CPU DNA: From the Apple IIe 6502 to the M1 MacBook Pro

This piece distills how central processing units (CPUs) work by tracing the evolution from the Apple IIe’s 6502 processor to the modern M1 in a MacBook Pro. It explains the shared architectural DNA of CPUs, including the fetch-decode-execute cycle, registers, program counter, and ALU, and shows how billions of transistors, cores, caches, and memory hierarchies cooperate to run software. The summary highlights the simplified analogy used to teach CPU operations, the role of compilers and assembly, and the differences between historic eight-bit designs and today's 64-bit, multi-core, pipelined processors. It also touches on alternatives like ASIs and FPGAs and the broader context of system on chip design.

Introduction

This article accompanies a detailed video that explains CPUs by comparing the Apple IIe era 6502 microprocessor with the modern M1 chip in a MacBook Pro. It emphasizes that all modern processors share a common architectural DNA, despite vast differences in scale and power.

Historical Roots and Modern Power

The Apple IIe used a 4,528-transistor 6502 capable of about 430,000 operations per second, a far cry from today’s 16‑billion-transistor M1 that can perform around 3 trillion calculations per second. Yet the underlying principles—fetch, decode, execute—are remarkably similar across generations.

The Shared DNA: Fetch-Decode-Execute

Every computer instruction passes through a three-step cycle: fetch an instruction using a program counter, decode the instruction with a decoder, and execute it with the ALU. This cycle is regulated by the processor’s clock and can be pipelined to handle multiple instructions concurrently, a key optimization in modern chips.

Hardware Building Blocks

Beyond the cycle, memory and data pathways, such as DRAM, caches, registers, and the program counter, organize how instructions and data move. The video uses an accessible analogy in which shelves, a cart, a table, and a calculator illustrate how data flows through a CPU core, with the program counter guiding the sequence of operations.

Instruction Sets and Complexity

Older CPUs offered limited instruction sets (the Apple IIe’s 6502 had about 56 instructions, the M1 supports around 354). This highlights how a surprisingly small core vocabulary can express nearly all modern software when combined with compilers that translate high level code into machine instructions.

From CPU to SoC

Modern processors are frequently part of a system on a chip, integrating CPU cores, a GPU, neural processing units, and other accelerators. The M1’s architecture includes multiple high performance cores, energy efficient cores, and dedicated hardware blocks, illustrating how a single chip can handle computation, graphics, and AI tasks in a highly integrated package.

Architectures and Alternatives

The video contrasts risk (RISC) architectures with CISC approaches, noting that modern smartphones tend to rely on RISC for efficiency while desktop CPUs historically used broader instruction sets. It also mentions alternatives like ASIs and FPGAs and touches on quantum computing as a future direction.

Conclusion

Understanding CPU DNA helps demystify how devices ranging from vintage computers to AI servers operate. It clarifies why modern chips, despite complexity, are built from a few core principles and a shared workflow, enabling trillions of operations per second across devices.