* extend(0x0000001200000004, 0x00000006) == 0x0000001200000006
* extend(0x0000001200000004, 0x00000002) == 0x0000001200000002
* extend(0x0000001200000004, 0xfffffffe) == 0x00000011fffffffe ! tsfhigh--
* extend(0x0000001200000004, 0x00000006) == 0x0000001200000006
* extend(0x0000001200000004, 0x00000002) == 0x0000001200000002
* extend(0x0000001200000004, 0xfffffffe) == 0x00000011fffffffe ! tsfhigh--
* extend(0x00000012fffffffc, 0xfffffffe) == 0x00000012fffffffe
* extend(0x00000012fffffffc, 0xfffffffa) == 0x00000012fffffffa
* extend(0x00000012fffffffc, 0x00000002) == 0x0000001300000002 ! tsfhigh++
* extend(0x00000012fffffffc, 0xfffffffe) == 0x00000012fffffffe
* extend(0x00000012fffffffc, 0xfffffffa) == 0x00000012fffffffa
* extend(0x00000012fffffffc, 0x00000002) == 0x0000001300000002 ! tsfhigh++