Microprocessor architecture

 Buses
 Registers
 ALU
 Flags
 Program Counter
 Stack Pointer
 Instruction registers and decoders
Timing and control circuitry

Busses
 Data Bus
 Address Bus
 Control Bus

Data Bus
8 bits wide (D0 D1…D7).So Bus Width is 8
“Bi-directional”.
Information flows both ways between the
microprocessor and memory or I/O.
The 8085 uses the data bus to transfer the
binary information.Word length is 8.
Word length = Bus Width
Control Bus
It must for Proper Operation
I/O single Control lines
Synchronize the operation between
microprocessor and external circuitry

Address Bus
16 bits wide (A0 A1…A15)
Therefore, the 8085 can access locations with numbers from
0 to 65,536. Or, the 8085 can access a total of 64K addresses.
“Unidirectional”.
Information flows out of the microprocessor and into the
memory or peripherals.
Divide into 2 part: A15 – A8 (upper) and
AD7 – AD0 (lower).
A15 – A8 : Unidirectional, known as ‘high order address’.
AD7 – AD0 : bidirectional and dual purpose (address and data
placed once at a time).
AD7 – AD0 also known as ‘low order address’.
To execute an instruction, at early stage AD7 – AD0 uses as
address bus and alternately as data bus for the next cycle.
The method to change from address bus to data bus known as
‘bus multiplexing’.

Registers
 Six 8 bit General
purpose Registers to
store the data
 16 Bit Registers pair
BC,DE,HL
H & L is used as data
pointer

ALU
 Heart of microprocessor
 arithmetic & logic operation. Such as ADD,OR etc.
Accumulator
8-bit register that is part of the ALU Used to store 8-bit data and
in performing 8-bit arithmetic and logical operations, and in
storing the results operations or they can be transferred to the
internal data bus for use elsewhere

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.
P-Parity flag
After an ALU operation if the result has an
even no of 1’s the p-flag is set. Otherwise it
is cleared. So, the flag can be used to
indicate even parity.

PROGRAM COUNTER (PC)
STACK POINTER (SP)

 8085 MPU has 3 pins that control or present the clock
signal.
 X1 and X2 pins determine the clock frequency.
 CLK OUT is a TTL square-wave output clock.
 The CLOCK OUT is one-half the crystal
frequency.
8085 Pinout

 8085 μp consists of 16 signal pins use as address
bus.
 Divide into 2 part: A15 – A8 (upper) and
AD7 – AD0 (lower).
 A15 – A8 : Unidirectional, known as ‘high order
address’.
 AD7 – AD0 : bidirectional and dual purpose
(address and data placed once at a time).
 AD7 – AD0 also known as ‘low order address’.
 To execute an instruction, at early stage AD7 –
AD0 uses as address bus and alternately as data
bus for the next cycle.
 The method to change from address bus to data
bus known as ‘bus multiplexing’.

Direct Memory Access (DMA)
 DMA is an IO technique where external IO device
requests the use of the MPU buses.
 Allows external IO devices to gain high speed access to
the memory.
 Example of IO devices that use DMA: disk memory system.
 HOLD and HLDA are used for DMA.
 If HOLD=1, 8085 will place it address, data and control
pins at their high-impedance.
 A DMA acknowledgement is signaled by HLDA=1.

20
MPU Communication and Bus Timing
Figure 3: Moving data form memory to MPU using instruction MOV C, A
(code machine 4FH = 0100 1111)

21
 The Fetch Execute Sequence :
1. The μp placed a 16 bit memory address from PC
(program counter) to address bus.
– Figure 4: at T1
– The high order address, 20H, is placed at A15 – A8.
– the low order address, 05H, is placed at AD7 - AD0 and
ALE is active high.
– Synchronously the IO/M is in active low condition to show it
is a memory operation.
2. At T2 the active low control signal, RD, is activated so as
to activate read operation; it is to indicate that the MPU
is in fetch mode operation.
MPU Communication and Bus
Timing

22
Figure 4: 8085 timing diagram for Opcode fetch cycle for MOV C, A .
MPU Communication and Bus Timing

Microprocessor architecture

More Related Content

What's hot (19)

Similar to Microprocessor architecture (20)

Recently uploaded (20)

Microprocessor architecture