The Thread Swappable Data (TSD) control block contains per-thread status and control information that resides in swappable memory and therefore is not required for reference out of context of the related thread. The resident memory counterpart to the TSD is the TCB (Thread Control Block).

The vast majority of the TSD is used as the ring 0 stack when a thread makes a privilege level transition to ring 0 via a call gate descriptor. The base of the ring 0 stack will therefore include the ring 3 call gate stack frame on entry to ring 0 (which is usually kernel or device driver code).

In the debug kernel a dummy page prefixes the used part of the TSD in order to catch ring 0 stack faults.

Other information contained in the TSD includes GTD instance data for the corresponding thread's context. This comprises descriptors for:

28: The LDT descriptor.

30: Base selector for ring 0 process instance data, which includes the ring 0 stack, TCBs and PTDA.

38: Floating point emulator instance data

40: FS mapping to the TIB

When an an inter-process thread context switches, descriptors 30 - 40 are loaded into the GDT from the TSD. When an intra-process thread context switches, descriptors 28 - 40 are loaded into the GDT from the TSD.

Refer to the following for related information

P Kernel Debugger and Dump Formatter command.
TSD control block format

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