OpenOCD - Open On-Chip Debugger Reference Manual, Paperback

The Open On-Chip Debugger (OpenOCD) aims to provide debugging, in-system programming and boundary-scan testing for embedded target devices. It does so with the assistance of a debug adapter, which is a small hardware module which helps provide the right kind of electrical signaling to the target being debugged. These are required since the debug host (on which OpenOCD runs) won't usually have native support for such signaling, or the connector needed to hook up to the target.