High Bit of State Register

On day 11 Sep 97 Vladimir Yudin had the good fortune to write to All:

VY> Here people sent me a lot of letters saying that my emulator is slow
VY> working with disks. So, today, unexpectedly for myself, I decided this question
VY>! :) Here's what's not clear - in the controller's status register, the most significant
VY> bit is responsible for the readiness of the disk drive (source book by Larchenko and
VY> Rodionov), I always had it on, today I started resetting it at the
VY> end of the command execution and everything crashed, so what is this
VY> bit responsible for?

It really is responsible for the readiness...
=== Cut ===
b7 Not ready This bit when set indicates the drive is not ready. When
reset it indicates the drive is ready. This bit is an
inverted copy of the Ready input and logically ORed with MR.
=== Cut ===

Andrew