Practical - 1
- Write the working of 8085 simulator GNUsim8085 and basic architecture of 8085 along with small introduction.
Aim:- Write the working of 8085 simulator GNUsim8085 and basic architecture of 8085 along with small introduction.
8085 Architecture
We have tried to depict the architecture of 8085 with this following image −
8085 is pronounced as "eighty-eighty-five" microprocessor. It is an 8-bit
microprocessor designed by Intel in 1977 using NMOS technology.
It has the following configuration −
• 8-bit data bus
• 16-bit address bus, which can address upto 64KB
• A 16-bit program counter
• A 16-bit stack pointer
• Six 8-bit registers arranged in pairs: BC, DE, HL
• Requires +5V supply to operate at 3.2 MHZ single phase clock
It is used in washing machines, microwave ovens, mobile phones, etc.
The working of 8085 simulator GNUsim8085:-
8085 consists of the following functional units −
Accumulator
It is an 8-bit register used to perform arithmetic, logical, I/O & LOAD/STORE operations.
It is connected to internal data bus & ALU.
Arithmetic and logic unit
As the name suggests, it performs arithmetic and logical operations like Addition,
Subtraction, AND, OR, etc. on 8-bit data.
General purpose register
There are 6 general purpose registers in 8085 processor, i.e. B, C, D, E, H & L.
Each register can hold 8-bit data.
These registers can work in pair to hold 16-bit data and their pairing combination is like B-C, D-E & H-L.
Program counter
It is a 16-bit register used to store the memory address location of the next
instruction to be executed. Microprocessor increments the program whenever an
instruction is being executed, so that the program counter points to the memory
address of the next instruction that is going to be executed.
Stack pointer
It is also a 16-bit register works like stack, which is always incremented/decremented by 2 during push & pop operations.
Temporary register
It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.
Flag register
It is an 8-bit register having five 1-bit flip-flops, which holds either 0
or 1 depending upon the result stored in the accumulator.
These are the set of 5 flip-flops −
Sign (S)
Zero (Z)
Auxiliary Carry (AC)
Parity (P)
Carry (C)
Its bit position is shown in the following table − D7 D6 D5 D4 D3 D2 D1 D0
Instruction register and decoder
It is an 8-bit register. When an instruction is fetched from memory then it
is stored in the Instruction register. Instruction decoder decodes the information
present in the Instruction register.
Timing and control unit
It provides timing and control signal to the microprocessor to perform operations.
Following are the timing and control signals, which control external and internal circuits −
Control Signals: READY, RD’, WR’, ALE
Status Signals: S0, S1, IO/M’
DMA Signals: HOLD, HLDA
RESET Signals: RESET IN, RESET OUT
Interrupt control
As the name suggests it controls the interrupts during a process. When a microprocessor
is executing a main program and whenever an interrupt occurs, the microprocessor
shifts the control from the main program to process the incoming request.
After the request is completed, the control goes back to the main program.
There are 5 interrupt signals in 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.
Serial Input/output control
It controls the serial data communication by using these two instructions:
SID (Serial input data) and SOD (Serial output data).
Address buffer and address-data buffer
The content stored in the stack pointer and program counter is loaded into
the address buffer and address-data buffer to communicate with the CPU.
The memory and I/O chips are connected to these buses; the CPU can exchange
the desired data with the memory and I/O chips.
Address bus and data bus
Data bus carries the data to be stored. It is bidirectional, whereas address bus
carries the location to where it should be stored and it is unidirectional.
It is used to transfer the data & Address I/O devices.
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