High level language and low level language programming

  • High level programming:

    • In CS170/CS171, you were writing computer programs in a high level programming language -- e.g.: Java

    • High level language programming uses statement constructs

      E.g.: 

        if ( x < 0 )
     x = -x;

  • Low level programming:

    • Low level programming does not have any language constructs

    • Low level programming does not have:   assignment statements, if, while, etc

High level language and low level language programming

  • High level programming:

      • The computer cannot (directly) execute a computer program written in a high level programming language

      • A high level language program must be compiled (= translated) into machine code first

      • A compiler (e.g.: javac) will translate statements in a high level programming language into a series of machine instructions that achieve the result of the statements.

  • Low level programming:

      • There is a one-to-one correspondence between: 

         Low level instruction <---> Binary machine instruction code

The historical development of computer programming languages

The first generation programming languages: (reference: click here )

  • In the pioneer days (~1945), a computer program was written using computer instructions in binary codes:

             

    The programmers would enter the computer instruction codes (= binary numbers) by flipping switches !!!

The historical development of computer programming languages

The 2nd generation programming languages: (Wikipedia: click here )

  • ~1950: introduction of assembler programming language

  • Assembler language uses English mnemomics to represent binary machine instructions:

          

  • A simple translation program (called the assembler) translates each mnemomic into one (1) machine instruction

The historical development of computer programming languages

The 3rd generation or "High level" programming languages: (Wikipedia: click here )

  • In 1957, the first high level programming language (Fortran) was available

  • A very complex translation program called a compiler is used to translate the statements into binary computer (machine) instructions in a 2 step process:

          

Example to show you the entire program compilation process - DEMO

The programmer write a program in a High level language program (C language): 

 int x, y, z;

 int main( )
 {
   z = x + y;
 }

 

DEMO: /home/cs255001/demo/translation-process/prog.c

Example to show you the entire program compilation process - DEMO

The programmer write a program in a High level language program (C language): 

 int x, y, z;

 int main( )
 {
   z = x + y;
 }

The compiler will translate the (C) program into an equivalent program in assembler code: 

main:
        str     fp, [sp, #-4] // These assembler codes
        add     fp, sp, #0    // perform: z = x + y
        ldr     r3, .L3
        ldr     r2, [r3]
	... (many more, omitted for brevity)

One (1) statement will translate into many assembler instructions !!

DEMO: /home/cs255001/bin/cc255 -S prog.c

Example to show you the entire program compilation process - DEMO

The program in assembler code is further processed by the assembler: 

main:
        str     fp, [sp, #-4]
        add     fp, sp, #0
        ldr     r3, .L3
        ldr     r2, [r3]
	...

Example to show you the entire program compilation process - DEMO

The program in assembler code is further processed by the assembler: 

main:
        str     fp, [sp, #-4]
        add     fp, sp, #0
        ldr     r3, .L3
        ldr     r2, [r3]
	...

The assembler will then translate the assembler codes into (binary) machine instruction codes: 

00000000 <main>:
 0:   e52db004  <-- binary number written in hexadcimal 
 4:   e28db000  // These are (binary) machine codes !!!
 8:   e59f3028   
 ...

DEMO: /home/cs255001/bin/cc255 -c prog.s (dump255 prog.o)

Why is assembler programming a core course in Computer Science

You can expose the details the following Computer Science concepts using assembler programming:

  • What happens inside the computer when you define a variable in your program ?
  • How does a computer program use array variables ?
  • How does a computer program use linked list variables ?
  • What happens inside the computer when an assignment statement is executed ?
  • What happens inside the computer when an if-statement is executed ?
  • What happens inside the computer when a method passes parameters and returns a value
  • What happens inside the computer when a parameter is passed by value and passed by reference ?
  • What happens inside the computer when a recursive function/method is executed ?

  • Etc, etc
 

All these concepts and more will be explained in CS255 - and illustrated with assembler codes