Bitness

Execute an API or DLL call of the wrong bitness from within any of these programs and the result is an unrecoverable run-time error.

Thunking allows parameters to be pushed correctly on the stack, enables a DLL of a different bitness to load in your process, and converts memory addresses...

Since both user mode and kernel mode have the same "bitness," 16-bit and 32-bit stacks do not need to be distinguished.

Table 7.1 summarizes the bitness requirements for TSPs under the various versions of TAPI and Windows.