context

By providing an abstraction of the current execution state in the current thread, including:

• the stack (with local variables)
• stack pointer
• all registers
• CPU flags
• the instruction pointer

a execution context represents a specific point in the application's execution path.

A context switch between threads requires system calls (involving the OS kernel),
which can cost more than thousand CPU cycles on x86 CPUs.
By contrast, transferring control vias callcc()/continuation requires
only few CPU cycles because it does not involve system calls as it is done within a single thread.