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Lecture 9 (The Stack and Procedures)

Lecture 9 (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.

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Lecture 9 (The Stack and Procedures)

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  1. Lecture 9 (The Stack and Procedures)

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

  3. 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

  4. 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

  5. 16-bit register or memory location 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. 4

  6. 1234 5678 AX BX Offset 00F8 00FA 00FC 00FE 0100 Offset 00F8 00FA 00FC 00FE 0100 Offset 00F8 00FA 00FC 00FE 0100 5678 SP 1234 1234 SP SP Empty Stack After PUSH AX AFTER PUSH BX The PUSH Instruction 5

  7. 16-bit register (except IP) or memory location 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. 6

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

  9. 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

  10. 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. 9

  11. 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 Procedure Declaration • Syntax (except the main procedure): • name PROC type • ; body of the procedure • RET • name ENDP 15

  12. 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

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

  14. The CALL Instruction • 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 SP 0012 SP Before CALL After CALL 12

  15. The RET Instruction • 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

  16. 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

  17. 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 15

  18. INDEC / OUTDEC Procedures • OUTDEC PROC • PUSH AX • PUSH BX • PUSH CX • PUSH DX • OR AX,AX • JGE @END_IF1 • PUSH AX • 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 • PUSH DX • INC CX • OR AX,AX • JNE @REPEAT1 • MOV AH,2 • @PRINT_LOOP: • POP DX • OR DL,30H • INT 21H • LOOP @PRINT_LOOP • POP DX • POP CX • POP BX • POP AX • RET • OUTDEC ENDP 16

  19. 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 • INT 21H • CMP AL,'-' • JE @MINUS • CMP AL,'+' • JE @PLUS • JMP @REPEAT2 • @MINUS: • MOV CX,1 • @PLUS: • INT 21H • @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

  20. 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 INT 21H MOV DL,0AH INT 21H JMP @BEGIN INDEC ENDP ;;;;;;;;;;;;;;;;;;;;;;;;;END READ;;;;;;;;; 18

  21. 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

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