06 stackler ve makrolar1

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Programming 8086 Part IV

Stacks, Macros

1

Microprocessors


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Contents

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Stacks

Macros


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Stack

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Stack is an area of memory for keepingtemporary data

Stack is used by CALLinstruction to keep return

address for procedure, RETinstruction gets this

value from the stack and returns to that offset

The stack can also be used to keep any other

data

There are two instructions that work with thestack:

PUSH- stores 16 bit value in the stack.

POP- gets 16 bit value from the stack.


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PUSH

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Syntax for PUSHinstruction:PUSH REG

PUSH SREG

PUSH memory

PUSH immediate (works only on 80186 and later)

REG: AX, BX, CX, DX, DI, SI, BP, SP

SREG: DS, ES, SS, CS

memory: [BX], [BX+SI+7], 16 bit variable, etc. immediate: 5, -24, 3Fh, 10001101b, etc.


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POP

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Syntax for POPinstruction:POP REG

POP SREG

POP memory

REG: AX, BX, CX, DX, DI, SI, BP, SP

SREG: DS, ES, SS, (except CS)

memory: [BX], [BX+SI+7], 16 bit variable, etc.


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LIFO

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The stack uses LIFO(Last In First Out)

algorithm, this means

that if we push these

values one by oneinto the stack:

1, 2, 3, 4, 5

the first value that we

will get on pop willbe 5, then 4, 3, 2, and

only then 1.


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Using the Stack

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It is very important to do equal number of PUSHs

and POPs, otherwise the stack maybe corrupted and

it will be impossible to return to operating system.

PUSHand POPinstruction are especially usefulbecause we don't have too much registers to operate

with, so here is a trick:

Store original value of the register in stack

(using PUSH). Use the register for any purpose.

Restore the original value of the register from stack

(using POP).


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A Simple Stack Example

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ORG 100h

MOV AX, 1234h

PUSH AX ; store value of AX in stack.

MOV AX, 5678h ; modify the AX value.POP AX ; restore the original value of AX.

RET

END


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Exchanging Values Using Stack

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ORG 100hMOV AX, 1234h ; store 1234h in AX.

MOV BX, 5678h ; store 5678h in BX

PUSH 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.

RET

END


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SS and SP

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The stack memory area is set by SS(Stack Segment) register,and SP(Stack Pointer) register.

Generally operating system sets values of these registers onprogram start.

The current address pointed by SS:SPis called the top of thestack.

For COMfiles stack segment is generally the code segment, andstack pointer is set to value of 0FFFEh. At the addressSS:0FFFEh stored a return address for RETinstruction that isexecuted in the end of the program.

You can visually see the stack operation by clicking on [Stack]button on emulator window. The top of the stack is marked with

"


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PUSH and POP

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"PUSH source " instruction does the following: Subtract 2from SPregister.

Write the value of source to the address SS:SP.

"POP dest inat ion " instruction does the following:

Write the value at the

address SS:SPto dest inat ion .

Add 2to SPregister.


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Initial State of the Stack

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SS

FFFF

FFFE

FFFD

FFFC

FFFB

FFFA

0000

(TOS) SP

1234H

5678H

AX

BX


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After PUSH AX

13

12

34

SS

FFFF

FFFE

FFFD

FFFC

FFFB

FFFA

0000

(TOS) SP

1234H

5678H

AX

BX


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After PUSH BX

14

12

34

56

78

SS

FFFF

FFFE

FFFD

FFFC

FFFB

FFFA

0000

(TOS) SP

1234H

5678H

AX

BX


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After POP AX

15

12

34

56

78

SS

FFFF

FFFE

FFFD

FFFC

FFFB

FFFA

0000

(TOS) SP

5678H

5678H

AX

BX


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After POP BX

16

12

34

56

78

SS

FFFF

FFFE

FFFD

FFFC

FFFB

FFFA

0000

(TOS) SP

5678H

1234H

AX

BX


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Macros

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Macros are just like procedures, but not really

Macros look like procedures, but they exist only

until your code is compiled

After compilation all macros are replaced with real

instructions

If you declared a macro and never used it in your

code, compiler will simply ignore it.


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Macro Definition

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name MACRO [parameters,...]

ENDM


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Unlike procedures, macros should be defined above

the code that uses it.

MyMacro MACRO p1, p2, p3

MOV AX, p1

MOV BX, p2

MOV CX, p3ENDM

ORG 100h

MyMacro 1, 2, 3

MyMacro 4, 5, DXRET

The code on the left is

expanded into:

MOV AX, 00001hMOV BX, 00002h

MOV CX, 00003h

MOV AX, 00004h

MOV BX, 00005h

MOV CX, DX


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Macros and Procedures

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1. When you want to use a procedure you shoulduse CALL instruction, for example:

CALL MyProc

When you want to use a macro, you can just type itsname. For example:

MyMacro2. You should use stack or any general purpose

registers to pass parameters to procedure.

To pass parameters to macro, you can just type themafter the macro name. For example: MyMacro 1, 2, 3


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Macros and Procedures

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3. Macros end with ENDMdirective andprocedures end with ENDPdirective.

4. Procedures are located at some specific

address in memory but macros are expanded

directly in program's code.


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Labels in Macros

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Macros are expanded directly in code, therefore ifthere are labels inside the macro definition you

may get "Duplicate declaration" error when macro

is used for twice or more.

To avoid such problem, use LOCALdirectivefollowed by names of variables, labels or

procedure names.


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Example

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MyMacro2 MACRO

LOCAL label1, label2

CMP AX, 2

JE label1

CMP AX, 3

JE label2

label1:

INC AX

label2:

ADD AX, 2

ENDM

ORG 100h

MyMacro2

MyMacro2

RET

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