HackMD
Let's compare real-world examples of Von Neumann and Harvard architectures using two popular microcontroller families: STM32 and Arduino AVR.
1. Arduino AVR (Harvard Architecture)
Example: ATmega328P (used in Arduino Uno)
- Architecture: Harvard
- Code (Program) Memory: Flash memory (e.g. 32 KB)
- Data Memory: SRAM (e.g. 2 KB)
- Instruction and Data buses: separate
- Instruction word size: 16 bits
- Data word size: 8 bits
Implications:
- Can fetch instructions and read/write data at the same time
- Efficient for simple embedded control tasks
- Code and data stored in different physical memory regions
Example Behavior:
When you use pgm_read_byte() to read a constant from flash memory, it’s because code and data memory are separate—normal pointers can’t access program memory directly.
2. STM32 (Von Neumann-Like Architecture)
Example: STM32F103 (Blue Pill) or STM32F4 series
- Architecture: Modified Harvard / Von Neumann-like
- Unified memory space using bus matrix
- Code and data often reside in the same address space
- Uses caching and pre-fetch to overcome bottlenecks
- Instruction and data fetches may share buses at certain points
Implications:
- Simpler software model (code and data are addressable with standard pointers)
- Easier to use C/C++ pointers on both RAM and Flash
- Slight performance tradeoff compared to pure Harvard for some real-time tasks
Side-by-Side Comparison:
Summary:
