is set to value of <B>0FFFEh</B>.At the address <B>SS:0FFFEh</B>stored a return address for <B>RET</B> instruction that is executed in the endof the program.<BR><BR>You can visually see the stack operation by clicking on [<B>Stack</B>] buttonon emulator window. The top of the stack is marked with "<B><</B>" sign.<BR><BR><BR><HR><CENTER><A HREF="asm_tutorial_08.html"><B> <<< Previous Part <<< </B></A> <A HREF="asm_tutorial_10.html"><B> >>> Next Part >>> </B></A></CENTER><HR><BR></FONT></TD></TR></TABLE></BODY></HTML>⌨️ 快捷键说明
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