Memory-Mapped I/O (MMIO)

There are two broad architectural approaches to CPU–device access:

1. Separate I/O space with special I/O instructions
2. Memory-mapped I/O (MMIO): map device registers into the normal address space so the CPU uses ordinary load/store

RISC-V commonly uses MMIO.

In MMIO, the CPU simply reads/writes an address; hardware address decoding decides whether that access goes to RAM/ROM or to a device register.

Figure 9.1: Example system address partitioning (RAM/ROM ranges and an MMIO range for devices).

A key behavioral difference:

• Memory reads/writes primarily store/retrieve data.
• MMIO reads/writes often trigger device behavior or query device state.

So repeated reads may change over time, and writes may have immediate side effects.