The pins of a 8085 microprocessor can be classified into seven
groups.
1.Address bus
A15 – A8 at pin no. 28 to pin no. 21, it carries the most significant 8-
bits of memory and I/O address. The microprocessor can address
directly 65536 memory locations or 64k memory locations using 16-
address lines (A15-A0). Pin no. 28 to pin no. 21 give us the higher
order 8-bits of the address (A15-A8).
AD7–AD0 at pin no. 19 to pin no. 12 are used for dual purpose. It is
time multiplexed lower 8-bit address bus (A7-A0) and 8-bit data bus
(D7-D0).
2.Control and status signals
These signals are used to identify the nature of operation. There are 3
control signal and 3 status signals.
Three control signals are RD, WR & ALE.
• RD(Read control signal) − This signal indicates that the selected
I/O or memory device is to be read and is ready for accepting data
available on the data bus.
• WR(Write control signal) − This signal indicates that the data on
the data bus is to be written into a selected memory or I/O location.
• ALE(Address latch Enable) − It is a positive going pulse generated
when a new operation is started by the microprocessor. When the
pulse goes high, it indicates address, that is means ALE=1. When
the pulse goes down it indicates data, that is means ALE=0.
