In other words, a microprocessor is the chip containing some control and logic circuits that is capable of making arithmetic and logical decisions based on input data and produces the corresponding arithmetic or logical output.
Application of Microprocessor
Microprocessors are used in many devices in the modern world, but the application of the microprocessors can be broadly classified into two groups i.e. reprogrammable and embedded.Personal computers, Servers, notebook, smartphones etc. are reprogrammable application of microprocessors.
Home appliances. Machinery, automated systems, communication devices are examples of embedded application of microprocessor. The microprocessor inside the embedded system are also known as the Microcontrollers.
Evolution of Microprocessors
Intel developed and delivered the first commercially viable microprocessor way back in the early 1970’s: the 4004 and 4040 devices. The 4004 was not very powerful and all it could do was add and subtract with 4-bit data only at a time. Intel rapidly followed their 4-bit offerings with their 8008 and 8080 eight-bit CPUs.
Intel started facing competition from Motorola, MOS Technology, and an upstart company formed by disgruntled Intel employees, Zilog. To compete, Intel produced the 8085 microprocessor. To the software engineer, the 8085 was essentially the same as the 8080 but had lots of hardware improvements that made it easier to design into a circuit.
During late 70’s Motorola’s 6800 series was easier to program, MOS Technologies’ 65xx family was also easier to program but very inexpensive, and Zilog’s Z80 chip was upward compatible with the 8080 with lots of additional instructions and other features. By 1978 most personal computers were using the 6502 or Z80 chips, not the Intel offerings.
The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which appeared around 1982 for the first time). If we are familiar with the PC market and its history, we know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. Intel makes all of these microprocessors and all of them are improvements of design base of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!
Von Neumann Architecture:
It is named after the mathematician and early computer scientist John Von Neumann. It is known as stored program architecture. The computer has single storage system (memory) for storing data as well as program to be executed. It is widely used computer architecture. The other architecture is Harvard Architecture which have separate memory for data and program respectively.
The control unit (CU) has overall responsibility for the system. It controls various activities within the microprocessor.
The arithmetic-logic unit (ALU), as its name also suggests, performs arithmetic (addition, subtraction, etc.) and logic (and, or, etc.) operations. The registers are small memory within the CPU. This is the only storage to which the ALU has direct access. To manipulate any values from memory they must first be loaded into registers.
Memory
Memory is device where both program code and data reside while the program is in execution. This is the stored program concept, often attributed to John von Neumann. Memory (or Main Memory) is sometimes referred to as Primary Storage, as opposed to Secondary Storage, which usually refers to disk drives, tape drives and the like. The major difference between primary and secondary storage is access time.Input and Output
Microprocessor takes in input from the input devices as well as from the data stored in the secondary storage. It gives output through output device after processing according to the instruction.The Memory Subsystem
Memory is where the computer stores information and (running) programs. Memory is made up of bits: the bit is the smallest unit of storage, it can hold one of two values: 1 and 0 (zero). Bits are grouped into larger units called bytes (8 bits). Group of Bytes are called as words.There are two important attributes associated with every chunk of memory: Its address and
Its contents
Memory Operations
Memory is capable of doing two things:1. Deliver the current contents of a designated memory cell to the CPU (memory read).
2. Store new contents in a designated memory cell (memory write).
For the first operation (known as a memory read) the CPU must pass two pieces of information to the memory subsystem:
The address of the memory cell to be read.
An indication that a read is requested.
The memory subsystem, having retrieved the information, must then forward it on to the CPU.
In order to carry out the second operation (a memory write), the CPU must pass:
The address of the memory cell to be stored into.
The new contents to be stored.
An indication that a write is requested.
In order to perform these operations Processor needs following registers and control signal:
The Memory Address Register (MAR): Which contains the address of the requested cell, the one we want either to read from or to write into.
The Memory Data Register (MDR): The memory subsystem places the retrieved contents of memory (read) and the CPU places the new contents of memory (write).
A Read/Write Line (R/W): Used to indicate whether the required operation is a read or a write.
Input/ Output Subsystem:
In order to communicate with anything outside the CPU/Memory system we need an I/O subsystem. This subsystem needs to be able to control many different types of device, for example: Screen, keyboard, printer, but they all have very different characteristics.I/O Controllers
Compare to RAM, I/O devices are extremely slow. If the CPU had to wait for peripheral I/O to complete before carrying on then the CPU is going to spend a good deal of its time doing nothing useful.The solution is to use an I/O Controller, which is a special purpose processor which has a small memory buffer, and a control logic to control the I/O device (e.g. move disk arm).
In its simplest form, the procedure for performing an I/O request (input or output, the scheme is the same) is as follows:
1. CPU sends I/O request information to the I/O controller. This is usually done by having the controller's registers accessible to the CPU.
2. The controller takes over responsibility for the I/O request. For example, on a disk read, the controller will be responsible for moving the arm to the appropriate cylinder. Data arriving from the disk will be buffered in the controller's internal memory and then by the process of Direct Memory Access or DMA it is transferred to RAM.
Harvard Architecture
Harvard architecture has separate data and instruction busses, allowing transfers to be performed simultaneously on both busses. A Von Neumann architecture has only one bus which is used for both data transfers and instruction fetches, and therefore data transfers and instruction fetches must be scheduled - they can not be performed at the same time.It is possible to have two separate memory systems for a Harvard architecture. As long as data and instructions can be fed in at the same time, then it doesn't matter whether it comes from a cache or memory. But there are problems with this. Compilers generally embed data (literal pools) within the code, and it is often also necessary to be able to write to the instruction memory space, for example in the case of self modifying code, or, if a debugger is used, to set software breakpoints in memory. If there are two completely separate, isolated memory systems, this is not possible. There must be some kind of bridge between the memory systems to allow this.
Use of caches:
At higher clock speeds, caches are useful as the memory speed is proportionally slower. Harvard architectures tend to be targeted at higher performance systems, and so caches are nearly always used in such systems.Von Neumann architectures usually have a single unified cache, which stores both instructions and data. The proportion of each in the cache is variable, which may be a good thing. It would in principle be possible to have separate instruction and data caches, storing data and instructions separately. This probably would not be very useful as it would only be possible to ever access one cache at a time.
Caches for Harvard architectures are very useful. Such a system would have separate caches for each bus. Trying to use a shared cache on a Harvard architecture would be very inefficient since then only one bus can be fed at a time. Having two caches means it is possible to feed both buses simultaneously....exactly what is necessary for a Harvard architecture.
This also allows to have a very simple unified memory system, using the same address space for both instructions and data. This gets around the problem of literal pools and self modifying code. What it does mean, however, is that when starting with empty caches, it is necessary to fetch instructions and data from the single memory system, at the same time. Obviously, two memory accesses are needed therefore before the core has all the data needed. This performance will be no better than a von Neumann architecture. However, as the caches fill up, it is much more likely that the instruction or data value has already been cached, and so only one of the two has to be fetched from memory. The other can be supplied directly from the cache with no additional delay. The best performance is achieved when both instructions and data are supplied by the caches, with no need to access external memory at all.
This is the most sensible compromise and the architecture used by certain Harvard processor cores. Two separate memory systems can perform better, but would be difficult to implement.
Basic Architecture of Microprocessor Based System
Control Unit
Arithmetic and Logic Unit
Memory
Input/ Output
CPU Registers
System Bus
CPU Registers:
In computer architecture, a processor register is a very fast computer memory used to speed the execution of computer programs by providing quick access to commonly use values-typically, the values being in the midst of a calculation at a given point in time.These registers are the top of the memory hierarchy, and are the fastest way for the system to manipulate data. In a very simple microprocessor, it consists of a single memory location, usually called an accumulator. Registers are built from fast multi- ported memory cell. They must be able to drive its data onto an internal bus in a single clock cycle. The result of ALU operation is stored here and could be re-used in a subsequent operation or saved into memory.
Registers are normally measured by the number of bits they can hold, for example, an “8-bit register” or a “32-bit register”. Registers are now usually implemented as a register file, but they have also been implemented using individual flip-flops, high speed core memory, thin film memory, and other ways in various machines.
There are several other classes of registers:
(a)
Accumulator:
It is most frequently used register used to store operand before the execution of an instruction and to store result after the execution of the instruction.(b)
General Purpose registers:
General purpose registers are used to store data, address and intermediate results during program execution. Its contents can be accessed through assembly programming. It can be further classified into Integer and Floating point registers.(c)
Special purpose Registers:
Users do not access these registers. These are used by computer system at the time of program execution. Some types of special purpose registers are given below:Memory Address Register (MAR):
It stores address of data or instructions to be fetched from memory or Input/ Output devices.Memory Buffer Register (MBR):
It stores instruction and data received from the memory and sent to be written in memory temporarily.Memory Data Register (MDR):
It is similar to MAR. It stores instruction and data received from the memory and to be sent to the memory for writing.Instruction Register (IR): Instructions are stored in instruction register. When one instruction is completed, next instruction is fetched from memory for processing.
Program Counter (PC): It stores the address of next instruction to be executed.
System Bus
The system bus is a cable which carries data communication between the major components of the computer, including the microprocessor. Not all of the communication that uses the bus involves the CPU, although naturally the examples used in this tutorial will center on such instances.The system bus consists of three different groups of wiring, called the data bus, control bus and address bus. These all have separate responsibilities and characteristics, which can be outlined as follows:
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