MOS 6502: How To Run Instructions

Dec 8, 2025 by Alex Johnson 34 views

Hey there, fellow retro-computing enthusiasts and curious minds! Ever wondered what makes those classic 8-bit machines tick? Today, we're diving deep into the heart of the MOS 6502 microprocessor, specifically focusing on the exciting moment when we can finally run instructions. This isn't just about theory; it's about getting your hands dirty (virtually, of course!) and understanding the practical steps involved in making this legendary chip execute commands. We're going to explore the prerequisites, the challenges, and the sheer satisfaction of seeing your code come to life on a simulated 6502 environment. So, buckle up, because we're about to shed some light on how to get those bytes flowing and those instructions executing!

The Crucial Prerequisites: Building the Foundation for Instruction Execution

Before we can even think about running instructions on our simulated MOS 6502, there's a significant amount of groundwork that needs to be laid. Think of it like building a house; you can't start hanging pictures before the walls are up and the roof is on, right? Similarly, our 6502 simulation needs several key components to be in place to function correctly. The prompt mentions that when items #10, #9, and #7 are complete, we should have enough 'blocks' to actually run instructions. This implies a modular approach to building the emulator, where each completed module contributes a vital piece of the puzzle. Let's break down what these 'blocks' likely represent and why they are so essential for instruction execution.

First, we have the CPU core itself. This is the brain of the operation, the actual implementation of the 6502's architecture. It needs to understand the instruction set, the various addressing modes, and how to manipulate registers like the Accumulator (A), Index registers (X and Y), the Program Counter (PC), and the Stack Pointer (SP). Without a functional CPU core, there's nothing to decode or execute instructions. This is the absolute bedrock upon which everything else is built.

Secondly, we need a robust memory system. The 6502 doesn't operate in a vacuum; it needs to read instructions from and write data to memory. This memory system, often referred to as RAM (Random Access Memory) or ROM (Read-Only Memory), is where your program code and data reside. In a real system, this would involve physical chips, address lines, and data lines. In our simulation, this is typically represented by an array or a data structure that mimics the 6502's address space (which is 64KB, or 0x0000 to 0xFFFF). The CPU core will constantly be interacting with this memory to fetch the next instruction, read operands, and store results. So, a working memory model is absolutely critical.

Thirdly, and arguably the most complex prerequisite for truly running instructions in a meaningful way, is the bus system. The bus is the communication highway between the CPU, memory, and other peripherals. It handles the transfer of addresses, data, and control signals (like read/write signals). In a real 6502 system, this would be a physical set of wires. In our simulation, it's a more abstract concept but no less vital. The CPU needs to signal which memory address it wants to access, whether it wants to read from or write to that address, and then either retrieve data from or send data to that location. A well-defined bus interface allows the CPU core and the memory model to communicate effectively. Without it, the CPU core might be able to decode instructions, but it wouldn't be able to fetch them or interact with data, making actual execution impossible.

These three components – the CPU core, the memory system, and the bus interface – are the fundamental 'blocks' that must be in place. The prompt's reference to items #10, #9, and #7 strongly suggests that these represent the development and testing of these very components. Once they are functional and integrated, we have the essential framework needed to begin the process of fetching, decoding, and executing the rich instruction set of the MOS 6502. The journey to running instructions is a testament to the careful, step-by-step construction of a complex system, where each completed part enables the next stage of development.

The Necessity of a Memory/Bus Mock: Bridging the Gap to Realistic Execution

As we've established, running instructions on the MOS 6502 requires a functional CPU, memory, and a way for them to communicate. However, creating a full, accurate simulation of a complete 1970s-era computer system with all its intricate peripheral chips and timing complexities can be an incredibly daunting task. This is where the concept of a memory/bus mock becomes not just useful, but absolutely essential. A mock, in this context, is a simplified or simulated representation of a component that allows us to test and develop other parts of the system in isolation, or with controlled interactions.

When we talk about mocking the memory and bus, we're essentially creating a controlled environment for the 6502 CPU to interact with. Instead of needing to perfectly emulate every nuance of a real RAM chip, a specific I/O device, or the precise signal timings of a physical bus, we create a simplified interface. This mock provides predictable responses to the CPU's requests. For example, when the CPU attempts to read from a specific memory address, the mock memory might return a pre-defined value. When the CPU writes data, the mock bus could simply log that data or signal that the write operation was successful, without needing to simulate the complex electrical signaling.

Why is this so crucial for running instructions? Firstly, it allows for rapid development and testing. Imagine trying to test your CPU's ability to execute a complex LDA (Load Accumulator) instruction that requires reading an operand from memory. Without a memory mock, you'd need a fully functional, albeit simplified, memory system. With a mock, you can simply tell the mock memory, "When the CPU reads from address $0200, return the value $42." This allows you to immediately verify if your CPU core correctly fetches the instruction, decodes it, reads the operand $42, and loads it into the Accumulator. You can create specific scenarios to test edge cases and different addressing modes without the overhead of a complete system.

Secondly, a memory/bus mock helps in isolating problems. If your 6502 simulation isn't behaving as expected when trying to run a particular sequence of instructions, a mock system makes debugging much easier. You can inspect the interactions between the CPU and the mock. Did the CPU request the correct address? Did it signal a read or a write correctly? Did the mock return the expected data? By controlling the environment, you can pinpoint whether the issue lies within the CPU's logic or in the way it's interacting with the simulated memory and bus. This drastically reduces the search space for bugs.

Thirdly, it provides a pathway to greater complexity. Once you have a stable CPU core running basic instructions using a memory/bus mock, you can gradually replace parts of the mock with more sophisticated simulations or even with emulations of actual hardware. You might start with a simple RAM mock, then introduce a mock for a specific I/O chip like a Programmable Interval Timer (PIT) or a Video Display Controller (VDC), and eventually, you could integrate more realistic bus timing and arbitration logic. The mock acts as a placeholder, a stand-in that enables progress while the more complex pieces are being developed or refined.

In essence, the memory/bus mock is the scaffolding that allows us to build and test the intricate logic of the MOS 6502 instruction execution. It provides a stable, predictable, and controllable environment, enabling developers to focus on the core functionality of the CPU and its instruction set without getting bogged down by the complexities of a full hardware emulation from the outset. It's a smart engineering approach that accelerates development and leads to a more robust final product. Without this crucial abstraction, the journey to running instructions would be significantly longer and far more prone to errors.

The Thrill of Execution: A Step-by-Step Glimpse into Running 6502 Instructions

Finally, the moment we've all been waiting for: running instructions on our simulated MOS 6502! With the CPU core, memory system, and a functional (even if mocked) bus interface in place, we can finally witness the magic unfold. This process, at its core, involves a continuous cycle: fetch, decode, execute. Let's break down what happens step-by-step when the 6502 is tasked with processing a piece of code. It's a fascinating dance of signals and data, orchestrated by the silicon heart of the processor.

The Fetch Cycle: Grabbing the Next Command

The journey begins with the Program Counter (PC). This special register holds the memory address of the next instruction to be executed. Initially, the PC is set to the starting address of your program (often from a reset vector or a jump instruction). The 6502 then initiates the fetch cycle:

Address Bus Output: The value currently stored in the PC is placed onto the address bus. This tells the memory system where to look for the instruction.
Read Signal Assertion: A read signal is asserted on the control bus. This signals to the memory that the CPU wants to read data from the specified address.
Memory Response: The memory system (or our mock) responds by placing the byte of data located at that address onto the data bus.
Data Bus Input: The 6502 reads the byte from the data bus and stores it internally. This byte is the opcode, the first byte of the instruction, which tells the CPU what operation to perform.
PC Increment: Crucially, after fetching the opcode, the PC is incremented. This prepares it to point to the next byte in memory, which might be an operand or part of a multi-byte instruction. If the instruction is one byte long, the PC will now point to the start of the next instruction. If it's two or three bytes, the subsequent bytes will be fetched in subsequent cycles, and the PC will be incremented accordingly after each fetch.

The Decode Cycle: Understanding the Command

Once the opcode is fetched and stored internally, the CPU enters the decode phase. This is where the processor's internal logic examines the opcode byte. It determines:

The Operation: What specific action needs to be performed (e.g., load data, add numbers, branch to a different part of the code)?
The Addressing Mode: How will the CPU access any necessary data (operands)? The 6502 has a rich set of addressing modes, including immediate, zero page, absolute, indexed, indirect, and implied. Each mode dictates how the PC and other registers are used to calculate the final memory address of the data, or if data is directly embedded in the instruction itself.

This decoding process is handled by the CPU's control unit, a complex piece of combinatorial logic that translates the opcode into a sequence of micro-operations required to perform the instruction.

The Execute Cycle: Performing the Action

This is where the actual work gets done. Based on the decoded instruction and its addressing mode, the CPU performs the required actions. This might involve:

Fetching Operands: If the instruction requires data from memory (e.g., LDA $1000), the CPU will perform additional memory read cycles, using the PC and potentially index registers (X, Y) to calculate the correct address ($1000 in this example). The data fetched is then placed into a temporary internal register or used directly.
Register Operations: Data might be moved between registers (e.g., TAX - Transfer Accumulator to X).
Arithmetic/Logic Operations: The Arithmetic Logic Unit (ALU) performs calculations. For example, in an ADC (Add with Carry) instruction, the ALU takes the value in the Accumulator, the fetched operand, and the Carry flag, and computes the sum, storing the result back in the Accumulator and updating status flags (like Carry, Zero, Negative).
Memory Writes: If the instruction modifies memory (e.g., STA $2000 - Store Accumulator at $2000), the CPU will output the address and the data to be written, and assert a write signal.
Branching/Jumping: For control flow instructions (like BEQ - Branch if Equal, or JMP - Jump), the CPU might modify the PC directly, causing subsequent fetches to occur from a different memory location. This is how programs change their execution path.
Interrupt Handling: If an interrupt signal is detected (like NMI or IRQ), the CPU will pause its current execution, save its state (PC and status flags) onto the stack, and jump to a specific interrupt handler routine.

Once the execute cycle is complete, the CPU returns to the fetch cycle, with the PC now pointing to the next instruction, ready to begin the entire process anew. This fetch-decode-execute cycle repeats billions of times per second in modern CPUs, but for the MOS 6502, even a few thousand times per second is a symphony of digital logic bringing software to life. Seeing this cycle successfully complete for various instructions is the ultimate reward for meticulously building an emulator.

Conclusion: The Journey Continues

Successfully running instructions on a simulated MOS 6502 is a monumental achievement in understanding how these classic processors work. It signifies that your CPU core, memory system, and bus interface are all playing together harmoniously. The use of a memory/bus mock is a testament to pragmatic engineering, allowing for focused development and easier debugging before tackling the full complexity of hardware emulation. Each successfully executed instruction, from the simplest LDA to a more complex JSR (Jump to Subroutine), is a validation of the hours spent crafting the logic.

This journey is just the beginning, of course. From here, one can explore more advanced topics like interrupts, DMA (Direct Memory Access), and the emulation of specific peripherals that defined the classic computing era. The ability to run instructions is the gateway to understanding and recreating the vibrant world of 8-bit computing.

For those eager to delve deeper into the fascinating world of microprocessors and emulation, I highly recommend exploring these excellent resources:

The official 6502 documentation and datasheets: Available through various retro-computing archives, these provide the definitive technical specifications. You can often find these by searching for '6502 datasheet' or '6502 programming manual' online.
Online 6502 simulators and emulators: Studying existing open-source emulators can provide invaluable insights into different implementation strategies. Websites like Wikipedia offer detailed overviews of the 6502 architecture and its history, providing excellent context. You can find comprehensive information on the MOS 6502 and its impact on computing history by visiting Wikipedia's page on the MOS Technology 6502.
Retro computing forums and communities: Engaging with other enthusiasts can provide support, answer specific questions, and share collective knowledge. Websites like the VCF Forums (Vintage Computer Federation) are fantastic places to connect.