Kunal Dawn

Device Drivers, Part 8: Accessing x86-Specific I/O-Mapped Hardware

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The second day in the Linux device drivers’ laboratory was expected to be quite different from the typical software-oriented class. Apart from accessing and programming architecture-specific I/O mapped hardware in x86, it had a lot to offer first-timers with regard to reading hardware device manuals (commonly called data sheets) and how to understand them to write device drivers. In contrast, the previous session about generic architecture-transparent hardware interfacing was about mapping and accessing memory-mapped devices in Linux without any device-specific details.

x86-specific hardware interfacing

Unlike most other architectures, x86 has an additional hardware accessing mechanism, through direct I/O mapping. It is a direct 16-bit addressing scheme, and doesn’t need mapping to a virtual address for access. These addresses are referred to as port addresses, or ports. Since this is an additional access mechanism, it has an additional set of x86 (assembly/machine code) instructions. And yes, there are the input instructions inbinw, and inl for reading an 8-bit byte, a 16-bit word, and a 32-bit long word, respectively, from I/O mapped devices, through ports. The corresponding output instructions are outboutw and outl, respectively. The equivalent C functions/macros (available through the header <asm/io.h>) are as follows:

u8 inb(unsigned long port);
u16 inw(unsigned long port);
u32 inl(unsigned long port);
void outb(u8 value, unsigned long port);
void outw(u16 value, unsigned long port);
void outl(u32 value, unsigned long port);

The basic question that may arise relates to which devices are I/O mapped and what the port addresses of these devices are. The answer is pretty simple. As per x86-standard, all these devices and their mappings are predefined. Figure 1 shows a snippet of these mappings through the kernel window /proc/ioports. The listing includes predefined DMA, the timer and RTC, apart from serial, parallel and PCI bus interfaces, to name a few.

x86-specific I/O ports

Figure 1: x86-specific I/O ports

Simplest: serial port on x86

For example, the first serial port is always I/O mapped from 0x3F8 to 0x3FF. But what does this mapping mean? What do we do with this? How does it help us to use the serial port? That is where a data-sheet of the corresponding device needs to be looked up.

A serial port is controlled by the serial controller device, commonly known as an UART (Universal Asynchronous Receiver/Transmitter) or at times a USART (Universal Synchronous/Asynchronous Receiver/Transmitter). On PCs, the typical UART used is the PC16550D.

Generally speaking, from where, and how, does one get these device data sheets? Typically, an online search with the corresponding device number should yield their data-sheet links. Then, how does one get the device number? Simple… by having a look at the device. If it is inside a desktop, open it up and check it out. Yes, this is the least you may have to do to get going with the hardware, in order to write device drivers. Assuming all this has been done, it is time to peep into the data sheet of the PC16550D UART.

Device driver writers need to understand the details of the registers of the device, as it is these registers that writers need to program, to use the device. Page 14 of the data sheet (also shown in Figure 2) shows the complete table of all the twelve 8-bit registers present in the UART PC16550D.

Registers of UART PC16550D

Figure 2: Registers of UART PC16550D

Each of the eight rows corresponds to the respective bit of the registers. Also, note that the register addresses start from 0 and goes up to 7. The interesting thing about this is that a data sheet always gives the register offsets, which then needs to be added to the base address of the device, to get the actual register addresses.

Who decides the base address and where is it obtained from? Base addresses are typically board/platform specific, unless they are dynamically configurable like in the case of PCI devices. In this case, i.e., a serial device on x86, it is dictated by the x86 architecture—and that precisely was the starting serial port address mentioned above—0x3F8.

Thus, the eight register offsets, 0 to 7, exactly map to the eight port addresses 0x3F8 to 0x3FF. So, these are the actual addresses to be read or written, for reading or writing the corresponding serial registers, to achieve the desired serial operations, as per the register descriptions.

All the serial register offsets and the register bit masks are defined in the header<linux/serial_reg.h>. So, rather than hard-coding these values from the data sheet, the corresponding macros could be used instead. All the following code uses these macros, along with the following:

#define SERIAL_PORT_BASE 0x3F8

Operating on the device registers
To summarise the decoding of the PC16550D UART data sheet, here are a few examples of how to do read and write operations of the serial registers and their bits.

Reading and writing the ‘Line Control Register (LCR)’:

u8 val;
val = inb(SERIAL_PORT_BASE + UART_LCR /* 3 */);
outb(val, SERIAL_PORT_BASE + UART_LCR /* 3 */);

Setting and clearing the ‘Divisor Latch Access Bit (DLAB)’ in LCR:

u8 val;
val = inb(SERIAL_PORT_BASE + UART_LCR /* 3 */);
/* Setting DLAB */
val |= UART_LCR_DLAB /* 0x80 */;
outb(val, SERIAL_PORT_BASE + UART_LCR /* 3 */);
/* Clearing DLAB */
val &= ~UART_LCR_DLAB /* 0x80 */;
outb(val, SERIAL_PORT_BASE + UART_LCR /* 3 */);

Reading and writing the ‘Divisor Latch’:

u8 dlab;
u16 val;
dlab |= UART_LCR_DLAB; // Setting DLAB to access Divisor Latch
val = inw(SERIAL_PORT_BASE + UART_DLL /* 0 */);
outw(val, SERIAL_PORT_BASE + UART_DLL /* 0 */);

Blinking an LED

To get a real experience of low-level hardware access and Linux device drivers, the best way would be to play with the Linux device driver kit (LDDK) mentioned above. However, just for a feel of low-level hardware access, a blinking light emitting diode (LED) may be tried, as follows:

Connect a light-emitting diode (LED) with a 330 ohm resistor in series across Pin 3 (Tx) and Pin 5 (Gnd) of the DB9 connector of your PC.

Pull up and down the transmit (Tx) line with a 500 ms delay, by loading and unloading theblink_led driver, using insmod blink_led.ko and rmmod blink_led, respectively.

Driver file blink_led.ko can be created from its source file blink_led.c by running make with the usual driver Makefile. Given below is the complete blink_led.c:

#include <linux/module.h>
#include <linux/version.h>
#include <linux/types.h>
#include <linux/delay.h>
#include <asm/io.h>
#include <linux/serial_reg.h>
#define SERIAL_PORT_BASE 0x3F8
int __init init_module()
    int i;
    u8 data;
    data = inb(SERIAL_PORT_BASE + UART_LCR);
    for (i = 0; i < 5; i++)
        /* Pulling the Tx line low */
        data |= UART_LCR_SBC;
        outb(data, SERIAL_PORT_BASE + UART_LCR);
        /* Defaulting the Tx line high */
        data &= ~UART_LCR_SBC;
        outb(data, SERIAL_PORT_BASE + UART_LCR);
    return 0;
void __exit cleanup_module()
MODULE_AUTHOR("Anil Kumar Pugalia <email_at_sarika-pugs_dot_com>");

Looking ahead

You might have wondered why Shweta is missing from this article? She bunked all the classes! Watch out for the next article to find out why.


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