Programming Assignment 2

1
Programming Assignment 2

• Virtual Machine Implementation

2
Goals

• Implement a Java-like virtual machine (you dont
  really have to know anything about the Java VM)
• Stack based machine
• Fetch instructions and execute them
• Return top of stack (if the program returns)

3
Process

• Read instructions from memory one-by-one
• Interpret them
• Find out what this instruction is about (using
  the opcodes table)
• OPCODE 1 Byte
• Read the operands (if available)
• OPERANDS 1-4 Bytes
• Update the PC accordingly
• Call appropriate procedure to handle the
  instruction.

4
General info on data segment

• .ascii str
• This stores str in memory, but without a null
  terminator.
• .asciiz str
• This stores str in memory, but with a null
  terminator. The "z" refers to zero, which is the
  ASCII code for the null character. This is how
  C-style strings are stored.
• .byte b1, ..., bn
• Store n bytes contiguously in memory (you get to
  pick n).
• .halfword h1, ..., hn
• Store n 16-bit halfwords contiguously in memory
  (you get to pick n).
• .word w1, ..., wn
• Store n 32-bit words contiguously in memory (you
  get to pick n).
• .space numBytes
• Reserves numBytes of space in memory.

5
Keep in mind

• Check for the validity of opcodes
• Make sure you dont go out of bounds
• In stack
• In variables memory

6
Stack implementation

• Example in class