(The Stack and Procedures) Lecture 8 (The Stack and Procedures)

Lecture Outline Introduction The Stack The PUSH Instruction The POP Instruction Terminology of Procedures INDEC / OUTDEC procedures 1

Introduction The stack segment of a program is used for temporary storage of data and addresses. PUSH and POP instructions are used to add and remove words from the stack. 2

The Stack A stack is a one-dimensional data structure. Items are added and removed from one end of the structure; that is, it processes in a “last-in-first-out” manner. A program must set aside a block of memory to hold the stack. Ex: .STACK 100H When the program is assembled and loaded in memory: SS will contain the segment number of the stack segment. SP is initialized to 100H, which represents the empty stack position. When the stack is not empty, SP contains the offset address of the top of the stack. 3

The PUSH Instruction To add a new word to the stack we PUSH it on. Syntax: PUSH source Execution of PUSH causes the following to happen: SP is decreased/decremented by 2. A copy of the source content is moved to the address specified by SS:SP. The source is unchanged. 16-bit register or memory location 4

The PUSH Instruction 1234 5678 AX BX Offset 00F8 00FA 00FC 00FE 0100 Empty Stack SP 1234 Offset 00F8 00FA 00FC 00FE 0100 After PUSH AX SP 1234 Offset 00F8 00FA 00FC 00FE 0100 AFTER PUSH BX SP 5678 5

The POP Instruction To remove the top item from the stack, we POP it. Syntax: POP destination Execution of POP causes the following to happen: The content of SS:SP (the top of the stack) is moved to the destination. SP is increased by 2. 16-bit register (except IP) or memory location 6

The POP Instruction 5678 1234 Offset 00F8 00FA 00FC 00FE 0100 FFFF 0001 CX DX 5678 1234 Offset 00F8 00FA 00FC 00FE 0100 After POP CX SP 0001 CX DX 5678 1234 Offset 00F8 00FA 00FC 00FE 0100 After POP DX SP CX DX 1234 Offset 00F8 00FA 00FC 00FE 0100 Stack SP 5678 7

Exercise 1 Write assembly code that uses the stack operations to swap the content of AX and DX. PUSH AX PUSH DX POP AX POP DX 8

Terminology of Procedures An assembly program can be structured as a collection of procedures. The MAIN procedure, contains the entry point to the program. To carry out a task, the main procedure calls one of the other It is also possible for these procedures to call each other, or for a procedure to call itself. When one procedure calls another, control transfers to the called procedure and its instructions are executed; the called procedure usually returns control to the caller at the next instruction after the call statement. 9

Procedure Declaration Syntax (except the main procedure): name PROC type ; body of the procedure RET name ENDP Name is the user-defined name of the procedure. The optional operand type is: NEAR: the statement that calls the procedure is in the same segment as the procedure itself, or FAR: the statement that calls the procedure is in a different segment. If type is omitted, NEAR is assumed. The RET (return) instruction causes control to transfer back to the calling procedure 15

Communication Between Procedures Assembly language procedures do not have parameter lists. It’s up to the programmer to devise a way for procedures to communicate. E.g. If there are only few input and output values, they can be placed in registers. 10

The CALL Instruction To invoke a procedure, the CALL instruction is used. 11

The CALL Instruction Before CALL After CALL Offset address 0010 0012 0200 Code segment MAIN PROC CALL PROC1 next instruction PROC1 PROC first instruction RET Offset address 0010 0012 0200 Code segment MAIN PROC CALL PROC1 next instruction PROC1 PROC first instruction RET IP IP Offset address 00FE 0100 Stack segment Offset address 00FE 0100 Stack segment 0012 SP SP Before CALL After CALL 12

The RET Instruction Before RET After RET Offset address 0010 0012 0200 0300 Code segment MAIN PROC CALL PROC1 next instruction PROC1 PROC first instruction RET Offset address 0010 0012 0200 0300 Code segment MAIN PROC CALL PROC1 next instruction PROC1 PROC first instruction RET IP IP Offset address 00FE 0100 Stack segment Offset address 00FE 0100 Stack segment 0012 SP SP Before RET After RET 13

INDEC / OUTDEC Procedures procedures used to read and print decimal data To invoke the two procedures, use CALL instruction inside the MAIN PROC . Example CALL INDEC . CALL OUTDEC 14

INDEC / OUTDEC Procedures Read character input from user and convert it to decimal stored in AX register Code of INDEC exist in file PGM9_3.ASM OUTDEC Display the decimal number in register AX to output screen Code of OUTDEC exist in file PGM9_1.ASM Include the two files using INCLUDE directive Syntax: INCLUDE C:\ASM\ PGM9_3.ASM INCLUDE C:\ASM\ PGM9_1.ASM 15

INDEC / OUTDEC Procedures PUSH AX PUSH BX PUSH CX PUSH DX OR AX,AX JGE @END_IF1 MOV DL,'-' MOV AH,2 INT 21H POP AX NEG AX @END_IF1: XOR CX,CX MOV BX,10D @REPEAT1: XOR DX,DX DIV BX INC CX JNE @REPEAT1 @PRINT_LOOP: POP DX OR DL,30H LOOP @PRINT_LOOP POP CX POP BX RET OUTDEC ENDP 16

INDEC / OUTDEC Procedures INDEC PROC ;;;;;;;;;;;;;;;;;;; READ DECIMAL NUMBER;;;;;;;;;;;; PUSH BX PUSH CX PUSH DX @BEGIN: MOV AH,2 MOV DL,'?' INT 21H XOR BX,BX XOR CX,CX MOV AH,1 CMP AL,'-' JE @MINUS CMP AL,'+' JE @PLUS JMP @REPEAT2 @MINUS: MOV CX,1 @PLUS: @REPEAT2: CMP AL,'0' JNGE @NOT_DIGIT CMP AL,'9' JNLE @NOT_DIGIT AND AX,000FH PUSH AX MOV AX,10 MUL BX POP BX 17

INDEC / OUTDEC Procedures Cont… ADD BX,AX MOV AH,1 INT 21H CMP AL,0DH JNE @REPEAT2 MOV AX,BX OR CX,CX JE @EXIT NEG AX @EXIT: POP DX POP CX POP BX RET @NOT_DIGIT: MOV AH,2 MOV DL,0DH MOV DL,0AH JMP @BEGIN INDEC ENDP ;;;;;;;;;;;;;;;;;;;;;;;;;END READ;;;;;;;;; 18

INDEC / OUTDEC Procedures MAIN PROGRAM MODEL SMALL .STACK 100H .CODE MAIN PROC ----------------------------------------- CALL INDEC CALL OUTDEC MOV AH, 4CH ; exit to DOS INT 21H MAIN ENDP INCLUDE C:ASM\PGM9_1.ASM INCLUDE C:ASM\PGM9_3.ASM END MAIN 19