Review:   What a computer looks like from inside  

  • The RAM (or Random Access Memory) stores/contains computer programs:

  • A computer programs consists of:

    1. Program instructions
    2. Program variables            ← Attention !!! 

  More review:   analogy between computer memory and mail boxes  

  • The main memory or RAM is similar to a collection of mail boxes:

    where:

    • Each mail box is uniquely identified by an integer mailbox number (= address)
    • Each mail box contains slip of paper can store a small number

  How the Java compiler works  

  • The Java compiler translates a Java program into executable machine code:
     

  • The machine code will be placed in computer memory for execution

  • The items inside the machine code program will be identified using memory addresses

  How the Java compiler works  

  • When the Java compiler finds a variable definition, it will allocate (= reserve) some memory cells to store the value:

  • These memory cells will be identified using memory addresses

  • Example: radius is placed in memory address 7000 and area in memory address 7008

  How the Java compiler works  

  • The Java compiler will remember the placement of the variables:
     

  • The Java compiler records the variable placement in its symbol table

  • Example content of the symbol table:    radius: 7000 and area: 7008

  How the Java compiler works  

  • When a variable is used in the program, the Java compiler use the location information to translate the instruction:

  • The Java compiler looks up the memory address of the variable using its identifier

  • Example translation:    radius = 20 ⇒ machine code that stores 20 into mem addr 7000

  How the Java compiler works  

  • When the Java program is run/executed, the effect of the instruction radius = 20 is: store the (binary) number 20 in memory location 7000:

  • Only the compiled machine code is used when a program is run

  • The source code is not used !