## Physical Device Driver Reference

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### 6h, Ah - Remarks

The physical device driver must perform the following actions:

Perform the requested function
Set the busy bit
Set the status WORD in the request header

For output on character devices, if the busy bit is returned set to 1, a subsequent Write request to the physical device driver has to wait for the completion of a currently active request. If the busy bit is set to 0, there is no current request. Therefore, a Write request would start immediately.

For input on character devices with a buffer, if the busy bit is returned set to 1, there are no characters currently buffered in the physical device driver. If the busy bit is set to 0, there is at least one character in the physical device driver buffer. The effect of busy bit=0 is that a read of one character does not need blocking. Devices that do not have an input buffer in the physical device driver should always return busy=0.