Linux I/O port programming mini-HOWTO
Author: Riku Saikkonen
Last modified: Dec 28 1997
This HOWTO document describes programming hardware I/O ports and wait
ing for small periods of time in user-mode Linux programs running on
the Intel x86 architecture.
1. Introduction
This HOWTO document describes programming hardware I/O ports and
waiting for small periods of time in user-mode Linux programs running
on the Intel x86 architecture. This document is a descendant of the
very small IO-Port mini-HOWTO by the same author.
This document is Copyright 1995-1997 Riku Saikkonen. See the Linux
HOWTO copyright
for details.
If you have corrections or something to add, feel free to e-mail me
(Riku.Saikkonen@hut.fi)...
Changes from the previous released version (Mar 30 1997):
· Clarified things regarding inb_p/outb_p and port 0x80.
· Removed information about udelay(), since nanosleep() provides a
cleaner way of using it.
· Converted to Linuxdoc-SGML, and reorganised somewhat.
· Lots of minor additions and modifications.
2. Using I/O ports in C programs
2.1. The normal method
Routines for accessing I/O ports are in /usr/include/asm/io.h (or
linux/include/asm-i386/io.h in the kernel source distribution). The
routines there are inline macros, so it is enough to #include
; you do not need any additional libraries.
Because of a limitation in gcc (present at least in 2.7.2.3 and below)
and in egcs (all versions), you have to compile any source code that
uses these routines with optimisation turned on (gcc -O1 or higher),
or alternatively #define extern to be empty before you #include
.
For debugging, you can use gcc -g -O (at least with modern versions of
gcc), though optimisation can sometimes make the debugger behave a bit
strangely. If this bothers you, put the routines that use I/O port
access in a separate source file and compile only that with
optimisation turned on.
Before you access any ports, you must give your program permission to
do so. This is done by calling the ioperm() function (declared in
unistd.h, and defined in the kernel) somewhere near the start of your
program (before any I/O port accesses). The syntax is ioperm(from,
num, turn_on), where from is the first port number to give access to,
and num the number of consecutive ports to give access to. For
example, ioperm(0x300, 5, 1) would give access to ports 0x300 through
0x304 (a total of 5 ports). The last argument is a Boolean value
specifying whether to give access to the program to the ports (true
(1)) or to remove access (false (0)). You can call ioperm() multiple
times to enable multiple non-consecutive ports. See the ioperm(2)
manual page for details on the syntax.
The ioperm() call requires your program to have root privileges; thus
you need to either run it as the root user, or make it setuid root.
You can drop the root privileges after you have called ioperm() to
enable the ports you want to use. You are not required to explicitly
drop your port access privileges with ioperm(..., 0) at the end of
your program; this is done automatically as the process exits.
A setuid() to a non-root user does not disable the port access granted
by ioperm(), but a fork() does (the child process does not get access,
but the parent retains it).
ioperm() can only give access to ports 0x000 through 0x3ff; for higher
ports, you need to use iopl() (which gives you access to all ports at
once). Use the level argument 3 (i.e., iopl(3)) to give your program
access to all I/O ports (so be careful --- accessing the wrong ports
can do all sorts of nasty things to your computer). Again, you need
root privileges to call iopl(). See the iopl(2) manual page for
details.
Then, to actually accessing the ports... To input a byte (8 bits) from
a port, call inb(port), it returns the byte it got. To output a byte,
call outb(value, port) (please note the order of the parameters). To
input a word (16 bits) from ports x and x+1 (one byte from each to
form the word, using the assembler instruction inw), call inw(x). To
output a word to the two ports, use outw(value, x). If you're unsure
of which port instructions (byte or word) to use, you probably want
inb() and outb() --- most devices are designed for bytewise port
access. Note that all port access instructions take at least about a
microsecond to execute.
The inb_p(), outb_p(), inw_p(), and outw_p() macros work otherwise
identically to the ones above, but they do an additional short (about
one microsecond) delay after the port access; you can make the delay
about four microseconds with #define REALLY_SLOW_IO before you
#include . These macros normally (unless you #define
SLOW_IO_BY_JUMPING, which is probably less accurate) use a port output
to port 0x80 for their delay, so you need to give access to port 0x80
with ioperm() first (outputs to port 0x80 should not affect any part
of the system). For more versatile methods of delaying, read on.
There are man pages for ioperm(2), iopl(2), and the above macros in
reasonably recent releases of the Linux manual page collection.
2.2. An alternate method: /dev/port
Another way to access I/O ports is to open() /dev/port (a character
device, major number 1, minor 4) for reading and/or writing (the stdio
f*() functions have internal buffering, so avoid them). Then lseek()
to the appropriate byte in the file (file position 0 = port 0x00, file
position 1 = port 0x01, and so on), and read() or write() a byte or
word from or to it.
Naturally, for this to work your program needs read/write access to
/dev/port. This method is probably slower than the normal method
above, but does not need compiler optimisation nor ioperm(). It
doesn't need root access either, if you give a non-root user or group
access to /dev/port --- but this is a very bad thing to do in terms of
system security, since it is possible to hurt the system, perhaps even
gain root access, by using /dev/port to access hard disks, network
cards, etc. directly.
3. Interrupts (IRQs) and DMA access
You cannot use IRQs or DMA directly from a user-mode process. You need
to write a kernel driver; see The Linux Kernel Hacker's Guide
for details and
the kernel source code for examples.
Also, you cannot disable interrupts from within a user-mode program.
4. High-resolution timing
4.1. Delays
First of all, I should say that you cannot guarantee user-mode
processes to have exact control of timing because of the multi-tasking
nature of Linux. Your process might be scheduled out at any time for
anything from about 10 milliseconds to a few seconds (on a system with
very high load). However, for most applications using I/O ports, this
does not really matter. To minimise this, you may want to nice your
process to a high-priority value (see the nice(2) manual page) or use
real-time scheduling (see below).
If you want more precise timing than normal user-mode processes give
you, there are some provisions for user-mode `real time' support.
Linux 2.x kernels have soft real time support; see the manual page for
sched_setscheduler(2) for details. There is a special kernel that
supports hard real time; see for
more information on this.
4.1.1. Sleeping: sleep() and usleep()
Now, let me start with the easier timing calls. For delays of multiple
seconds, your best bet is probably to use sleep(). For delays of at
least tens of milliseconds (about 10 ms seems to be the minimum
delay), usleep() should work. These functions give the CPU to other
processes (``sleep''), so CPU time isn't wasted. See the manual pages
sleep(3) and usleep(3) for details.
For delays of under about 50 milliseconds (depending on the speed of
your processor and machine, and the system load), giving up the CPU
takes too much time, because the Linux scheduler (for the x86
architecture) usually takes at least about 10-30 milliseconds before
it returns control to your process. Due to this, in small delays,
usleep(3) usually delays somewhat more than the amount that you
specify in the parameters, and at least about 10 ms.
4.1.2. nanosleep()
In the 2.0.x series of Linux kernels, there is a new system call,
nanosleep() (see the nanosleep(2) manual page), that allows you to
sleep or delay for short times (a few microseconds or more).
For delays <= 2 ms, if (and only if) your process is set to soft real
time scheduling (using sched_setscheduler()), nanosleep() uses a busy
loop; otherwise it sleeps, just like usleep().
The busy loop uses udelay() (an internal kernel function used by many
kernel drivers), and the length of the loop is calculated using the
BogoMips value (the speed of this kind of busy loop is one of the
things that BogoMips measures accurately). See
/usr/include/asm/delay.h) for details on how it works.
4.1.3. Delaying with port I/O
Another way of delaying small numbers of microseconds is port I/O.
Inputting or outputting any byte from/to port 0x80 (see above for how
to do it) should wait for almost exactly 1 microsecond independent of
your processor type and speed. You can do this multiple times to wait
a few microseconds. The port output should have no harmful side
effects on any standard machine (and some kernel drivers use it). This
is how {in|out}[bw]_p() normally do the delay (see asm/io.h).
Actually, a port I/O instruction on most ports in the 0-0x3ff range
takes almost exactly 1 microsecond, so if you're, for example, using
the parallel port directly, just do additional inb()s from that port
to delay.
4.1.4. Delaying with assembler instructions
If you know the processor type and clock speed of the machine the
program will be running on, you can hard-code shorter delays by
running certain assembler instructions (but remember, your process
might be scheduled out at any time, so the delays might well be longer
every now and then). For the table below, the internal processor speed
determines the number of clock cycles taken; e.g., for a 50 MHz
processor (e.g. 486DX-50 or 486DX2-50), one clock cycle takes
1/50000000 seconds (=200 nanoseconds).
Instruction i386 clock cycles i486 clock cycles
nop 3 1
xchg %ax,%ax 3 3
or %ax,%ax 2 1
mov %ax,%ax 2 1
add %ax,0 2 1
(Sorry, I don't know about Pentiums; probably close to the i486. I
cannot find an instruction which would use one clock cycle on an i386.
Use the one-clock-cycle instructions if you can, otherwise the
pipelining used in modern processors may shorten the times.)
The instructions nop and xchg in the table should have no side
effects. The rest may modify the flags register, but this shouldn't
matter since gcc should detect it. nop is a good choice.
To use these, call asm("instruction") in your program. The syntax of
the instructions is as in the table above; if you want multiple
instructions in a single asm() statement, separate them with
semicolons. For example, asm("nop ; nop ; nop ; nop") executes four
nop instructions, delaying for four clock cycles on i486 or Pentium
processors (or 12 clock cycles on an i386).
asm() is translated into inline assembler code by gcc, so there is no
function call overhead.
Shorter delays than one clock cycle are impossible in the Intel x86
architecture.
4.1.5. rdtsc for Pentiums
For Pentiums, you can get the number of clock cycles elapsed since the
last reboot with the following C code:
______________________________________________________________________
extern __inline__ unsigned long long int rdtsc()
{
unsigned long long int x;
__asm__ volatile (".byte 0x0f, 0x31" : "=A" (x));
return x;
}
______________________________________________________________________
You can poll this value to delay for as many clock cycles as you want.
4.2. Measuring time
For times accurate to one second, it is probably easiest to use
time(). For more accurate times, gettimeofday() is accurate to about a
microsecond (but see above about scheduling). For Pentiums, the rdtsc
code fragment above is accurate to one clock cycle.
If you want your process to get a signal after some amount of time,
use setitimer() or alarm(). See the manual pages of the functions for
details.
5. Other programming languages
The description above concentrates on the C programming language. It
should apply directly to C++ and Objective C. In assembler, you have
to call ioperm() or iopl() as in C, but after that you can use the I/O
port read/write instructions directly.
In other languages, unless you can insert inline assembler or C code
into the program or use the system calls mentioned above, it is
probably easiest to write a simple C source file with functions for
the I/O port accesses or delays that you need, and compile and link it
in with the rest of your program. Or use /dev/port as described above.
6. Some useful ports
Here is some programming information for common ports that can be
directly used for general-purpose TTL (or CMOS) logic I/O.
If you want to use these or other common ports for their intended
purpose (e.g., to control a normal printer or modem), you should most
likely use existing drivers (which are usually included in the kernel)
instead of programming the ports directly as this HOWTO describes.
This section is intended for those people who want to connect LCD
displays, stepper motors, or other custom electronics to a PC's
standard ports.
If you want to control a mass-market device like a scanner (that has
been on the market for a while), look for an existing Linux driver for
it. The Hardware-HOWTO
is a good
place to start.
is a good source for more
information on connecting devices to computers (and on electronics in
general).
6.1. The parallel port
The parallel port's base address (called ``BASE'' below) is 0x3bc for
/dev/lp0, 0x378 for /dev/lp1, and 0x278 for /dev/lp2. If you only want
to control something that acts like a normal printer, see the
Printing-HOWTO .
In addition to the standard output-only mode described below, there is
an `extended' bidirectional mode in most parallel ports. For
information on this and the newer ECP/EPP modes (and the IEEE 1284
standard in general), see and
. Remember that since
you cannot use IRQs or DMA in a user-mode program, you will probably
have to write a kernel driver to use ECP/EPP; I think someone is
writing such a driver, but I don't know the details.
The port BASE+0 (Data port) controls the data signals of the port (D0
to D7 for bits 0 to 7, respectively; states: 0 = low (0 V), 1 = high
(5 V)). A write to this port latches the data on the pins. A read
returns the data last written in standard or extended write mode, or
the data in the pins from another device in extended read mode.
The port BASE+1 (Status port) is read-only, and returns the state of
the following input signals:
· Bits 0 and 1 are reserved.
· Bit 2 IRQ status (not a pin, I don't know how this works)
· Bit 3 ERROR (1=high)
· Bit 4 SLCT (1=high)
· Bit 5 PE (1=high)
· Bit 6 ACK (1=high)
· Bit 7 -BUSY (0=high)
(I'm not sure about the high and low states.)
The port BASE+2 (Control port) is write-only (a read returns the data
last written), and controls the following status signals:
· Bit 0 -STROBE (0=high)
· Bit 1 AUTO_FD_XT (1=high)
· Bit 2 -INIT (0=high)
· Bit 3 SLCT_IN (1=high)
· Bit 4 enables the parallel port IRQ (which occurs on the low-to-
high transition of ACK) when set to 1.
· Bit 5 controls the extended mode direction (0 = write, 1 = read),
and is completely write-only (a read returns nothing useful for
this bit).
· Bits 6 and 7 are reserved.
(Again, I am not sure about the high and low states.)
Pinout (a 25-pin female D-shell connector on the port) (i=input,
o=output):
1io -STROBE, 2io D0, 3io D1, 4io D2, 5io D3, 6io D4, 7io D5, 8io D6,
9io D7, 10i ACK, 11i -BUSY, 12i PE, 13i SLCT, 14o AUTO_FD_XT,
15i ERROR, 16o -INIT, 17o SLCT_IN, 18-25 Ground
The IBM specifications say that pins 1, 14, 16, and 17 (the control
outputs) have open collector drivers pulled to 5 V through 4.7 kiloohm
resistors (sink 20 mA, source 0.55 mA, high-level output 5.0 V minus
pullup). The rest of the pins sink 24 mA, source 15 mA, and their
high-level output is min. 2.4 V. The low state for both is max. 0.5 V.
Non-IBM parallel ports probably deviate from this standard. For more
information on this, see
.
Finally, a warning: Be careful with grounding. I've broken several
parallel ports by connecting to them while the computer is turned on.
It might be a good thing to use a parallel port not integrated on the
motherboard for things like this. (You can usually get a second
parallel port for your machine with a cheap standard `multi-I/O' card;
just disable the ports that you don't need, and set the parallel port
I/O address on the card to a free address. You don't need to care
about the parallel port IRQ, since it isn't normally used.)
6.2. The game (joystick) port
The game port is located at port addresses 0x200-0x207. For
controlling normal joysticks, there is a kernel-level joystick driver,
see , filename
joystick-*.
Pinout (a 15-pin female D-shell connector on the port):
· 1,8,9,15: +5 V (power)
· 4,5,12: Ground
· 2,7,10,14: Digital inputs BA1, BA2, BB1, and BB2, respectively
· 3,6,11,13: ``Analog'' inputs AX, AY, BX, and BY, respectively
The +5 V pins seem to often be connected directly to the power lines
in the motherboard, so they should be able to source quite a lot of
power, depending on the motherboard, power supply and game port.
The digital inputs are used for the buttons of the two joysticks
(joystick A and joystick B, with two buttons each) that you can
connect to the port. They should be normal TTL-level inputs, and you
can read their status directly from the status port (see below). A
real joystick returns a low (0 V) status when the button is pressed
and a high (the 5 V from the power pins through an 1 Kohm resistor)
status otherwise.
The so-called analog inputs actually measure resistance. The game port
has a quad one-shot multivibrator (a 558 chip) connected to the four
inputs. In each input, there is a 2.2 Kohm resistor between the input
pin and the multivibrator output, and a 0.01 uF timing capacitor
between the multivibrator output and the ground. A real joystick has a
potentiometer for each axis (X and Y), wired between +5 V and the
appropriate input pin (AX or AY for joystick A, or BX or BY for
joystick B).
The multivibrator, when activated, sets its output lines high (5 V)
and waits for each timing capacitor to reach 3.3 V before lowering the
respective output line. Thus the high period duration of the
multivibrator is proportional to the resistance of the potentiometer
in the joystick (i.e., the position of the joystick in the appropriate
axis), as follows:
R = (t - 24.2) / 0.011,
where R is the resistance (ohms) of the potentiometer and t the high
period duration (seconds).
Thus, to read the analog inputs, you first activate the multivibrator
(with a port write; see below), then poll the state of the four axes
(with repeated port reads) until they drop from high to low state,
measuring their high period duration. This polling uses quite a lot of
CPU time, and on a non-realtime multitasking system like (normal user-
mode) Linux, the result is not very accurate because you cannot poll
the port constantly (unless you use a kernel-level driver and disable
interrupts while polling, but this wastes even more CPU time). If you
know that the signal is going to take a long time (tens of ms) to go
down, you can call usleep() before polling to give CPU time to other
processes.
The only I/O port you need to access is port 0x201 (the other ports
either behave identically or do nothing). Any write to this port (it
doesn't matter what you write) activates the multivibrator. A read
from this port returns the state of the input signals:
· Bit 0: AX (status (1=high) of the multivibrator output)
· Bit 1: AY (status (1=high) of the multivibrator output)
· Bit 2: BX (status (1=high) of the multivibrator output)
· Bit 3: BY (status (1=high) of the multivibrator output)
· Bit 4: BA1 (digital input, 1=high)
· Bit 5: BA2 (digital input, 1=high)
· Bit 6: BB1 (digital input, 1=high)
· Bit 7: BB2 (digital input, 1=high)
6.3. The serial port
If the device you're talking to supports something resembling RS-232,
you should be able to use the serial port to talk to it. The Linux
serial driver should be enough for almost all applications (you
shouldn't have to program the serial port directly, and you'd probably
have to write a kernel driver to do it); it is quite versatile, so
using non-standard bps rates and so on shouldn't be a problem.
See the termios(3) manual page, the serial driver source code
(linux/drivers/char/serial.c), and
for more information
on programming serial ports on Unix systems.
7. Hints
If you want good analog I/O, you can wire up ADC and/or DAC chips to
the parallel port (hint: for power, use the game port connector or a
spare disk drive power connector wired to outside the computer case,
unless you have a low-power device and can use the parallel port
itself for power, or use an external power supply), or buy an AD/DA
card (most of the older/slower ones are controlled by I/O ports). Or,
if you're satisfied with 1 or 2 channels, inaccuracy, and (probably)
bad zeroing, a cheap sound card supported by the Linux sound driver
should do (and it's quite fast).
With accurate analog devices, improper grounding may generate errors
in the analog inputs or outputs. If you experience something like
this, you could try electrically isolating your device from the
computer with optocouplers (on all signals between the computer and
your device). Try to get power for the optocouplers from the computer
(spare signals on the port may give enough power) to achieve better
isolation.
If you're looking for printed circuit board design software for Linux,
there is a free X11 application called Pcb that should do a nice job,
at least if you aren't doing anything very complex. It is included in
many Linux distributions, and available in
(filename pcb-*).
8. Troubleshooting
Q1.
I get segmentation faults when accessing ports.
A1.
Either your program does not have root privileges, or the
ioperm() call failed for some other reason. Check the return
value of ioperm(). Also, check that you're actually accessing
the ports that you enabled with ioperm() (see Q3). If you're
using the delaying macros (inb_p(), outb_p(), and so on),
remember to call ioperm() to get access to port 0x80 too.
Q2.
I can't find the in*(), out*() functions defined anywhere, and
gcc complains about undefined references.
A2.
You did not compile with optimisation turned on (-O), and thus
gcc could not resolve the macros in asm/io.h. Or you did not
#include at all.
Q3.
out*() doesn't do anything, or does something weird.
A3.
Check the order of the parameters; it should be outb(value,
port), not outportb(port, value) as is common in MS-DOS.
Q4.
I want to control a standard RS-232 device/parallel
printer/joystick...
A4.
You're probably better off using existing drivers (in the Linux
kernel or an X server or somewhere else) to do it. The drivers
are usually quite versatile, so even slightly non-standard
devices usually work with them. See the information on standard
ports above for pointers to documentation for them.
9. Example code
Here's a piece of simple example code for I/O port access:
______________________________________________________________________
/*
* example.c: very simple example of port I/O
*
* This code does nothing useful, just a port write, a pause,
* and a port read. Compile with `gcc -O2 -o example example.c',
* and run as root with `./example'.
*/
#include
#include
#include
#define BASEPORT 0x378 /* lp1 */
int main()
{
/* Get access to the ports */
if (ioperm(BASEPORT, 3, 1)) {perror("ioperm"); exit(1);}
/* Set the data signals (D0-7) of the port to all low (0) */
outb(0, BASEPORT);
/* Sleep for a while (100 ms) */
usleep(100000);
/* Read from the status port (BASE+1) and display the result */
printf("status: %d\n", inb(BASEPORT + 1));
/* We don't need the ports anymore */
if (ioperm(BASEPORT, 3, 0)) {perror("ioperm"); exit(1);}
exit(0);
}
/* end of example.c */
______________________________________________________________________
10. Credits
Too many people have contributed for me to list, but thanks a lot,
everyone. I have not replied to all the contributions that I've
received; sorry for that, and thanks again for the help.