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| Internal architecture of 8085 |
The intel
8085 is complete 8-bit parallel processing unit. The main component of the 8085
A as shown in figure above are the array of registers. The arithmetic logic
units, encoder, decoder timing and control circuits. These are linked by an
internal data bus. The major components of the block diagram are described as
follows.
(1) Accumulator
The
ACC(register) is an 8-bit is an accessible to the program. Actually, the 8085
microprocessor is an accumulator based microprocessor between one of the
operation depends on accumulation itself. So it plays key role in the operation
of 8085 A.
When data
is read from input ports, it is first moved to the accumulator. When data is
output to the output ports, it must be 1st placed in the accumulator
almost all arithmetic c logic functions are performed on the accumulator.
(2) Temporary Register
It is an
8-bit register not accessible to the programmer while executing the instruction.
The 8085 A often places the data into temporary register for a period.
(3) Instruction
Register
The
instruction register receives the operation codes (op-code) of instruction from
internal data bus and passes it to the instruction decodes and machine cycle
encoder circuit decodes the op-codes. So that the HP knows which type of
operation is to be performed before executing it. Instruction register is not
accessible to the programmer.
(4) General
Purpose Register
These four
8-bit registers are accessible to programmer and hence can be used to store
data during a program executing. These can be used individual as 8-bit register
or in pairs BC and DE as 16-bit register. Their content may be incremented or
decremented and combined logically with the content of the accumulator.
(5) Register
H,L
Register H
and L are 8-bit register that can be used in the same manner of scratch pad
register. In addition the register H and L are utilized in indirect addressing
mode. In these mode the memory location whose address is specified by the
content of the register because a programmer can use them to load or transfer
data from the register by using instructions.
(6) Stack
pointer
A stack is
an area of memory set assigned for the purpose of storing data by an operation
known as stacking. As data is stored and retrieved, its location is specified
by a 16-bit register known as the stack pointer.
The
beginning of stack is defined by loading 16-bit address in the stack pointer
register.
(7) Program
Counter
A computer
program consists the sequence of coded instruction. These instructions are
stored sequentially in the memory. As the 8085 begins to execute the
instruction, the memory address of first instruction to be executed is placed
on the register called program counter. So the microprocessor uses this
register to sequence the execution of instruction.
Its
function is to point to the memory address from which the next byte of the
instruction is to be fetched depending upon the instruction type the program
counter is incremented by 1,2,3 to point to the next memory location.
(8) Flags
Register
consisting of five flip flops, each holding status of different stage
separately is known as flag register and each flip flops are called flags. In
the process of performing arithmetic and logic operation, the 8085 can set or
reset 1 or more of the flags. These flags are carry flag, syn flag, zero flag,
auxiliary flag and parity flag.
Normally,
the program is executed sequentially by obtaining instruction and data from
memory sequential. However, the microprocessor can be told to branch to a
specified address on the condition that anyone of the flag is set or reset.
The states
of the flag indicate the reset of arithmetic and logic operation which in term
can be used for decision making process. Different flags can be described
follows:
. Carry
flags: If the last operation generates a carry, the carry flag is set or high.
E.g.
1101
1
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S
|
Z
|
X
|
AC
|
X
|
P
|
X
|
CR
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