Apple OS startup options

To bootup with diagnostics: 

To use any of these key combinations, press and hold the keys immediately after pressing the power button to turn on your Mac, or immediately after your Mac begins to restart. Keep holding until the described behavior occurs.
If your Mac is using a firmware password, all of these key combinations are disabled, except as noted below.

Hold during startup	Description
Shift (⇧)	Start up in safe mode.
Option (⌥)	Start up to Startup Manager, which allows you to choose other startup disks, if available. If your Mac is using a firmware password, you're asked to enter the password first.
C	Start up from an available CD, DVD, or USB thumb drive that contains a valid operating system for your Mac. Or use Startup Manager, as described above.
D	Start up from the built-in Apple Hardware Test or Apple Diagnostics utility, depending on your Mac model. Or use Option-D to start up from this utility over the Internet.
N	Start up from a compatible NetBoot server, if available. To use the default boot image on the NetBoot server, hold down Option-N instead.
Command-R	Start up from the built-in macOS Recovery system. Or use Option-Command-R or Shift-Option-Command-R to start up from macOS Recovery over the Internet. macOS Recovery installs different versions of macOS, depending on the key combination you use while starting up.
Option-Command (⌘)-P-R	Reset NVRAM (or PRAM). If your Mac is using a firmware password, this combination causes your Mac to start up from macOS Recovery instead. Turn off the firmware password from macOS Recovery, then restart and reset NVRAM.
Command-S	Start up in single-user mode.
T	Start up in target disk mode.
X	Start up from your macOS startup disk when your Mac would otherwise start up from a non-macOS startup disk, such as a Windows partition. Or use Startup Manager, as described above.
Command-V	Start up in verbose mode.
Eject (⏏), F12, mouse button, or trackpad button	Eject removable media, such as an optical disc.

These combinations work with Intel-based Mac computers. Other Mac keyboard shortcuts are available after your Mac has started up.


Apple website

microphone shutdown - Skype

Have Skype running on Windows and microphone keeps shutting off after 10 mintues:


the fix:
Control panel -> Hardware and Sound -> power options -> change plan settings -> advanced power settings -> USB settings -> USB selective suspend settings: DISABLED


from Microsoft