Service 00h or 10h (Read Char):

If there is no key in the keybuffer on entry, these services invoke Int 15h, Fcn 4102h with BH equal to the low byte of KBGET, BL=0, and ES:DI pointing at KBPUT. This has the effect of hanging inside the Int 15h, Fcn 4102h code until a key is present in the keybuffer. Eventually, if there is no keyboard activity, this service will Auto-Timeout. When a key is eventually fetched from the keybuffer, the Auto-Timeout counter, TimOutCnt is reset.