Using I/O Ports

I/O ports are the means by which drivers communicate with many devices out there—at least part of the time. This section covers the various functions available for making use of I/O ports; we also touch on some portability issues.

Let us start with a quick reminder that I/O ports must be allocated before being used by your driver. As we discussed in Section 2.5.1 in Chapter 2, the functions used to allocate and free ports are:

#include <linux/ioport.h>
int check_region(unsigned long start, unsigned long len);
struct resource *request_region(unsigned long start, 
       unsigned long len, char *name);
void release_region(unsigned long start, unsigned long len);

After a driver has requested the range of I/O ports it needs to use in its activities, it must read and/or write to those ports. To this aim, most hardware differentiates between 8-bit, 16-bit, and 32-bit ports. Usually you can’t mix them like you normally do with system memory access.[32]

A C program, therefore, must call different functions to access different size ports. As suggested in the previous section, computer architectures that support only memory-mapped I/O registers fake port I/O by remapping port addresses to memory addresses, and the kernel hides the details from the driver in order to ease portability. The Linux kernel headers (specifically, the architecture-dependent header <asm/io.h>) define the following inline functions to access I/O ports.


From now on, when we use unsigned without further type specifications, we are referring to an architecture-dependent definition whose exact nature is not relevant. The functions are almost always portable because the compiler automatically casts the values during assignment—their being unsigned helps prevent compile-time warnings. No information is lost with such casts as long as the programmer assigns sensible values to avoid overflow. We’ll stick to this convention of “incomplete typing” for the rest of the chapter.

unsigned inb(unsigned port); , void outb(unsigned char byte, unsigned port);

Read or write byte ports (eight bits wide). The port argument is defined as unsigned long for some platforms and unsigned short for others. The return type of inb is also different across architectures.

unsigned inw(unsigned port); , void outw(unsigned short word, unsigned port);

These functions access 16-bit ports (word wide); they are not available when compiling for the M68k and S390 platforms, which support only byte I/O.

unsigned inl(unsigned port); , void outl(unsigned longword, unsigned port);

These functions access 32-bit ports. longword is either declared as unsigned long or unsigned int, according to the platform. Like word I/O, “long” I/O is not available on M68k and S390.

Note that no 64-bit port I/O operations are defined. Even on 64-bit architectures, the port address space uses a 32-bit (maximum) data path.

The functions just described are primarily meant to be used by device drivers, but they can also be used from user space, at least on PC-class computers. The GNU C library defines them in <sys/io.h>. The following conditions should apply in order for inb and friends to be used in user-space code:

  • The program must be compiled with the -O option to force expansion of inline functions.

  • The ioperm or iopl system calls must be used to get permission to perform I/O operations on ports. ioperm gets permission for individual ports, while iopl gets permission for the entire I/O space. Both these functions are Intel specific.

  • The program must run as root to invoke ioperm or iopl [33] Alternatively, one of its ancestors must have gained port access running as root.

If the host platform has no ioperm and no iopl system calls, user space can still access I/O ports by using the /dev/port device file. Note, though, that the meaning of the file is very platform specific, and most likely not useful for anything but the PC.

The sample sources misc-progs/inp.c and misc-progs/outp.c are a minimal tool for reading and writing ports from the command line, in user space. They expect to be installed under multiple names (i.e., inpb, inpw, and inpl and will manipulate byte, word, or long ports depending on which name was invoked by the user. They use /dev/port if ioperm is not present.

The programs can be made setuid root, if you want to live dangerously and play with your hardware without acquiring explicit privileges.

String Operations

In addition to the single-shot in and out operations, some processors implement special instructions to transfer a sequence of bytes, words, or longs to and from a single I/O port or the same size. These are the so-called string instructions, and they perform the task more quickly than a C-language loop can do. The following macros implement the concept of string I/O by either using a single machine instruction or by executing a tight loop if the target processor has no instruction that performs string I/O. The macros are not defined at all when compiling for the M68k and S390 platforms. This should not be a portability problem, since these platforms don’t usually share device drivers with other platforms, because their peripheral buses are different.

The prototypes for string functions are the following:

void insb(unsigned port, void *addr, unsigned long count); , void outsb(unsigned port, void *addr, unsigned long count);

Read or write count bytes starting at the memory address addr. Data is read from or written to the single port port.

void insw(unsigned port, void *addr, unsigned long count); , void outsw(unsigned port, void *addr, unsigned long count);

Read or write 16-bit values to a single 16-bit port.

void insl(unsigned port, void *addr, unsigned long count); , void outsl(unsigned port, void *addr, unsigned long count);

Read or write 32-bit values to a single 32-bit port.

Pausing I/O

Some platforms—most notably the i386—can have problems when the processor tries to transfer data too quickly to or from the bus. The problems can arise because the processor is overclocked with respect to the ISA bus, and can show up when the device board is too slow. The solution is to insert a small delay after each I/O instruction if another such instruction follows. If your device misses some data, or if you fear it might miss some, you can use pausing functions in place of the normal ones. The pausing functions are exactly like those listed previously, but their names end in _p; they are called inb_p, outb_p, and so on. The functions are defined for most supported architectures, although they often expand to the same code as nonpausing I/O, because there is no need for the extra pause if the architecture runs with a nonobsolete peripheral bus.

Platform Dependencies

I/O instructions are, by their nature, highly processor dependent. Because they work with the details of how the processor handles moving data in and out, it is very hard to hide the differences between systems. As a consequence, much of the source code related to port I/O is platform dependent.

You can see one of the incompatibilities, data typing, by looking back at the list of functions, where the arguments are typed differently based on the architectural differences between platforms. For example, a port is unsigned short on the x86 (where the processor supports a 64-KB I/O space), but unsigned long on other platforms, whose ports are just special locations in the same address space as memory.

Other platform dependencies arise from basic structural differences in the processors and thus are unavoidable. We won’t go into detail about the differences, because we assume that you won’t be writing a device driver for a particular system without understanding the underlying hardware. Instead, the following is an overview of the capabilities of the architectures that are supported by version 2.4 of the kernel:

IA-32 (x86)

The architecture supports all the functions described in this chapter. Port numbers are of type unsigned short.

IA-64 (Itanium)

All functions are supported; ports are unsigned long (and memory-mapped). String functions are implemented in C.


All the functions are supported, and ports are memory-mapped. The implementation of port I/O is different in different Alpha platforms, according to the chipset they use. String functions are implemented in C and defined in arch/alpha/lib/io.c. Ports are unsigned long.


Ports are memory-mapped, and all functions are supported; string functions are implemented in C. Ports are of type unsigned int.


Ports are memory-mapped, and only byte functions are supported. No string functions are supported, and the port type is unsigned char *.


The MIPS port supports all the functions. String operations are implemented with tight assembly loops, because the processor lacks machine-level string I/O. Ports are memory-mapped; they are unsigned int in 32-bit processors and unsigned long in 64-bit ones.


All the functions are supported; ports have type unsigned char *.


Similar to the M68k, the header for this platform supports only byte-wide port I/O with no string operations. Ports are char pointers and are memory-mapped.


Ports are unsigned int (memory-mapped), and all the functions are supported.


Once again, I/O space is memory-mapped. Versions of the port functions are defined to work with unsigned long ports.

The curious reader can extract more information from the io.h files, which sometimes define a few architecture-specific functions in addition to those we describe in this chapter. Be warned that some of these files are rather difficult reading, however.

It’s interesting to note that no processor outside the x86 family features a different address space for ports, even though several of the supported families are shipped with ISA and/or PCI slots (and both buses implement different I/O and memory address spaces).

Moreover, some processors (most notably the early Alphas) lack instructions that move one or two bytes at a time.[34] Therefore, their peripheral chipsets simulate 8-bit and 16-bit I/O accesses by mapping them to special address ranges in the memory address space. Thus, an inb and an inw instruction that act on the same port are implemented by two 32-bit memory reads that operate on different addresses. Fortunately, all of this is hidden from the device driver writer by the internals of the macros described in this section, but we feel it’s an interesting feature to note. If you want to probe further, look for examples in include/asm-alpha/core_lca.h.

How I/O operations are performed on each platform is well described in the programmer’s manual for each platform; those manuals are usually available for download as PDF files on the Web.

[32] Sometimes I/O ports are arranged like memory, and you can (for example) bind two 8-bit writes into a single 16-bit operation. This applies, for instance, to PC video boards, but in general you can’t count on this feature.

[33] Technically, it must have the CAP_SYS_RAWIO capability, but that is the same as running as root on current systems.

[34] Single-byte I/O is not as important as one may imagine, because it is a rare operation. In order to read/write a single byte to any address space, you need to implement a data path connecting the low bits of the register-set data bus to any byte position in the external data bus. These data paths require additional logic gates that get in the way of every data transfer. Dropping byte-wide loads and stores can benefit overall system performance.

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