1. Unit 1
8085 Microprocessor

2. Microprocessor (CPU)
Multipurpose programmable clock driven, Register based electronic device.
Programmable integrated device (silicon chip) that has computing & decision making capabilities
Communicates & operates in binary numbers 0 & 1, called bits
Has a fixed set of instructions in the form of binary patterns – machine language
Difficult for humans to remember machine language – each instruction is represented using abbreviated names (mnemonics)

3. Organization of A Microprocessor-based System (Computer)
CPU – Central Processing Unit
Memory
ROM – Read Only Memory
RAM – Random Access Memory
I/O
Keyboard
Display Device
Clock – Square Wave Oscillator (Timing)
System Bus

4. Memory Symbolic representation Word length 1 word instruction
Word: no. of bits micro-P recognizes and processes at a time ( bit ).
Instruction: combination of bit patterns with specific meaning known to micro-P.
Program: Set of all instructions.
address

5. I/O Microprocessor’s connection to the outside world
Input: Keyboard, mouse
Output: Monitor, printer

6. System Bus – wires connecting memory & I/O to microprocessor
Address Bus
Unidirectional
Identifying peripheral or memory location
Data Bus
Bidirectional
Transferring data
Control Bus
Synchronization signals
Timing signals
Control signal

7. Actions performed by microprocessor:
CPU – Memory
CPU – I/O
Data Processing
Arithmetic operations
Logical operations
Control
Jump
Interrupts

8. Basic Concepts of Microprocessors
Differences between:
Microcomputer – a computer with a microprocessor as its CPU. Includes memory, I/O etc.
Microprocessor – silicon chip which includes ALU, register circuits & control circuits
Microcontroller – silicon chip which includes microprocessor, memory & I/O in a single package.

10. Grouping of Signals of 8085 Microprocessor
Power supply and frequency signals
Address bus signals
Data bus signals
Control and status signals
Externally initiated signals & external signal acknowledgement
Serial I/O port signals

11. Address bus signals, Data bus signals
AD0-AD7, A8-A15
16 address lines – 2 sets
Most significant bits (A8-A15) – single directional
Least significant bits (AD0-AD7) – bidirectional
Multiplexed with the bits of bi-directional data bus
It is used as both address and data bus

12. Control and status signals
Status lines:
IO/M
Differentiate I/O and memory applications
High – I/O
Low – Memory
S1, S0 – status signals, to indicate the type of machine cycle in progress
Control lines:
RD, WR & INTA
RD – data on the data bus to be read into processor
WR – data on the data bus to be written to processor
INTA – acknowledge an INTR interrupt _ _ _ _ _

13. Externally initiated signals & signal acknowledgement
Reset In – reset CPU
Hold – suspend CPU operation
Ready – CPU go into wait state, to sync with slower devices
Signal acknowledgement
Reset out – high once CPU is rest
HLDA – acknowledges hold signal

15. Intel 8085 Microprocessor Microprocessor consists of:
Control unit: control microprocessor operations.
ALU: performs data processing function.
Registers: provide storage internal to CPU.
Interrupts
Internal data bus

16. Registers General Purpose Registers Special Purpose Registers
B, C, D, E, H & L (8 bit registers)
Can be used singly
Or can be used as 16 bit register pairs
BC, DE, HL
H & L can be used as a data pointer (holds memory address)
Special Purpose Registers
Accumulator (8 bit register)
Store 8 bit data
Store the result of an operation
Store 8 bit data during I/O transfer

17. Flag Register
8 bit register – shows the status of the microprocessor before/after an operation
S (sign flag), Z (zero flag), AC (auxillary carry flag), P (parity flag) & CY (carry flag)
Sign Flag
Used for indicating the sign of the data in the accumulator
The sign flag is set if negative (1 – negative)
The sign flag is reset if positive (0 – positive) D7 D6 D5 D4 D3 D2 D1 D0 S Z X AC P CY

18. Zero Flag Carry Flag Is set if result obtained after an operation is 0
Is set following an increment or decrement operation of that register
Carry Flag
Is set if there is a carry or borrow from arithmetic operation
Carry
Borrow

19. Auxillary Carry Flag Parity Flag
Is set if there is a carry out of bit 3
Parity Flag
Is set if parity is even
Is cleared if parity is odd

20. 16 – Bit Registers Program Counter Stack Pointer
A pointer to the next instruction to be executed
Contains the 16-bit memory address of the next instruction
Updated after processor has fetched the instruction
Stack Pointer
Stack – an area in memory in which temporary info is stored
Stack – FILO (First In Last Out) basis
Holds the address of the top of the stack

21. Non Programmable Registers
Instruction Register & Decoder
Instruction is stored in IR after fetched by processor
Decoder decodes instruction in IR
Internal Clock generator
3.125 MHz internally
6.25 MHz externally

22. Basic Working of a Microprocessor
Instructions are stored sequentially in memory
Microprocessor
Fetches instruction from memory
Decodes instruction
Executes instruction

23. Interrupts of 8085 Microprocessor
Maskable Interrupts
Microprocessor can ignore or delay interrupt request
INTR – General purpose interrupt
RST 5.5, RST 6.5, RST 7.5 – Restart interrupts, higher priorities
Nonmaskable Interrupts
Enabled by default
Cannot be disabled
Microprocessor must respond to it immediately
TRAP – highest priority