---
name: "cm"
section: "4"
platform: "linux"
package: "freebsd-manpages"
version: "6.0-5"
release: "Debian 4.0 (Etch)"
platforms: ["linux"]
url: "https://new.carta.tech/cm/4"
---

# cm(4)

SMC Arcnet Ethernet device driver

## NAME

**cm** – SMC Arcnet Ethernet device driver

## SYNOPSIS

To compile support for the

**cm** driver into your kernel, place the following lines in your kernel configuration file:

**device isa** **device cm**

Alternatively, to load the [cm(4)](/cm/4.md) driver at boot time, place the following line in [loader.conf(5)](/loader.conf/5.md):

```
if_cm_load="YES"
```

## DESCRIPTION

The **cm** driver provides support for the Arcnet ISA network adapters.

This driver also has quirks preset in the [device.hints(5)](/device.hints/5.md) file for card detection.

## HARDWARE

The **cm** driver supports the following card models:

- SMC90c26

- SMC90c56

- SMC90c66 in '56 compatibility mode.

## IMPLEMENTATION NOTES

When the *NOARP* flag is set on the **cm** interface, it does not employ the address resolution protocol described in [arp(4)](/arp/4.md) to dynamically map between Internet and Ethernet addresses on the local network. Instead it uses the least significant 8 bits of the IP address as the hardware address like described in RFC 1051 and RFC 1201.

With the IFF\_LINK0 flag cleared IP/ARP/RARP encoding is done according to RFC 1201 that is, with Packet Header Definition Standard header and packet type 212 / 213. The MTU is normally 1500. The IFF\_LINK0 flag is cleared by default.

With the IFF\_LINK0 flag set, IP and ARP encoding is done according to the deprecated RFC 1051 encoding, that is with simple header, packet type 240 / 241, and the MTU is 507.

When switching between the two modes, use

```
ifconfig interfacename down up
```

to switch the MTU.

## DIAGNOSTICS

The following driver specific error messages may be reported:

- reset: card reset, link addr = 0x%02x (cm%d)

  The card is being reset and a new link address assigned.

- srint: restarted rx on buf cm%d

  The rx buffer has been emptied and will be reset.

## SEE ALSO

[netintro(4)](/netintro/4.md), [watchdog(4)](/watchdog/4.md), [device.hints(5)](/device.hints/5.md), [ifconfig(8)](/ifconfig/8.md), [watchdog(8)](/watchdog/8.md)

## HISTORY

The **cm** device was ported from NetBSD by Max Khon ⟨fjoe@FreeBSD.org⟩ and first appeared in FreeBSD 4.6. This manual page first appeared in FreeBSD 5.3.

## AUTHORS

This manual page was written by Tom Rhodes ⟨trhodes@FreeBSD.org⟩. The IMPLEMENTATION NOTES section was submitted by Max Khon ⟨fjoe@FreeBSD.org⟩ and originated from NetBSD.

## BUGS

The **cm** driver code could do with a bit of improvement, it would be nice if some one could come along and take care of this.

The IMPLEMENTATION NOTES section is specific to all Arcnet devices (see *sys/net/if\_arcsubr.c*) and should be moved to a more generic location.