Table of Contents

• CPU-SIM
  • CPU
  • Memory
  • Timer
  • Interrupt processing
  • Instruction Set
  • How to build and run
  • Save the Snail Game
  • Listing of my Files

This is a academic project I did a while back, it simulates a simple computer system consisting of a CPU and memory. The CPU and memory are simulated in separate processes and use pipes to communicate.

• supports registers: PC, SP, IR, AC, X, Y.
• supports the instructions shown below.
• runs the user program at address 0.
• Instructions are fetched into the IR from memory. The operand can be fetched into a local variable.
• Each instruction is executed before the next instruction is fetched.
• The user stack resides at the end of user memory and grows down toward address 0.
• The system stack resides at the end of system memory and grows down toward address 0.
• There is no hardware enforcement of stack size.
• The program ends when the End instruction is executed. The 2 processes should end at that time.
• The user program cannot access system memory (exits with error message).

• consists of 2000 integer entries, 0-999 for the user program, 1000-1999 for system code.
• supports two operations:
  • read(address) - returns the value at the address
  • write(address, data) - writes the data to the address
• Memory initializes itself by reading a program file.

• A timer interrupts the processor after every X instructions, where X is a command-line parameter.

• There are two forms of interrupts: the timer and a system call using the int instruction.
• The stack is switched to the system stack.
• SP and PC registers is saved on the system stack. (The handler may save additional registers).
• A timer interrupt causes execution at address 1000.
• The int instruction causes execution at address 1500.
• Interrupts are disabled during interrupt processing to avoid nested execution.
• The iret instruction returns from an interrupt.

Because I'm too lazy to make a build file, do the following to build: open a terminal and cd into the cpu-sim directory run the following command:

find . -name '*.cpp' ! -name 'Test.cpp' | xargs g++ -o cpu-sim

The command will find and compile all the cpp files except the poorly hacked together unit test file, which won't compile unless you have CUTE test framework library available.

An example of how to run the program with the snail game:

./cpu-sim snailGame.txt 150

Interrupt timer interval: supports numbers on interval [1, 2147483647]

The snail races to a stamper, if the stamper is shut when the snail reaches it, then the snail is going to get squashed. The rate at which the stamper opens and slams is controlled by how often the interrupt handler gets called. the interrupt handler is called based on the timer parameter that you pass to the program. So in order to win you need to find a timer interval that allows the snail to reach the stamper with it open.

IMPORTANT NOTE: Game animation depends on how printing a char is implemented in the CPU simulator. The buffer needs to be disabled or flushed each time after printing the character.

snailGame.txt - Save the Snail Game. Please note the comments on the top of the source file if the game does not animate in your own CPU sim implementation.

Source files: Main.cpp - contains the main function. Does some parameter checking and parses the program source into a string vector to pass to ComputerSim. ComputerSim.cpp - sets up the pipes and forks. It creates the CPU or Memory objects depending on if it is a parent or child. Also passes the program string vector to the Memory. Memory.cpp - initializes the memory from the string vector. Signals the CPU when it is ready and then listens to commands from the cpu to either read or write. Cpu.cpp - fetches instructions from the memory and executes them.