---
name: "amdmsr"
section: "4"
platform: "openbsd"
package: "man"
version: "8.0-current-20261101"
release: "OpenBSD 8.0-current (2026-11-01)"
platforms: ["openbsd"]
url: "https://new.carta.tech/amdmsr"
---

# amdmsr(4)

access MSRs on certain AMD Geode LX systems

## NAME

**amdmsr** – access MSRs on certain AMD Geode LX systems

## SYNOPSIS

**amdmsr0 at mainbus?**

## DESCRIPTION

The **amdmsr** device provides access to the Model Specific Registers (MSRs) on systems with an AMD Geode LX CPU and graphics processor.

### IOCTL INTERFACE

MSRs can be read or written via **ioctl**() calls performed on */dev/amdmsr*. Declarations and data types are to be found in ⟨machine/amdmsr.h⟩.

Read or write requests are performed using:

```
struct amdmsr_req {
	u_int32_t addr;	/* 32-bit MSR address */
	u_int64_t val;	/* 64-bit MSR value */
};
```

The *RDMSR* ioctl is used to read an MSR register.

The *WRMSR* ioctl is used to write an MSR register.

## SEE ALSO

[ioctl(2)](/ioctl/2.md)

## HISTORY

**amdmsr** support was added in OpenBSD 4.4.