---
name: "ntb"
section: "4"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["freebsd"]
url: "https://new.carta.tech/ntb"
---

# ntb(4)

Non-Transparent Bridge subsystem

## NAME

**ntb** – Non-Transparent Bridge subsystem

## SYNOPSIS

**device** **ntb**

In [loader.conf(5)](/loader.conf/5.md): **hw.ntb.debug\_level** **hint.ntb\_hw.%d.config** **ntb\_load="YES"**

## DESCRIPTION

Non-Transparent Bridges connect two computer systems with PCIe link(s), providing each of them limited access to others memory space, scratchpad registers and interrupts. The **ntb** subsystem uses those resources provided in generic way by hardware drivers and splits them between several functions, according to specified configuration.

## LOADER TUNABLES

The following tunables are settable from the [loader(8)](/loader/8.md):

- *hw.ntb.debug\_level*

  Driver debug level. The default value is 0, higher means more verbose.

- *hint.ntb\_hw.**%d**.config*

  Configures a set of NTB functions, separated by commas, and their resource allocation. Each function can be configured as: "\[\<name\>\]\[:\<mw\>\[:\<spad\>\[:\<db\>\]\]\]", where: *name* is a name of the driver to attach (empty means any), *mw* is a number of memory windows to allocate (empty means all available), *spad* is a number of scratchpad registers to allocate (empty means all available), *db* is a number of doorbells to allocate (empty means all available). The default configuration is empty string, which means single function with all available resources, allowing any driver to attach.

## SEE ALSO

[if\_ntb(4)](/if_ntb/4.md), [ntb\_hw\_amd(4)](/ntb_hw_amd/4.md), [ntb\_hw\_intel(4)](/ntb_hw_intel/4.md), [ntb\_hw\_plx(4)](/ntb_hw_plx/4.md), [ntb\_transport(4)](/ntb_transport/4.md)

## AUTHORS

The **ntb** subsystem was developed by Intel and originally written by Carl Delsey ⟨*carl@FreeBSD.org*⟩. Later improvements were done by Conrad E. Meyer ⟨*cem@FreeBSD.org*⟩ and Alexander Motin ⟨*mav@FreeBSD.org*⟩.