⚙️ CESSER

16-bit CPU Emulator & Mini Assembler (Pascal)

Low-level • Educational • Stack-based • ISA Emulator

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📌 Overview

CESSER (Custom Emulated Stack-based System ExecuteR)
is a 16-bit CPU emulator written in FreePascal, complete with a mini assembler, RAM, stack, and subroutine support.

This project simulates how a real processor works internally — from instruction decoding, register manipulation, stack operations, to program execution.

CESSER is built for learning low-level systems, computer architecture, and emulator design.

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✨ Features

✅ 16-bit Virtual CPU
✅ Custom Instruction Set Architecture (ISA)
✅ Built-in Mini Assembler
✅ 4KB RAM Memory
✅ Stack Pointer & Subroutines (CALL / RET)
✅ Conditional Jumps (JZ, JNZ)
✅ Arithmetic Instructions
✅ Interactive CLI Emulator

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🧠 CPU Architecture

┌─────────────────────────────┐
│           CESSER CPU        │
├──────────────┬──────────────┤
│ Registers    │ R0 R1 R2 R3  │
│              │ SP PC STATUS │
├──────────────┴──────────────┤
│ RAM          │ 4 KB (Word)  │
├─────────────────────────────┤
│ Stack        │ Grows Down   │
├─────────────────────────────┤
│ ISA          │ Custom       │
└─────────────────────────────┘

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🧱 Instruction Set (ISA)

Instruction	Description
MOV	Move data
ADD	Addition
SUB	Subtraction
MUL	Multiplication
CMP	Compare values
LOAD	Load from memory
STORE	Store to memory
JMP	Unconditional jump
JZ	Jump if zero
JNZ	Jump if not zero
CALL	Call subroutine
RET	Return from subroutine
PUSH	Push to stack
POP	Pop from stack
PRINT	Debug output
HALT	Stop execution

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📂 Project Structure

CESSER/
└── PROCESSER/
    └── cesser.pas   # CPU Emulator + Assembler + VM

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🔄 Execution Flow

Assembly Code
     ↓
Mini Assembler
     ↓
Bytecode in RAM
     ↓
Fetch → Decode → Execute
     ↓
CPU Registers & Memory Updated

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🚀 Getting Started

🔧 Requirements

• FreePascal Compiler (FPC)
• Terminal / Command Prompt

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⚙️ Compile

fpc cesser.pas

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▶ Run

./cesser

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🖥️ Emulator Commands

Command	Description
load <file>	Assemble & load program
run	Start CPU execution
reset	Reset CPU state
help	Show help
quit	Exit emulator

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🧪 Example Assembly Program

; Factorial of 5
MOV R0, 5
MOV R1, 1
CALL 20
JMP 30

20:
CMP R0, 1
JZ 25
MUL R1, R0
SUB R0, 1
JMP 20

25:
RET

30:
PRINT R1
HALT

Expected output:

[CPU OUT] R1 = 120

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🧵 Stack & Subroutines

CESSER implements a real stack model:

• CALL pushes return address to stack
• RET pops address back into PC
• PUSH / POP manipulate stack directly

This allows modular programs, loops, and function-like behavior.

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🎯 Learning Outcomes

By building and using CESSER, you learn:

• How CPUs fetch & execute instructions
• Register-based computation
• Stack memory mechanics
• Assembly-level program flow
• Emulator & VM architecture
• Low-level systems programming

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⚠️ Notes

• This project is educational
• Instruction encoding is simplified
• Label handling is minimal
• Designed for clarity, not performance

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📜 License

MIT License — free to use, modify, and learn from.

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Developed by norct 👾