Microprocessor notes 4th sem, Aktu

What is a microprocessor?

A microprocessor is the most important unit within a computer system and is responsible for processing the unique set of instructions and processes.
A microprocessor is designed to execute logical and computational tasks with typical operations such as addition/subtraction, interprocess and device communication, input/output management, etc.
A microprocessor is composed of integrated circuits that hold thousands of transistors; exactly how many depends on its relative computing power.

Evolution of Microprocessor

Microprocessors were categorized into five generations: 
First, second, third, fourth, and fifth generations. 

Microprocessor characteristics are as follows:

1. First Generation: The microprocessors that were introduced in 1971 to 1972 were referred to as the first generation systems. First-generation microprocessors processed their instructions serially—they fetched the instruction, decoded it, then executed it. When an instruction was completed, the microprocessor updated the instruction pointer and fetched the next instruction, performing this sequential drill for each instruction in turn.
2. Second Generation: By the late 1970s, enough transistors were available on the IC to usher in the second generation of microprocessor sophistication: 16-bit arithmetic and pipelined instruction processing.

Motorola’s MC68000 microprocessor, introduced in 1979, is an example. Another example is Intel’s 8080. This generation is defined by overlapped fetch, decode, and execute steps (Computer 1996). As the first instruction is processed in the execution unit, the second instruction is decoded and the third instruction is fetched.

The distinction between the first and second-generation devices was primarily the use of newer semiconductor technology to fabricate the chips. This new technology resulted in a five-fold increase in instruction, execution, speed, and higher chip densities.

3. Third Generation: The third generation, introduced in 1978, was represented by Intel’s 8086 and the Zilog Z8000, which were 16-bit processors with minicomputer-like performance. The third generation came about as IC transistor counts approached 250,000.

Motorola’s MC68020, for example, incorporated an on-chip cache for the first time and the depth of the pipeline increased to five or more stages.

This generation of microprocessors was different from the previous ones in that all major workstation manufacturers began developing their own RISC-based microprocessor architectures (Computer, 1996).

4. Fourth Generation: As the workstation companies converted from commercial microprocessors to in-house designs, microprocessors entered their fourth generation with designs surpassing a million transistors.
Leading-edge microprocessors such as Intel’s 80960CA and Motorola’s 88100 could issue and retire more than one instruction per clock cycle.
5. Fifth Generation: Microprocessors in their fifth generation, employed decoupled superscalar processing, and their design soon surpassed 10 million transistors. In this generation, PCs are a low-margin, high-volume-business dominated by a single microprocessor.

Operations of Microprocessor:

The main sample operations and instruction sets of a microprocessor include:
ARITHMETIC LOGIC UNIT: Do addition, subtraction, multiplication, and division) and logic operations (AND, OR, and XOR) on data stored in its registers
INPUT AND OUTPUT: accept input from devices, such as a mouse, keyboard, or scanner
MEMORY: store and access binary instructions in memory, page file
CONTROL UNIT: fetch execute routine using ALU & memory registers
INFORMATION EXCHANGE: communicate between peripherals using a system bus.

Addressing Modes of 8085 Microprocessor

1. Register Addressing Mode
In this mode, the operands are registers of the microprocessor. The operation is performed within different registers of the microprocessor. Examples of register addressing modes are given below.
ADD L
MOV C, D
SUB L
2. Immediate Addressing Mode
In this mode, the 8 bit or 16-bit data is specified in the instruction itself as one of the operands. Examples of immediate addressing modes are given below.
ADI 05H
MVI A, 33H
LXI H, 2050H
3. Direct Addressing Mode
In this addressing mode, one of the operands is data stored in the memory. The memory address of the data is directly given in the instruction itself. Examples of direct addressing are given below.
STA 3050H
OUT 02H
LDA 2345H
4. Indirect Addressing Mode
In this mode also one of the operands is the data stored in the memory. The memory address of the data is specified by the register pair.
STAX B   //memory address is specified by BC register pair
LDAX D   //memory address is specified by DE register pair
MOV M, C   //memory address is specified by HL register pair
This was all about addressing the modes of the 8085 microprocessor. If you found anything missing or incorrect in about article then please mention it by commenting below.

Interrupts on Microprocessor:

An interrupt is a mechanism by which an I/O or instruction can suspend the normal execution of the processor and get itself serviced. Generally, a particular task is assigned to that interrupt signal. In the microprocessor-based system, the interrupts are used for data transfer between the peripheral devices and the microprocessor.

Interrupt Service Routine (ISR)

A small program or a routine that when executed services the corresponding interrupting source is called as an ISR.

Maskable Interrupt

An interrupt that can be disabled by writing some instruction is known as Maskable Interrupt otherwise it is called Non-Maskable Interrupt.

Execution of Interrupt

When there is an interrupt requests to the Microprocessor then after accepting the interrupts Microprocessor sends the INTA (active low) signal to the peripheral. The vectored address of a particular interrupt is stored in the program counter. The processor executes an interrupt service routine (ISR) addressed in the program counter.


There are two types of interrupts used in the 8085 Microprocessor:

  1. Hardware Interrupts
  2. Software Interrupts

Data Transfer Scheme of 8085 Microprocessor

We know that data transfer may take place between microprocessor and memory, microprocessor and I/O devices and memory & I/O devices. As we know not much of the problems arise for the data communication between microprocessor and memory as same technology is used in the manufacturing of memory and microprocessor.  
Data Transfer
1. Parallel data transfer    2. Serial data transfer

Parallel data transfer scheme

Parallel data transfer scheme is faster than serial I/O transfer. in parallel data transfer 8-bit data send all together with 8 parallel wire. In 8085 microprocessor mainly three types of parallel data transfer scheme we observed. Those are

Serial I/O mode transfer

For these devices and for these reasons serial I/O mode is used. In serial I/O mode transfer a single bit of data on a single line at a time. For serial I/O data transmission mode, 8-bit parallel word is converted to a stream of eight serial bit using a parallel-to-serial converter. Similarly, in serial reception of data, the microprocessor receives a stream of 8-bit one by one which are then converted to 8- bit parallel word using serial-to-parallel converter. For this purpose data transfer schemes of 8085 microprocessor are introduced.

Instruction and Data Flow

An instruction is a binary pattern designed to perform a specific function. The list of entire instructions is called the instruction set. The instruction set determines what function the microprocessor can perform.

The following notations are used in the description of the instructions:
R = 8085 8-bit registers (B, C, D, E, H, L)
M = memory register (location) pointed by value HL
Rs = register source
Rd = register destination (B, C, D, E, H, L)
Rp = register pair (BC, DE, HL)
( ) = contents of
The 8085 instruction set can be classified into the following five categories: 
1. Data Transfer (copy) Instructions
These instructions perform the following six operations:
  • Load 8-bit number in a register.
  • Load 16-bit number in a register pair.
  • Copy from register to register.
  • Copy between register and memory.
  • Copy between I/O and accumulator.
  • Copy between registers and stack memory.
MVI R, 8-bit
MOV Rd, Rs
LXI Rp, 16-bit 
OUT 8-bit
IN 8-bit
LDA 16-bit
STA 16-bit
LDAX Rp
STAX Rp
MOV R, M
MOV M, R
 
2. Arithmetic Instructions
The frequently used arithmetic operations are: Add, Subtract, Increment (add 1), Decrement (subtract 1) ADD R
ADI 8-bit
ADD M
SUB R
SUI 8-bit
SUM M
INR R
INR M
DCR R
DCR M
INX Rp
DCX Rp
3. Logical and Bit Manipulation Instructions
These instructions include the following operations: AND, OR, X-OR, Compare, Rotate bits
ANA R
ANI 8-bit
ANA M
ORA R
ORI 8-bit
ORA M
XRA R
XRI 8-bit
XRA M
CMP R
CPI 8-bit
4. Branching Instructions
The following instructions change the program sequence.
JMP 16-bit
JZ 16-bit
JNZ 16-bit
JC 16-bit
JNC 16-bit
CALL 16-bit
RET
5. Miscellaneous Instructions
There are a number of instructions related with data transfer among the register, the stack operation instructions and interrupt operations of 8085 MP which are kept in this group. They are:
PUSH, POP
EI, DI
6. Machine Control Instructions
These instructions affect the operation of the processor.
HLT, NOP
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