Microprocessor
The output data lines of microprocessor and memories are usually tristated because

 It increases the speed of data transfer over the data bus
 More than one device can transmit information over the data bus by enabling only one device at a time
 The data line can be multiplexed for both input and output
 More than one device can transmit over the data bus at the same time

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Microprocessor
HLDA signal in 8085 performs the following operation:

 Indicates that the CPU has received the HOLD request and that it will relinquish the bus in the next clock cycle.
 Indicates that another master is requesting the use of the address and data buses.
 Hold the data in the accumulator until the microprocessor is turned OFF.
 Indicates that the CPU has not received the HOLD request.

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