How Debuggers Work: Getting and Setting x86 Registers, Part 1

In this article, I would like to shortly describe the methods used to dump and restore the different kinds of registers on 32-bit and 64-bit x86 CPUs. The first part will focus on General Purpose Registers, Debug Registers and Floating-Point Registers up to the XMM registers provided by the SSE extension. I will explain how their values can be obtained via the ptrace(2) interface.

The ptrace(2) API is commonly used in all modern BSD systems and Linux, as all of them derive it from the original form designed and implemented in 4.3BSD. The primary focus in this article is on the FreeBSD and NetBSD systems. Nevertheless, the users of other Operating Systems such as OpenBSD, DragonFly BSD or Linux can still benefit from this article as the basic principles are the same and the code examples are intended to be easily adapted to other platforms.

A single CPU (in modern hardware: CPU core or CPU thread, if hyperthreading is available) can execute only one program thread at a time. In order to be able to run multiple processes and threads quasi-simultaneously, the Operating System must perform context switching — that is periodically suspend the currently running thread, save its state, restore the saved state of another thread and resume it. Saving and restoring the values of the processor’s registers play an important part in context switching. It is important that this process is fully transparent to the process being switched, and in a properly implemented kernel there should be no side effects that are perceptible to the program.

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