AMR (Audio Modem Riser), CNR (Communications and Network Riser) and ACR (Advanced Communications Riser) are slots that you can find on your motherboard that have the same goal: to install HSP (Host Signal Processing) devices to your PC. These devices can be modems, sound cards and network cards.

HSP devices lack self-control, it is the computer CPU that controls the device. As a result, the device is fairly cheap, because it doesn't have any fancy circuitry. But, on the other hand, the PC loses a little bit of its performance, since the processor will waste a little bit of its time controlling the device. This kind of device is also commonly refered as "software-controlled" (e.g., "soft modems").

Usually AMR, CNR and ACR devices are offered by the motherboard manufacturer as an option to the motherboard. For example, your motherboard can have a AMR modem as an option. You can also buy such devices on the market. An AMR modem costs USD 15 or less, but they are not so easy to find.

You will only find one of these slots at a time, i.e., if a motherboard has AMR slot, you won't find a CNR nor an ACR slot on it.

Let's take a look at the physical differences between these slots.