8085 Microprocessor Simulator

A lightweight Intel 8085 microprocessor simulator written in C++. The project emulates the core components of the 8085 CPU, supports execution of assembly programs, and includes a small assembler that converts assembly code into machine code.

This project was built to understand CPU architecture, instruction decoding, and emulator design.

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Features

CPU Emulation

The simulator implements the core behavior of the Intel 8085 processor:

• 8-bit registers A B C D E H L

• 16-bit registers PC (Program Counter) SP (Stack Pointer)

• Flags register

  • Zero (Z)
  • Sign (S)
  • Parity (P)
  • Carry (CY)
  • Auxiliary Carry (AC structure defined)

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Instruction Execution

The CPU supports execution of several 8085 instruction groups.

Data Transfer

• MOV r1,r2
• MVI r,data

Arithmetic

• ADD r
• SUB r
• ADI data
• SUI data

Logical

• ANA r
• XRA r
• ORA r
• ANI data
• ORI data
• CMP r

Register Operations

• INR r
• DCR r

Control Flow

• JMP addr
• JZ addr
• JNZ addr
• CALL addr
• RET

Machine Control

• NOP
• HLT

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Debugging and Trace Output

Each instruction execution prints:

• Program Counter
• Opcode
• Executed instruction
• Register state

Example output:

PC=0000 OPCODE=06
MVI executed
A=00
B=05
C=00
D=00
E=00
H=00
L=00

This helps trace program execution step-by-step.

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Assembler

The project includes a simple two-pass assembler that converts assembly programs into machine code.

Supported assembly instructions include:

MVI
MOV
ADD
SUB
INR
DCR
JMP
JZ
JNZ
HLT

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Label Support

The assembler supports labels for branching.

Example:

MVI B,05

LOOP:
DCR B
JNZ LOOP

HLT

The assembler automatically resolves label addresses.

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Project Structure

8085Simulator
│
├── assembler/
│   └── assembler.cpp
│
├── cpu/
│   ├── cpu.cpp
│   └── cpu.h
│
├── memory/
│   ├── memory.cpp
│   └── memory.h
│
├── program.asm
│
├── main.cpp
└── CMakeLists.txt

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CPU Architecture

The simulator models a simplified 8085 execution cycle:

Fetch → Decode → Execute

1. Fetch

   • Read opcode from memory using PC.

2. Decode

   • Identify instruction type using opcode masks.

3. Execute

   • Perform register, memory, or control operation.

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Example Assembly Program

MVI B,05

LOOP:
DCR B
JNZ LOOP

HLT

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Example Execution Trace

Assembly completed successfully

PC=0000 OPCODE=06
MVI executed

PC=0002 OPCODE=05
DCR executed

PC=0003 OPCODE=C2
JNZ executed

The loop continues until register B becomes zero, after which the CPU executes HLT.

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How to Run

1. Build the project

mkdir build
cd build
cmake ..
make

2. Run the simulator

./8085Simulator

3. Modify program.asm to test your own assembly programs.

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Learning Goals

This project demonstrates:

• CPU emulator design
• instruction decoding using bit masks
• assembler implementation
• program counter control
• stack operations
• debugging through execution tracing

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Future Improvements

Planned features:

• Memory instructions LDA, STA, LHLD, SHLD

• Instruction dispatch table (faster decoding)

• Memory viewer and debugger tools

• Graphical interface using Qt

• Full 8085 instruction set support

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Author

Ashutosh Upreti Computer Science Student Interested in systems programming, CPU architecture, and low-level software development.

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