dx              dx  xor    \ Clear high half of DX
      op: 6 [bp]      dx  mov    \ Task state selector in DX
      h# 12 [bp]      bx  mov    \ Global descriptor table address in BX
      dx              bx  add    \ Task state address in BX
      4 [bx]          dx  mov    \ Second word of task descriptor in DX
      h# 200 invert # dx  and    \ Clear task busy bit
      dx          4 [bx]  mov    \ Write back to descriptor
      h#  6 [bp]  ltr      \ Task register