The Territory Is On The Map
The Territory Is On The Map
In some architectures, talking to hardware requires speaking a different language. The CPU must use special instructions, reaching outside its normal memory space to poke at isolated peripheral ports. The map of memory and the territory of the physical hardware are kept strictly separate.
Plan 9 on modern system-on-chip architectures (like the ARM64 bcm64) resolves this separation using Memory-Mapped I/O (MMIO).
The physical address spaces of the peripheral controllers—the GPIO pins, the SPI buses, the UART consoles—are directly mapped into the kernel's virtual address space using iomap and vmap. To turn on an LED or read a sensor, the kernel does not issue a special hardware command; it simply writes a value to a specific memory address via a volatile pointer.
The hardware is not hidden behind an abstract interface; it is literally present in the memory layout. The physical world has been folded into the virtual map. To touch the memory is to touch the metal. The territory is on the map.