Hardwired and Microprogrammed Control Units in Computer Processors, Lecture notes of Computer Science

Download Hardwired and Microprogrammed Control Units in Computer Processors and more Lecture notes Computer Science in PDF only on Docsity! Unit 3: Processor Content 3.1 Hardwired and Micro programmed 3.2 Arithmetic Logic Unit 3.3 Types of Registers 3.4 Bus Architecture 3.5 Instruction Execution 3.6 Addressing Modes (Immediate Addressing, Direct Addressing, Register Addressing, Register Indirect Addressing, Indexed Addressing and Based-Indexed Addressing) Specific Objective • Explain hardwired implementation, micro-instruction sequencing and execution as well as application of micro-programming. • Elaborate Arithmetic and Logic unit; register types, bus architecture, instruction sets and executions. • Illustrate and explain different addressing modes with suitable examples. 3.1 Hardwired and Micro programmed Design of Control Unit Control unit generates timing and control signals for the operations of the computer. To execute an instruction, the control unit of the CPU must generate the required control signal in the proper sequence. There are two approaches used for generating the control signals in proper sequence, which is given below: Hardwired Control Unit The control unit whose control signals are generated by the hardware through a sequence of instruction is called a hardwired control Unit. It is implemented with the help of gates, flip flops, decoders etc. in the hardware. The inputs to control unit are the instruction register, flags, timing signals etc. This organization can be very complicated if we have to make the control unit large. If the design has to be modified or changed, all the combinational circuits have to be modified which is a very difficult task. The advantage of hardwired control is that is very fast. The disadvantage is that the instruction set and the control logic are directly tied together by special circuits that are complex and difficult to design or modify. If someone designs a hardwired computer and later decides to extend the instruction set, the physical components in the computer must be changed. This is prohibitively expensive, because not only must new chips be fabricated but also the old ones must be located and replaced. The characteristics of hardwired control units are as follows: • Hardwired control units are based on combinational circuits. • Fixed logic circuits that correspond directly to the Boolean expressions are used to generate the control signals. • Hardwired control is faster than micro-programmed control. • A controller that uses this approach can operate at high speed. Micro-programmed Control Unit Microprogramming is a second alternative for designing control unit of digital computer. The control unit whose control signals are generated by the data stored in control memory and constitutes a program on the small scale is called a micro-programmed control unit. It is implemented by using programming approach. In computer central processing units, micro-operations are detailed low-level instructions used in some designs to implement complex machine instructions. Each micro-operation is described in symbolic notation. This notation looks suspiciously like a programming language. In fact it is a language, known as a microprogramming language. The control variables at any given time can be represented by a string of 1's and 0's called a control word (which can be programmed to perform various operations on the component of the system). Each word in control memory contains within it a microinstruction. A sequence of micro operations is carried out by executing a program consisting of micro-instructions. A sequence of microinstructions constitutes a micro program or firmware. In this organization any modifications or changes can be done by updating the micro program in the control memory by the programmer. A micro-program is midway between hardware and software. It is easier to design in firmware than hardware, but it is more difficult to write a firmware program than a software program. A memory that is part of a control unit is referred to as a control memory The main advantages of the micro-programmed control are the fact that once the hardware configuration is established; there should be no need for further hardware or wiring changes. If we want to establish are different control sequence for the system, all we need to do is specify different set microinstructions for control memory. The hardware configuration should not be changed for different operations; the only thing that must be changed is the micro-program residing in control memory. Characteristics of micro programmed control units: ■ The control signals associated with operations are stored in special memory units inaccessible by the programmer as Control Words. ■ Control signals generated by a program are similar to machine language programs. ■ Micro-programmed control unit is slower in speed because of the time it takes to fetch microinstructions from the control memory.