3. ALU
The ALU performs the arithmetic and logical
operations. The operations performed by ALU
of 8085 are addition, subtraction, increment,
decrement, logical AND, OR, EXCLUSIVE -OR,
compare, complement and left / right shift .
4. Accumulator and
temporary register
• The accumulator and temporary register are
used to hold the data during an arithmetic /
logical operation. After an operation the
result is stored in the accumulator and the
flags are set or reset according to the result of
the operation.
5. TIMING & CONTROL UNIT
The timing and control unit synchronizes all the
microprocessor operations with the clock and
generates the control signals necessary for
communication between the microprocessor
and peripherals.
6. INSTRUCTION REGISTER &
DECODER
• When an instruction is fetched from memory it is
placed in instruction register. Then it is decoded
and encoded into various machine cycles.
7. STACK POINTER (SP)
• It is also a 16-bit register which is used as a
memory pointer.
• It maintains the address of the last byte that is
entered into stack.
8. PROGRAM COUNTER (PC)
• The program counter (PC) keeps track of
program execution.
• It is a special purpose register.
• A program counter stores the address of the
next instruction to be executed.
9. FLAG REGISTER
There are five flags in 8085, which are sign flag
(8), zero flag (Z), auxiliary carry flag (AC), parity
flag (P) and carry flag (CY). The bit positions
reserved for these flags in the flag register are
shown in figure below.
10. The Flags register
• There is also a flag register whose bits are affected by
the arithmetic & logic operations.
• S-sign flag
• The sign flag is set if bit D7 of the accumulator
is set after an arithmetic or logic operation.
• Z-zero flag
• Set if the result of the ALU operation is 0.
Otherwise is reset. This flag is affected by
operations on the accumulator as well as other
registers. (DCR B).
• AC-Auxiliary Carry
• This flag is set when a carry is generated from
bit D3 and passed to D4 . This flag is used only
internally for BCD operations.
11. .
• P-Parity flag
• After an ALU operation, if the result has an even #
of 1s, the p-flag is set. Otherwise it is cleared. So,
the flag can be used to indicate even parity.
• CY-carry flag
• This flag is set when a carry is generated from bit
D7 after an unsigned operation.
• OV-Overflow flag
• This flag is set when an overflow occurs after a
signed operation.
Unspecified
7 6 5 4 3 2 1 0SF ZF X AF X PF X CF
12. General Purpose
Registers
• 8085 consists of 6 special types of registers called
General Purpose Registers. These General
Purpose Registers are used to hold data like any
other registers .
• The GPR in 8085 processors are B,C,D,E,H,L.Each
registers can hold 8-bit data.
• They can work in pair such as B-C,D-E,H-L to store
16-bit data.
13. Temporary Register
This register acts as a temporary memory during
the arithmetic and logical operations.
Unlike other registers, this temporary registers
can only be accessed by the microprocessor and
it is completely inaccessible to programmers.
Temporary register is an 8-bit register.
14. PSW
• The combination of these 8-bits is called
Program Status Word(PSW).PSW and the
accumulator as a 16-bit unit for the stack
operation.
