Today's Topics

• Programming with High and Low-level Languages

• The C Program
• Combining ASM with C

• Stack Contents
• Complete Procedure Call Mechanism

• The ASM Program
• LINK and RUN

Programming with High and Low-Level Languages

• Why write in Assembly
  • Compact Code
  • Performance (Fast Execution) (no boot time)
  • Ideal for embedded controllers,
    where cost needs to minimized (CPU speed & memory size)
    • Calculators
    • Cellular Phones/Pagers
    • Arcade/Action Games (QUAKE, etc)
    • Robots, LegoBots
    • Automobiles
    • Elevators
    • Digital Cameras
    • Microwave Ovens
    • Laser Printers
    • Ubiquitous Computing

  • To understand how computers really work

• Why write in higher-level languages
  (C++/C/Pascal/BASIC/Fortran/JAVA/Perl)
  • Easier to write
  • Easier to understand
  • Easier to debug
  • Portable across machines

• Can we have the best of both worlds
  • YES! by combining Assembly through the linker.
  • Ultimately, all code becomes assembler
  • We just need a mechansim to call our code directly

Review

• Recall from lecture 8 that we have already learned to use for preserving registers and calling subroutines.

The C Program

• Define External Function Names

  extern void far MYSUB( int , int );

• Create functional parameters (MS C600 sizes)

  C Variable Type	# of Bytes
  char	1
  int, short, near*	2
  long, far*	4

• Write the main program (sub.c)

  #include <stdlib.h> #include <stdio.h> extern int far MYSUB(int x,int y); extern void far FXIT(); CSUB(int x,int y) { return x-y; } main() { int z; z = CSUB (0x6666,0x2222); printf("%x\n",z); z = MYSUB(0x6666,0x2222); printf("%x\n",z); FXIT(); }

• Compile the program

  • CL /c sub.c 

Combining ASM with C

• Use the stack to pass function arguments
  • The statement MYSUB( 0x6666 , 0x2222 )

    MYSUB( int x , int y ) { .. }

  • Pushes the following on the stack

    Value	Notes
    2222h	Right-most variable pushed first
    6666h	Left-most variable pushed last

  • C pushes arguments from right to left.

• CALL Subroutine
  • Far CALL to different code segment must push both segment and offset

    Value	Notes
    CS	Return Address (FAR)
    IP

• Use BP to index these data
  • PUSH BP
    MOV BP,SP

  • Mem	Value	Notes
    [BP+8]	22	Right-most parameter
    [BP+6]	66	Left-most parameter
    [BP+4]	CS	Return Address (FAR)
    [BP+2]	IP
    [BP]	BP'	Old value of BP

  • After saving BP, location of parameters remains fixed relative to BP

• Allocate stack memory for local variables
  • Define a local variable, i
    • In C:

      CSUB(int x , int y) { int i = 77; ... }

    • In ASM:

      MYSUB PROC NEAR ... SUB SP,2 MOV [BP-2],77 ... MYSUB ENDP

      
      

  • Mem	Value	Notes
    [BP-2]	77	Local Variable

• Use the stack to save register values
  • MS C600 expects SI, DI, DS, and SS to remain unchanged.

• Return result in accumulator
  • AL : Byte size
  • AX : Word size
  • DX:AX : Double

Stack Contents

Complete Procedure Call Mechanism

• Program writes function parameters to stack
  • PUSH Xp (Rightmost arguments of function)
  • ..
  • PUSH X1 (Leftmost arguments of function)

  • CALL saves program's return address on stack
    • PUSH CS (Far Procedure)
    • PUSH IP

    • Routine marks stack frame
      • PUSH BP
      • MOV BP,SP

      • Routine allocates stack memory for local variables
        • SUB SP,n

        • Routine saves registers it modifies
          • PUSH SI
          • PUSH DI
          • PUSH DS
          • PUSH SS
            • Subroutine Code
            • Additional CALLs, PUSHes, POPs

        • Routine restores registers it modifies
          • POP SS
          • POP DS
          • POP DI
          • POP SI

      • Routine frees stack memory for local variables
        • Add SP,n (or MOV SP,BP)

    • Routine restores original value of BP
      • POP BP

  • Subroutine Returns
    • RET (POP CS, POP IP) (Far Procedure)

• Program clears parameters from
  • SUB SP, p

The ASM Program

• Preceede procedure name with underscore

  _MYSUB

• Declare that procedure is PUBLIC

  PUBLIC _MYSUB 

• Write the procedure (mysub.asm)

  PAGE 75, 132 TITLE MYSUB John Lockwood 10/3/96 , Updated 9/29/97 EXTRN DOSXIT:near PUBLIC _MYSUB, _FXIT CSEG segment public assume cs:CSEG, ds:CSEG _MYSUB PROC FAR PUSH BP ; Stack Frame MOV BP,SP PUSH DS ; Save all registers modified MOV AX,CS MOV DS,AX ; Load segment register MOV AX,[BP+6] ; Leftmost Argument (x) SUB AX,[BP+8] ; Rightmost Argument (y) POP DS ; Restore modified registers POP BP ; Restore stack frame RET _MYSUB ENDP _FXIT PROC FAR CALL DOSXIT _FXIT ENDP CSEG ends end

• Assemble the program

  • MASM mysub.asm,,; 

• Alternatively, we could also defined a procedure using MASM's C-style procedure.
  See also: MASM /h references to INVOKE

LINK

• LINK SUB MYSUB,,,LIB291/MAP;
  Produces SUB.EXE

Run the program

• SUB
  4444
  (Exactly the result we expected!)

• Other Examples

• Pop Quiz

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