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Curso 2010-2011

Lesson 4. Stack, Procedures and Macros

Computer Structure and Organization

Graduated in Computer Sciences /

Graduated in Computer Engineering

Computer Structure and Organization Graduated in Computer Sciences / Graduated in Computer Engineering Automatic Department

Lesson 4: Stack, Procedures and Macros

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Computer Structure and Organization Graduated in Computer Sciences / Graduated in Computer Engineering Automatic Department

Contents

 Bit instructions

 The stack  How to define procedures  Type of procedures: NEAR y FAR  How to pass parameters to a procedure:

- Registers - The Stack  Structures and parameters passing  Macros  Procedures vs. Macros

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Computer Structure and Organization Graduated in Computer Sciences / Graduated in Computer Engineering Automatic Department

Bit instructions (I)

 Mnemonic: TEST

 Format: TEST target, source  Description: This funtion is a logical AND over source and target. The differences

is that the result is not stored in anywhere. This instruction only modifies flags register bits.

 Example: ; AX = 1234, BX = 0000 TEST AX, BX ; AX =1234, BX = 0000, Result = 0000 JZ Es_Cero ; IF Zero jumps to Es_Cero label

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Bit instructions (II)

 Mnemonic: CLI

 Format: CLI  Description: Disable masked interrupts handling. Unmasked interrupt can not be disabled  Example: - CLI

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Bit instructions (III)

 Mnemonic: STI  Format: STI  Description: Enable interrupts handling  Example: - STI

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Bit instructions (IV)

 Mnemonic: CLC  Format: CLC  Description: Carry flag is set to zero  Example: - CLC

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Bit instructions (V)

 Mnemonic: STC  Format: STC  Description: Carry flag is set to one  Example: - STC

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Bit instructions (VI)

 Mnemonic: CLD

 Format: CLD  Description: Direction flag is set to zero (used by string instructions)  Example: - CLD

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Bit instructions (and VII)

 Mnemonic: STD

 Format: STD  Description: Direction flag is set to one (used by string instructions)  Example: - STD

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The Stack

 Portion of memory that handles return addresses of procedures and interrupt services  LIFO structure (Last In First Out)  SS and SP are dedicate registers to point to the top of the stack  Each stack position is 16 bits length  The stack increases to lower memory positions addresses with PUSH  The stack decreases to upper memory positions addresses with POP STACK

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Definición de procedimientos  Procedures group instructions and allow to call them from different parts of the program without need to repeat them

 To call a procedure: CALL  Last instruciton of a procedure: RET  Return address of the procedure is stored on the stacl. Segment

address the procedure belongs to is also stored on the stack if the type of the procedure is FAR  To define a procedure folloing directives are used: PROC (begin of the procedure) y ENDP (end of the procedure)

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Computer Structure and Organization Graduated in Computer Sciences / Graduated in Computer Engineering Automatic Department

Types of procedures  Procedures have the following attributes: FAR (if it will be called from a different segment it belongs to) or NEAR (call is within the same segment)

 Main Procedure is always FAR (implícit) PrintString PROC FAR ; this procedure calls to 9h DOS MOV AH, 09h ; services to display a character string on the INT 21h ; string whose address is stored on DS:DX RET ; return of the procedure PrintString ENDP Ejemplo de definición de un procedimiento

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How to pass parameters to a procedure by using registers

 General purpose registers are used to pass and to recieve paramenters and results from a procedure

 We so few registers the i8086 has, it's needed to save used registers by the procedure before calling it  First set of instructions in a procedure must store on the stack all the registers that will be used in it.  Before returning from procedure save registers must be restored. All of them except those in which results will be returned

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How to pass parameters to a NEAR PROCEDURE. The stack (I)

Actions performed by CALLER procedure

 Parameters offset or variables themselves are in the same segment so it will be stored on the stack

Stack Structure

Caller Procedure: MOV AX, VARIABLE1 PUSH AX LEA AX, VARIABLE2 PUSH AX

CALL Nombre_Proc

IP (returning address) Offset VARIABLEn Offset Variable2 Offset Variable1

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How to pass parameters to a NEAR PROCEDURE. The stack (II)

Actions performed by CALLED procedure

1. BP register is stored on the stack (PUSH BP) BP will be used to

access parameters.

2. BP must point to SP (MOV BP, SP) 3. Parameters are recovered from the stack by using BP. The i

parameter offset will be: [BP] + 4 + 2 · (n - i)

Estructura de la pila CALLER procedure: Name_Proc PROC PUSH BP MOV BP,SP PUSH [used registers] MOV AX,[BP+4] ;VARIABLEn ... MOV BX,[BP+4+2 · (n-i)] BP register IP (returning address) Offset VARIABLEn

Offset Variable2

Offset Variable1

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How to pass parameters to a NEAR PROCEDURE. The stack (and III)

Actions performed by CALLED procedure

 RET parameter indicates the stack positions to be removed when returning from CALLED procedure.

 This number will be 2xnumber of passed parameters.  E.g. RET 8 if four parameters have been passed

CALLED proceure: POP [used registers] POP BP RET 2·n Name_Proc ENDP Stack structure BP register IP (returning address) Offset VARIABLEn

Offset Variable2

Offset Variable1

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How to pass parameters to a FAR PROCEDURE. The stack (I)

Actions to be performed by CALLER procedure

 Variables offset and segment must be stored on the stack because CALLER and CALLED procedures belongs to different memory segmentes

CALLER procedure: LEA AX, VARIABLE1 PUSH DS PUSH AX LEA AX, VARIABLE2 PUSH DS PUSH AX ... CALL PROCEDIMIENTO

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How to pass parameters to a FAR PROCEDURE. The stack (and II)

Actions performed by CALLED procedure

• BP register is stored on the stack (PUSH BP) BP will be used to access parameters.

• BP must point to SP (MOV BP, SP) • Parameters are recovered from the stack by using BP. The i

parameter offset will be: [BP] + 4 + 2 (n - i) and the segment whose belongs to: [BP] + 8 + 4 · (n - i)

RET parameter of the CALLED procedure:  RET parameter indicates the stack positions to be removed when

returning from CALLED procedure.

 This number will be 4xnumber of passed parameters.  E.g. RET 16 if four parameters have been passed

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Structs and parameters passing (I) Structures

 Structures will be defined with followin directives: STRUC (start of structure) and ENDS (ends of structure)

 Group different type and size data with a single name  StructureName.Field will be uses to access to structure records  RET argument and parameter passing can be easy by using

structures

DNI STRUC Name_and_Surname DB 50 DUP ('0') Address DB 60 DUP ('0') Age DB 0 Profession DB 15 DUP ('0') DNI ENDS

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Structs and parameters passing (II) Structure code and records for parameters passign to a NEAR procedure:

Estructura STRUC bp0 DW ? ; 2 bytes for storing BP return DW ? ; It's equivalent to a +2 bytes offset from the bebining of the ; structure (first DW named bp0) p2 DW ? ; It's equivalent to a +4 bytes offset from the beginning of the ; structure (DW + return) P1 DW ? ; It's equivalent to a +6 bytes offset from the beginning of the ; structure (DW + return + p2) Estructura ENDS NRET EQU OFFSET p1 - OFFSET retorn

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Computer Structure and Organization Graduated in Computer Sciences / Graduated in Computer Engineering Automatic Department

Structs and parameters passing (and III) Actions performed by NEAR CALLED procedure

1. BP register is stored on the stack (PUSH BP) BP will be used to

access parameters.

2. BP must point to SP (MOV BP, SP) 3. Accessing to parameters with BP and the structure 4. RET argument will be NRET

CALLED procedure Prueba PROC NEAR PUSH BP MOV BP, SP PUSH AX MOV AX, [BP].p1 ADD [BP].p2, AX POP AX POP BP RET NRET Prueba ENDP Example of previous defined structure

Estructura struc bp0 DW ? retorno DW ? p2 DW ? p1 DW ? Estructura ends NRET EQU OFFSET p1 - OFFSET retorno

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MACROS

 MACROS are names given to some pieces of code

 Code expansion instead of jumping is done  Extra parameters will be ignored if passed. So have

instructions that likely use them  Mind the labels inside a macro  Labels must be local inside the macro code

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Las macros (II)

 Mnemonic: MACRO

 Format: macro_name MACRO parameter_list  Description: name and parameters to be passed to the macro

are defined. Parameter names are separated by commas. The end of a macro definition is specified using ENDM directive without writing macro name

 Nombre y Format: ENDM  Description: to establish the end of a macro definition  Example: Add_three MACRO operand1, operand2, result ;macro body ENDM

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Las macros (III)

 Mnemonic: LOCAL  Format: LOCAL label,[label,...]  Description: indicate label names to be changed when

expending a macro. This method solves duplicate label names. Must be used after macro definition line and only use it on macros

 Example: Delay MACRO number local go_on mov cx, number go_on: loop go_on ENDM

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Procedures vs. Macros

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