MMIO and Bit Manipulation

Purpose

Guide agents through safe memory-mapped I/O: volatile semantics, read-modify-write patterns, bitfield pitfalls, alignment and endianness, and portable register access macros for bare-metal drivers.

When to Use

• Writing peripheral register drivers without HAL
• Fixing intermittent register corruption or stale reads
• Replacing C bitfields with explicit masks
• Porting drivers between little-endian MCUs
• Auditing ISR vs main-line register access

Workflow

1. MMIO fundamentals

Peripheral registers live at fixed addresses in the CPU memory map. The compiler must not cache reads/writes.