Recursive Factorial Program

Assignment Overview

This project demonstrates a recursive factorial calculation implemented in both C and assembly language, running on a custom 16-bit RISC CPU emulator. The assignment showcases:

1. C recursive function with a main driver program
2. Executable memory layout in the virtual CPU
3. Function call mechanism (CALL/RET instructions)
4. Recursion execution with stack frame management

## Quick Start

### 1. Build the Tools

bash
make clean
make all

This creates:

• build/assembler - Assembles .asm files to binary
• build/emulator - Executes binary programs on virtual CPU

2. Run the C Version

gcc -o build/factorial_c programs/factorial.c
./build/factorial_c

3. Run the Assembly Version

# Assemble
./build/assembler programs/factorial.asm /tmp/factorial.bin

# Execute
./build/emulator /tmp/factorial.bin

Output:

=== Starting Execution ===
F(5) = 120

=== Execution Complete ===
Instructions executed: 90

4. View Detailed Execution Trace

./build/emulator /tmp/factorial.bin -d

This shows:

• Each instruction executed
• Register values after each step
• Stack pointer changes during recursion
• Memory addresses and program counter

5. Run Interactive Demo

./demo.sh

This presents a step-by-step walkthrough of:

1. C program with recursion
2. Assembly translation
3. Memory layout diagram
4. Function call mechanism
5. Recursion trace with stack visualization
6. Actual execution trace

CPU Architecture

Registers

• R0: Return value / Accumulator
• R1: Function argument (n)
• R2: Temporary storage
• R3-R7: General purpose
• SP: Stack pointer (hardware managed)
• PC: Program counter (hardware managed)

Memory Map (64KB Address Space)

• 0x0000-0x7FFF: Code section (32KB)
• 0x8000-0xEFFF: Data section (28KB)
• 0xF000-0xF0FF: Memory-mapped I/O (256 bytes)
• 0xF100-0xFFFF: Stack (3.75KB, grows downward)

Instruction Set (Subset Used)

• MOVI: Move immediate to register
• MOV: Move register to register
• PUSH/POP: Stack operations
• CALL/RET: Function calls
• ADD/SUB/MUL/DIV: Arithmetic
• CMPI: Compare immediate
• JZ/JNZ: Conditional jumps
• STORE: Memory-mapped I/O output
• HALT: Stop execution