Functional
block diagram of the 8085 Microprocessor
Accumulator
It is a 8-bit register which is used to perform
arithmetical and logical operation. It stores the output of any operation. It
also works as registers for I/O accesses.
Temporary Register
It is a 8-bit register which is used to hold the
data on which the accumulator is computing operation. It is also called as
operand register because it provides operands to ALU.
Registers
These are general purposes registers.
Microprocessor consists 6 general purpose registers of 8-bit each named as
B,C,D,E,H and L. Generally theses registers are not used for storing the data
permanently. It carries the 8-bits data. These are used only during the
execution of the instructions.
These registers can also be used to carry the 16 bits data by making the pair of 2 registers. The valid register pairs available are BC,DE HL. We can not use other pairs except BC,DE and HL. These registers are programmed by user.
These registers can also be used to carry the 16 bits data by making the pair of 2 registers. The valid register pairs available are BC,DE HL. We can not use other pairs except BC,DE and HL. These registers are programmed by user.
ALU
ALU performs the arithmetic operations and logical
operation.
Flag Registers
It consists of 5 flip-flop which changes its
status according to the result stored in an accumulator. It is also known as
status registers. It is connected to the ALU.
There are five flip-flops in the flag register are as follows:
There are five flip-flops in the flag register are as follows:
1.
Sign(S)
2.
zero(z)
3.
Auxiliary carry(AC)
4.
Parity(P)
5.
Carry(C)
The bit position of the flip flop in flag register
is:
D7
|
D6
|
D5
|
D4
|
D3
|
D2
|
D1
|
D0
|
S
|
Z
|
AC
|
P
|
CY
|
All of the three flip flop set and reset according
to the stored result in the accumulator.
1. Sign-
If D7 of the result is 1 then sign flag is set
otherwise reset. As we know that a number on the D7 always decides the sign of
the number. If D7 is 1: the number is negative. If D7 is 0: the number is
positive.
2. Zeros (Z)-
If the result stored in an accumulator is zero
then this flip flop is set otherwise it is reset.
3. Auxiliary carry (AC)-
If any carry goes from D3 to D4 in the output then
it is set otherwise it is reset.
4. Parity (P)-
If the no of 1's is even in the output stored in
the accumulator then it is set otherwise it is reset for the odd.
5. Carry (C)-
If the result stored in an accumulator generates a
carry in its final output then it is set otherwise it is reset.
Instruction registers (IR)
It is a 8-bit register. When an instruction is
fetched from memory then it is stored in this register.
Instruction Decoder
Instruction decoder identifies the instructions.
It takes the information’s from instruction register and decodes the
instruction to be performed.
Program Counter
It is a 16 bit register used as memory pointer. It
stores the memory address of the next instruction to be executed. So we can say
that this register is used to sequencing the program. Generally the memory has
16 bit addresses so that it has 16 bit memory. The program counter is set to
0000H.
Stack Pointer
It is also a 16 bit register used as memory
pointer. It points to the memory location called stack. Generally stack is a
reserved portion of memory where information can be stores or taken back together.
Timing and Control Unit
It provides timing and control signal to the
microprocessor to perform the various operations. It has three control signals.
It controls all external and internal circuits. It operates with reference to
clock signal. It synchronizes all the data transfers.
There are three control signals:
There are three control signals:
1.
ALE-Arithmetic Latch Enable, It provides control signal to
synchronize the components of microprocessor.
2.
RD- This is active low used for reading operation.
3.
WR-This is active low used for writing operation.
There are three status signal used in
microprocessor S0, S1 and IO/M. It changes its status according the provided
input to these pins.
Serial Input Output Control-
There are two pins in this unit. This unit is used
for serial data communication.
Interrupt Unit
There are 6 interrupt pins in this unit. Generally
an external hardware is connected to these pins. These pins provide interrupt signal sent by external hardware
to microprocessor and microprocessor sends acknowledgement for receiving the interrupt signal. Generally
INTA is used for acknowledgement.
Register Section
Many registers has been used in microprocessor.
PIPO shift register it consists of PIPO (Parallel Input Parallel Output)
register.
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