The document discusses the Microprocessor 8085. It describes the architecture of the 8085, which is divided into registers, an arithmetic logic unit, an instruction decoder, address buffers, interrupt control, and timing/control circuitry. It details the registers of the 8085 including general purpose, temporary, special purpose, and 16-bit registers like the program counter and stack pointer. The document also examines the ALU, instruction decoder, addressing mechanisms, interrupt handling, serial I/O, and timing control circuitry of the 8085 microprocessor.

1. Microprocessor 8085
Dr. Nilesh Bhaskarrao Bahadure
https://www.sites.google.com/site/nileshbbahadure/home
July 25, 2021
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2. Overview
1 Basics of Microprocessor 8085
Architecture of 8085
Registers of 8085
Arithmetic Logic Unit of 8085
Instruction Decoder
Address buffer
Address/data buffer
Increment / decrement address latch
Interrupt control
Serial input / output control
Timing and control circuitry
Pin configuration of 8085
Control & Status signal
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3. Architecture of 8085
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4. Architecture of 8085
Architecture of 8085 divided in the following groups
Registers
Arithmetic and logic unit
Instruction decoder
Address buffer
Address / data buffer
Increment / decrement address latch
Interrupt control
Serial input / output control
Timing and control circuitry
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5. Registers of 8085
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6. Registers of 8085
Registers of 8085 can be classified as
General purpose registers
Temporary registers
1 Temporary data registers
2 W and Z register
Special purpose registers
1 Accumulator
2 Flag register
3 Instruction registers
16 Bit registers
1 Program counter
2 Stack pointer
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7. Special Purpose Registers of 8085 I
1 Accumulator
it is a 8 bit register. It is extensively used in arithmetic, logical, load
and store operations as well as input / output (I/O) operations.
Most of the times the result of arithmetic and logical operations is
stored in register A. hence it is also identified as accumulator.
2 Flag Register
It is an 8 - bit register, in which five of the bits carry significant
information in the form of flags: S (sign flag), Z (zero flag), AC
(Auxiliary carry flag), P (Parity flag) and CY (carry flag).
3 Instruction Register
in a typical processor operation, the processor first fetches the opcode
of instruction from memory. The CPU stores this opcode in a register
called an instruction register. The opcode is further sent to the
instruction decoder to select one of the 256 alternatives.
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8. Special Purpose Registers of 8085 II
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9. 16 - bit registers
1 Program Counter (PC):
Program is sequence of instructions. As mentioned earlier,
microprocessor fetches these instructions from the memory and
executes them sequentially. The program counter is a special purpose
register which, at a given time, stores the address of the next
instruction to be fetched. Program counter acts as a pointer to the
next instruction. How processor increments PC depends on the nature
of the instruction, for one byte instruction it increments PC by one, for
two byte instruction it increments PC by two and for three byte
instruction it increments PC by three such that PC always points to
the address of the next instruction.
2 Stack Pointer (SP):
the stack pointer is a reserved area of the memory in the RAM where
temporary information may be stored. A 16 bit stack pointer is used
to hold the address of the most recent stack entry.
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10. ALU of 8085
1 The 8085’s ALU performs arithmetic and logical operations on 8 bit
variables. The arithmetic unit performs bitwise fundamental
arithmetic operations such as addition and subtraction. The logic unit
performs logical operations such as complement, AND, OR and EX -
OR, as well as rotate and clear. The ALU also looks after the
branching decisions.
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11. Instruction Decoder
1 The processor first fetches the opcode of instruction from memory
and stores this opcode in the instruction register. It is then sent to
the instruction decoder. The instruction decoder decodes it and
accordingly gives the timing and control signals which controls the
register, the data buffers, ALU and external peripherals signals
depending on the nature of the instruction.
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12. Address Buffer
1 This is an 8 - bit unidirectional buffer. It is used to drive external high
order address bus (A15 - A8). It is also used to tri - state the high
order address bus under certain conditions such as reset, hold, halt
and when address lines are not in use.
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13. Address/data Buffer
1 This is an 8 - bit bidirectional buffer. It is used to drive multiplexed
address / data bus, i.e. low order addresses bus (A7 - A0) and data
bus (D7 - D0). It is also used to tri - state the multiplexed address /
data bus under certain conditions such as reset, hold, halt and when
the bus is not in use.
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14. Increment / decrement address latch
1 This 16 - bit register is used to increment or decrement the contents
of PC or stack pointer as a part of execution of instruction related to
them.
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15. Interrupt control
1 The processor fetches, decodes and executes instruction in a sequence.
Sometimes it is necessary to have processor the automatically
executes one of a collection of special routines whenever special
conditions exists within a program or the microcomputer system.
2 The most important thing is that, after execution of the special
routine, the program control must be transferred to the program
which processor was executing before the occurrence of the special
condition. The occurrence of the special condition is referred as
interrupt.
3 The interrupt control block has five interrupt inputs RST 5.5, RST
6.5, RST 7.5, TRAP and INTR and one acknowledge signal INTA.
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16. Serial input / output control
1 In situations like, data transmission over long distance and
communication with cassette tapes or a CRT terminal it is necessary
to transmit data bit by bit to reduce the cost of cabling. In serial
communication one bit is transferred at a time over a single line. The
8085’s serial I/O control provides two lines, SOD and SID for serial
communication. The serial output data (SOD) line is used to send
data serially and serial input data (SID) line is used to receive data
serially.
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17. Timing and control circuitry
1 The control circuitry in the processor 8085 is responsible for all
operations. The control circuitry and hence the operations in 8085
are synchronized with the help of clock signal. Along with the control
of fetching and decoding operations and generating appropriate
signals for the instruction executions control circuitry also generates
signals required to interface external devices to the processor 8085.
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18. Pin configuration of 8085
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19. Control & Status signal
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20. Thank you
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