Port I/O allows the cpu to communicate with an external device. There are 65536
available I/O port addresses. To send data to I/O port mapped devices, you can
use the instruction
OUTS). To receive data from I/O port mapped
devices, you can use the instruction
Usually, I/O ports can’t be accessed from userland, but this behavior can be customized, with the IOPL value in EFLAGS and the TSS.
In K, you will need I/O ports for:
- The serial driver
- The PIC
- The PIT
- The Keyboard
Memory-mapped I/O is accessed as normal memory, and access to memory-mapped is protected the same way the memory is protected (segmentation or pagination).
Memory-mapped devices have an address range of physical memory allowed to them. It implies that the memory-mapped I/O address space has the same size as the physical address space of your CPU.
In K, you will use memory-mapped I/O for the VGA framebuffer.
- Intel manual 1: 18
- Intel manual 3: 20.2.8
- Intel manual 3: 2.3