Microprocessor 8085 basics
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The
microprocessor 8085 is a complex IC of
sequential circuits. The sequential circuits are designed to do some operation
depending on what is the input on their lines. The vital inputs on the lines are
what therefore determines what operation will be done by the sequential circuits
inside it. The operations can be very complex and therefore this chip is also
called a processor. We can
make the processor execute a programme to do useful things for us. These inputs
then could alternately, be called instructions. The inputs that we talked of so
far are actually voltages to represent 1s and 0s. They can only be 1s or 0s,
they are called digital values. The number of input lines that the processor
provides are some estimation of the largeness of the instruction set that it can
support. The microprocessor 8085 has a 8 bit input (actually called the input lines of the data
bus), meaning that we could have only a maximum of 2^8=256 different combination
of 1s and 0s as inputs to the processor. Well! The 8085 supports almost that
many different instructions. The programme that we desire to be executed must be
loaded into consecutive locations of memory chips. The memory is not part of the
8085 processor. The memory chips are again sequential circuits consisting of
flip flops which are capable of storing digital values. Since we would be
interested in storing a huge number of such digital values, a large number of
these memories are packed together with a scheme of addresses, so that we can
address them individually. Generally the memories are arranged in large numbers
of 8 bit bunches each. The addressing is also done by the voltages (1s or 0s) on
the address bus. The 8085 has a address bus which is 16 bit wide. Therefore it
can put 2^16 different digital values on it, and therefore it can address a
maximum of 2^16 different address locations. And then we ask the
processor to execute those instructions from a particular memory location
onwards. It goes on executing those instructions one after another. And that is
all it does. The memory location from which it is to pick up the next
instruction for execution is maintained in an internal memory location
(Register) called Programme Counter (PC). As would be expected, it has to hold a
16 bit memory address.
As soon as the power is turned
on, the 8085 does a reset of its programme counter. It is reset to 0000H ,on
start up. The address bus
is connected in a parallel fashion to the entire memory. When the voltages on
the address bus are indicating at 0000H, only that memory location is activated.
And whatever is the content of that memory location is now floated on the data
bus. The data bus feeds that value back to the processor. This is also called a
opcode fetch cycle (an instruction fetch cycle). The kit executes that
instruction and waits for the next instruction.
