Assembly and High Level Languages

1
Assembly and High Level Languages
2
Creating Software

• Now that we have a computer to work with, how do
  we create software to solve our problems?
• One solution use machine language instructions

3
Machine Language

• Recall machine language instructions are the
  binary patterns that a processor natively
  recognizes
• What's the problem with this approach?
• Solution ABSTRACTION!

4
Assembly Language

• Rather than binary patterns, what if we used
  mnemonics (essentially words) to represent
  instructions?
• Ex add instead of 101010
• What do we gain?

5
Assembly Language (cont)

• Consists of mnemonic names for instructions
• Each instruction corresponds to exactly one
  machine language instruction
• Another piece of software called the assembler
  then translates ASM code into machine language

6
The Assembly Code to Machine Code Translation
7
But....

• Still tied to a particular piece of hardware
• Why?
• Thus each assembly language is specific to a
  particular architecture

8
The Structure of Assembly

• A typical line has four parts
• Label (optional)
• Instruction
• Address (sometimes optional)
• Comment (optional)

9
Structure (cont)

• The address field holds the symbolic address of
  the memory location being used
• The label is a symbolic representation of an
  instruction or data value

10
Pseudo Ops

• Operations performed by the assembler
• Example
• .DATA associates labels with addresses and can
  build integer values

11
.DATA Example

• FIVE .DATA 5
• Associates an memory address with the label
  FIVE, and puts the numeric value 5 into that
  location
• Can now do
• LOAD FIVE
• But why not LOAD 5?
• All psuedo-ops come after the HALT

12
Overall Structure of An Assembly Program
.BEGIN ...... some instructions....... HALT
.... some pseudo ops (such as .DATA
declarations).....END
13
Example

• Write an assembly program to do
• A B C - 7

14
Example 2

• Write a program which loops 10 times
• Roughly equivalent to
• For a 0 to 10 do ....

15
The Assembler

• Since each assembly instruction corresponds to
  exactly one machine language instruction, the
  assembler only has to
• Convert mnemonic op codes to binary
• Convert symbolic addresses to binary
• Do the pseudo-ops
• Save output to a file

16
Good Enough?

• As much of an improvement as it is, there are
  still some drawbacks to assembly language.
• Such as?

17
Assembly Drawbacks

• Manual memory management
• Microscopic view of tasks
• Not like natural-language
• MACHINE-SPECIFIC

18
High Level Programming Languages

• Designed to address some of the shortcomings of
  assembly
• Many memory details are abstracted away
• Many problem-specific details are abstracted away
• Less machine specific
• Closer to natural language

19
This is not a course in Java

• This is not a course in Java
• This is not a course in Java
• This is not a course in Java
• This is not a course in Java
• This is not a course in Java
• This is not a course in Java

20
Examples of HLPLs

• Java (but this is not a course in it)
• C (but this is not a course in C)
• Scheme/LISP
• Prolog
• FORTRAN
• Etc, etc, etc

21
Concepts

• Variables
• Identifiers vs keywords
• Statements

22
Statement Types

• Four main categories of statements
• Input
• Output
• Assignment
• Control

23
Assignment Statements

• Assign a value to an identifier (typically a
  variable)
• Ex a 3 b
• 3 b is evaluated
• Result is assigned to a
• What about
• 3 a b

24
Control Statements

• Three control mechanisms
• Sequential
• Conditional
• Looping
• The three define the flow of control in a program

25
Divide and Conquer

• Break a problem into smaller pieces
• Solve each piece and combine results to solve the
  overall problem

26
Methods / Functions / Procedures / Subroutines

• A rose by any other name....

27
Methods / Functions / Procedures / Subroutines

• A convenient way to encapsulate some
  computation, which can then be used without
  worrying about its implementation. With ...
  functions it is possible to ignore how a job is
  done knowing what is done is sufficient. (C
  Programming Language, Kernighan and Ritchie)

28
Parts of Functions

• Functions can be identified by their
• Name
• Return type (if any)
• Parameter (or argument) List

29
Example

• integer Foo (boolean a, integer x)
• Defines a function called Foo which takes two
  arguments a boolean value and an integer, and
  returns an integer

30
A Question....
integer x 3 Foo (integer y) y y
3 What is the value of x after doing "Foo(x)"?
31
Call-by.... Mechanisms

• In programming languages there are two main
  parameter passing mechanisms
• Call by Value each argument to a function is
  copied, and that copy passed to the function
• Call by Reference the address of each parameter
  to a function is passed in to the function

32
Call By Value

• Using the Foo(x) example....

33
Call By Reference

• Using the Foo(x) example....

34
In Practical Terms

• In call-by-value, any changes to a variable are
  not seen outside of the called function
• In call-by-reference, changes to a variable are
  seen outside of the called function

35
In Practical Terms (cont)

• From an efficiency standpoint
• Call by value has to make a copy of each
  parameter thus it can be slower