What You Will Learn

Prerequisites

This lecture assumes you have completed the two earlier lectures in this mini-series covering I/O memory basics and memory-mapped I/O. Some familiarity with x86 architecture is helpful, since port-mapped I/O is primarily an x86 concept.

What Is Port-Mapped I/O?

Unlike memory-mapped I/O, where a peripheral’s registers simply occupy addresses inside the normal address space, port-mapped I/O uses a completely separate, smaller address space that the CPU can only reach through dedicated instructions. On x86 processors these are the IN and OUT assembly instructions, and the kernel exposes them to driver code through C wrapper functions rather than requiring you to write inline assembly yourself.

Step 1: Reserving an I/O Port Range

Just like with memory-mapped I/O, you must reserve a port range before using it, so the kernel’s bookkeeping reflects that your driver owns it. On kernel 6.x the managed variant is preferred:

#define MYDEV_IOPORT_BASE  0x300
#define MYDEV_IOPORT_SIZE  8

static int mydevice_probe_pmio(struct pci_dev *pdev)
{
    if (!devm_request_region(&pdev->dev, MYDEV_IOPORT_BASE,
                              MYDEV_IOPORT_SIZE, "mydevice"))
        return -EBUSY;

    /* Port range is now reserved for this driver */
    return 0;
}

As with the memory-mapped case, the devm_ prefixed version automatically releases the port range when the driver is detached, so you do not need a matching manual release call in a normal teardown path.

Step 2: Reading and Writing Ports

Once the range is reserved, you access individual ports with a small family of functions rather than a pointer. There is no mapping step here at all, since ports are not part of the regular address space.

A Simple Original PMIO Example

Here is a small, original status-check pattern showing the same read-modify-write idea from the MMIO lecture, adapted to ports instead of a mapped pointer:

#define STATUS_PORT_OFFSET  0x02
#define READY_BIT           BIT(1)

static bool mydevice_is_ready(unsigned int base_port)
{
    u8 status = inb(base_port + STATUS_PORT_OFFSET);

    return status & READY_BIT;
}

static void mydevice_reset(unsigned int base_port)
{
    outb(0x01, base_port + STATUS_PORT_OFFSET);
}

Real-World Use Case

Port-mapped I/O today mostly survives in legacy PC hardware paths: the classic PC parallel port, some older serial UART implementations, and a handful of very old PCI devices that never migrated to a pure memory-mapped BAR layout. Most new silicon, including new x86 peripherals, has moved entirely to memory-mapped I/O, so treat this lecture as important background knowledge rather than something you will use in every driver you write.

MMIO vs PMIO: Which Should You Use?

Common Mistakes and Troubleshooting

Key Takeaways

Frequently Asked Questions