The 8085 architecture contains several functional blocks including registers, an ALU, instruction decoder, address buffer, and timing/control circuitry. It has general purpose registers, temporary registers, and special purpose registers like the program counter, stack pointer, and flags register. The flags register tracks the sign, zero, auxiliary carry, parity, and carry flags. The ALU performs arithmetic and logic operations on data in the accumulator and temporary register. The address and data buses use time-shared address/data lines to transfer instructions and data. Instructions are fetched from memory by the program counter and executed by decoding and performing the operation.

1. 8085 Architecture
Varun Sukheja
Student 6th semester
Department of Computer Science & Engineering
Lakshmi Narain College of Technology and Science

4. •Functional Blocks
• Registers
• ALU
• Instruction Decoder
• Address Buffer
• Address/Data Buffer
• Increment/ Decrement
Address Latch
• Interrupt Control
• Serial I/O Control
• Timing and control
circuitry
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5. •Registers
• General purpose
Registers
• Temporary Registers
• Special Purpose
Register
• 16 Bit Registers
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6. •The Flags register
• There is also the flags 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.
• 0- + Ve 1- -Ve
• 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.
• 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.
• CY-carry flag
• CY = carry is set when result generates a carry. Also a borrow flag.
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7. •PROGRAM COUNTER (PC) AND STACK
POINTER (SP)
• These are two 16-bit registers used to hold memory addresses.
• PC:
• The function of the PC is to point to the memory address from
which the next byte is to be fetched.
• When a byte (machine code) is being fetched, the program
counter is incremented by one to point to the next memory
location.
• SP:
• It points to a memory location in R/W memory, called the stack.
• The beginning of the stack is defined by loading a 16-bit address
in the stack pointer.
• The PC will automatically update when calling to /returning from
Subroutines.

8. •The ALU
• In addition to the arithmetic & logic circuits, the ALU includes
the accumulator, which is part of every arithmetic & logic
operation.
• Also, the ALU includes a temporary register used for holding
data temporarily during the execution of the operation. This
temporary register is not accessible by the programmer.
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9. •The Address and Data Busses
• The address bus has 8 signal lines A8 – A15 which are
unidirectional.
• The other 8 address bits are multiplexed (time shared) with the
8 data bits.
• So, the bits AD0 – AD7 are bi-directional and serve as A0 –
A7 and D0 – D7 at the same time.
• During the execution of the instruction, these lines carry
the address bits during the early part, then during the
late parts of the execution, they carry the 8 data bits.
• In order to separate the address from the data, we can use
a latch to save the value before the function of the bits
changes.
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10. •How Instructions are Executed
• All instructions (of a program) are stored in memory.
• To run a program, the individual instructions must be read
from the memory in sequence,and executed.
– Program counter puts the 16‐bit memory address of the
instruction on the address bus
– Control unit sends the Memory Read Enable signal to access
the memory
– The 8‐bit instruction stored in memory is placed on the data
bus and transferred to the instruction decoder
– Instruction is decoded and executed
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