Syscall API: thread-control-block

• mapped to MD register where possible for cheap userspace access:

  • i386: %gs, amd64: %fs, hppa: %cr27, powerpc: %r2, sparc/sparc64: %g7

  • not yet: alpha, arm

• <machine/tcb.h> provides macros to hide the differences

• thread library creates the TCB for the initial thread and calls TCB_SET() during library .init function