Lecture 9 (The Stack and Procedures)

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

Introduction 2 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.

3 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.

4 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

5 The PUSH Instruction Offset 00F8 00FA 00FC 00FE 0100 Empty Stack SP AXBX 1234 Offset 00F8 00FA 00FC 00FE 0100 After PUSH AX SP 1234 Offset 00F8 00FA 00FC 00FE 0100 AFTER PUSH BX SP5678

6 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 bit register (except IP) or memory location

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

8 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

9 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 procedures. 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.

15 Procedure Declaration Syntax (except the main procedure): name PROC type ; body of the procedure RET name ENDP 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. Name is the user-defined name of the procedure. The RET (return) instruction causes control to transfer back to the calling procedure

10 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.

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

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

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

14 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

15 INDEC / OUTDEC Procedures INDEC 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

16 INDEC / OUTDEC Procedures OUTDECPROC PUSHAX PUSHBX PUSHCX PUSHDX ORAX,AX PUSHAX MOVDL,'-' MOVAH,2 INT21H POPAX XORCX,CX XORDX,DX DIVBX PUSHDX INCCX ORAX,AX POPDX ORDL,30H INT21H POPDX POPCX POPBX POPAX RET OUTDECENDP

17 INDEC / OUTDEC Procedures INDECPROC ;;;;;;;;;;;;;;;;;;; READ DECIMAL NUMBER;;;;;;;;;;;; PUSHBX PUSHCX MOVAH,2 MOVDL,'?' INT21H XORBX,BX XORCX,CX MOVAH,1 INT21H CMPAL,'-' CMPAL,'+' @MINUS: CMPAL,'0' CMPAL,'9' ANDAX,000FH PUSHAX MOVAX,10 MULBX POPBX

ADDBX,AX MOVAH,1 INT21H CMPAL,0DH MOVAX,BX ORCX,CX POPDX POPCX POPBX MOVAH,2 MOVDL,0DH INT21H MOVDL,0AH INT21H INDECENDP ;;;;;;;;;;;;;;;;;;;;;;;;;END READ;;;;;;;;; INDEC / OUTDEC Procedures Cont… 18

MODELSMALL.STACK100H.CODE MAINPROC CALL INDEC CALL OUTDEC MOVAH, 4CH; exit to DOS INT 21H MAINENDP INCLUDE C:ASM\PGM9_1.ASM INCLUDE C:ASM\PGM9_3.ASM ENDMAIN INDEC / OUTDEC Procedures MAIN PROGRAM 19