Introduction to Machine And Assembly Language

1
Introduction to Machine And Assembly Language

• Modular Programming Procedures
• Dr. Richard Ford

2
Okay

• Weve looked at some of the procedures used in
  the book
• Now, we need to learn how to define our own
  procedures
• But the best thing is, we already know how

3
Remember MAIN?

• Main PROC
• main ENDP
• Well, thats a procedure (mostly)!
• All we need to do is add a RET instruction, so
  the assembler knows were done

4
Um why?

• Main is special
• It ends with an exit command
• This is an alias for a call to ExitProcess a
  Windows function
• Exit INVOKE ExitProcess, 0
• So, our procedures need to finish too hence the
  RET instruction
• Chris DONT WORRY ABOUT INVOKE YET!!!

5
Procedures in General

• A procedure receives registers from the calling
  code unchanged (this is a lie)
• So, if EBX 123456h when we call a proc, it
  starts off with that value in the proc
• Similarly if a register gets changed in a proc,
  its changed in the calling program too BE WARNED

6
Example PROC

• SumOf PROC add eax, ebx add eax, ecx
  retSumOf ENDP
• NOTE eax is changed in the calling code
• Helpful to add comments (see p.160)

7
Remember that Stack thing?

• Vital for understanding how procedures work
• CALL is used to call a procedure it pushes the
  address of the next instruction on the stack, and
  JMPs to the procedure in memeory
• RET JMPs back to the address we stored on the
  stack (probably youll see)
• DEMO CALL.ASM

8
The Stack in Nested Procs

• If a PROC calls a PROC calls a PROC what will the
  stack look like?
• Look at NESTED.ASM
• What will the stack look like at the bottom of
  the program?

9
Global Labels

• Easy just use a

10
Saving and Restoring Registers

• MASM makes it easy (though I /never/ use it)
• USES Operator
• ArraySum USES esi ecx
• Would save esi and ecx and restore them at the
  end of the procedure
• I prefer manually PUSHing and POPing, but hey,
  Im old fashioned
• Lets modify NESTED.ASM and look in NTSD

11
Lets look at our first real program

• Integer summation program with keyboard input

12
Assignment 3

• You need to write a program that accepts strings
  from the keyboard - up to a maximum of 16
  strings. Each string can have a max length of 30
  characters. String entry should stop when the
  string done is entered.
• The program should then sort these strings
  alphabetically and print them out in order
• The program should be case insensitive to make it
  easier!
• The printout show each string in a different
  color.
• HINT This is just a bubblesort with a custom
  string comparison routine and a more complex swap
  function