arch – architecture–specific information and control

bind –a #P /dev

This device presents textual information about PC hardware and allows user–level control of the I/O ports on x86–class machines.

Reads from cputype recover the processor type and clock rate in MHz. Reads from archctl yield at least data of this form:
cpu AMD64 2201 pge
pge on
coherence mfence
cmpswap cmpswap486
i8253set on
cache default uc
cache 0x0 1073741824 wb
cache 0x3ff00000 1048576 uc

Where AMD64 is the processor type, 2201 is the processor speed in MHz, and pge is present only if the `page global extension' capability is present; the next line reflects its setting. coherence is followed by one of mb386, mb586, mfence or nop, showing the form of memory barrier used by the kernel. cmpswap is followed by cmpswap386 or cmpswap486, reflecting the form of `compare and swap' used by the kernel. i8253set is a flag, indicating the need to explicitly set the Intel 8253 or equivalent timer. There may be lines starting with cache that reflect the state of memory caching via MTRRs (memory– type region registers). The second word on the line is default or a C–style number which is the base physical address of the region; the third is a C–style length of the region; and the fourth is one of uc (for uncachable), wb (write–back), wc (write–combining), wp (write–protected), or wt (write–through). A region may be a subset of another region, and the smaller region takes precedence. This may be used to make I/O registers uncachable in the midst of a write–combining region mostly used for a video framebuffer, for example. Control messages may be written to archctl and use the same syntax as the data read from archctl. Known commands include cache, coherence, i8253set, and pge.

Reads from 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.

Reads from 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.

Reads and writes to iob, iow, and 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.

The following code reads from an x86 byte I/O port.
inportb(unsigned 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%4.4ux): %r", port);
return data;

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