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+.TH ARCH 3
+.SH NAME
+arch \- architecture-specific information and control
+.SH SYNOPSIS
+.nf
+.B bind -a #P /dev
+
+.B /dev/cputype
+.B /dev/ioalloc
+.B /dev/iob
+.B /dev/iol
+.B /dev/iow
+.B /dev/irqalloc
+.SH DESCRIPTION
+.PP
+This device presents textual information about PC hardware and allows
+user-level control of the I/O ports on x86-class and DEC Alpha machines.
+.PP
+Reads from
+.I cputype
+recover the processor type and clock rate.
+.PP
+Reads from
+.I ioalloc
+return I/O ranges used by each device, one line
+per range.  Each line contains three fields separated by white space: first address
+in hexadecimal,
+last address, name of device.
+.PP
+Reads from
+.I irqalloc
+return the enabled interrupts, one line per
+interrupt.  Each line contains three fields separated by white space:
+the trap number, the IRQ it is assigned to, and the name of
+the device using it.
+.PP
+Reads and writes to
+.IR iob ,
+.IR iow ,
+and
+.I iol
+cause 8-bit wide, 16-bit wide, and 32-bit wide requests to
+I/O ports.
+The port accessed is determined by the byte offset of the
+file descriptor.
+.SH EXAMPLE
+The following code reads from an x86 byte I/O port.
+.IP
+.EX
+uchar
+inportb(long port)
+{
+    uchar data;
+
+    if(iobfd == -1)
+        iobfd = open("#P/iob", ORDWR);
+
+    seek(iobfd, port, 0);
+    if(read(iobfd, &data, sizeof(data)) != sizeof(data))
+        sysfatal("inportb(0x%4.4x): %r\en", port);
+    return data;
+}
+.EE
+.SH SOURCE
+.B /sys/src/9/pc/devarch.c