Control memory | control word,,control memory,address register,sequencer,pipeline register

The major functional parts in a digital computer are -Central Processing unit (CPU), Memory, and Input-output. The main digital hardware functional units of CPU are control unit, arithmetic and logic unit, and registers. The function of the control unit in a digital computer is to initiate sequences of micro operations. The number of different types of micro operations that are available in a given system is finite. The complexity of the digital system is derived from the number of sequences of micro operations that are performed.Two methods of implementing control unit are hardwired control and micro- programmed control. The design of hardwired control involves the use of fixed instructions, fixed logic blocks of and or arrays, encoders, decoders, etc. The key characteristics of hardwired control logic are high-speed operation, expensive, relatively complex, and no flexibility of adding new instructions. Example CPUs with hardwired logic control are Intel 8085, Motorola 6802, Zilog 80, and any RISC (Reduced Instruction Set Computer) CPUs. When the control signals are generated by hardware using conventional logic design techniques, the control unit .is said to be hardwired. Micro-programming is a second alternative for designing the. control unit of a digital computer. The principle of micro-programming is an elegant and systematic method for controlling the micro operation sequences in a digital computer. For example, CPUs with micro-programmed control unit are Intel 8080, Motorola 68000, and any CISC (Complex Instruction Set Computer) CPUs.
The control function that specifies a micro-operation is a binary variable. When it is in one binary state, the corresponding micro-operation is executed.


A control variable in die opposite binary state does not change the state of the register to the system. The active state of a control variable may be either the 1 state or the 0 state, depending on the application. In a bus-organized system, the control signals that specify Micro-operations are groups of bits that select the paths in multiplexers, decoders and arithmetic logic units.
The-control unit initiates a series of sequential steps of micro-operations during any given time. certain micro-operations are to be initiated. while others initiate idle. The control variables at any given time can be represented by a string of it's and 0's called a control word. As such. control word can be programmed to perform various operations on the components of the system.

A control unit whose binary control variables are stored in memory is called micro-programmed control unit. Each word in control memory a contains within it micro-instruction. The micro-instruction specifies one or more micro-operations for the system. A sequence of micro-instructions constitutes a micro-program. 'Since alterations of the micro-program are not needed once the control units is operation, the control memory can be a read-only memory (ROM). The content of the words in ROM are.fixed and cannot be altered by. simple programming since no writing capability is available in the ROM. ROM word are made permanent during the hardware production of the unit. the use of a micro-program involves placing all control variables in words of ROM for use by the control unit through successive read operations. The content of the word in ROM at a given address specifies a micro-instruction.

A more advanced development known as dynamic micro-programming permits a micro-program to be loaded initially from an auxiliary memory such as a magnetic disk. Control units that use dynamic micro-programming employ a writable control memory. This type of memory can be use for writing ( to change the micro-program) but is used mostly for reading. A memory that is part of a control unit is referred to as a control memory.

A computer that employs a micro-programmed control unit will have two separate memories a main memory and a control memory. The main memory is available to the user for storing thee programs.The contents of main memory may alter when the data are manipulated and every time that the program is changed.The user's program in main memory consists of machine instructions and data .in contrast, the control memory holds a fixed micro-program that can not be altered by the occasional user. The micro-program consists of micro instructions that specify various internal control signals for execution of register micro operations. Each machine instruction initiates a series of micro instruction in control memory. These micro-instructions generate the micro-operation fetch the instruction from main memory to evaluate the effective address, to execute the operation specified by the instruction, and to return control to the fetch phase in order to repeat the cycle for the next instruction:


The general configuration of a micro-programmed control unit is demon started. The control memory is assumed to be a ROM, within which all control information is permanently stored. The control memory address register specifies the address of the micro-instruction and the control data register holds the micro instruction read from memory.
The micro-instruction contains a control word that specifies one or more micro- operations for the data processor..Once these operations are executed, the control must determine the next address. The location of the next micro-instruction may be the one next in sequence, or it may be located somewhere else in the control memory. For this reason it is necessary to use some bits of the present micro-instruction to control the generation of the address of the next micro-instruction. The next address may also be a function of external input conditions. While the micro-operations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next micro-instruction. Thus a micro-instruction contains bits for initiating micro-operations in the data processor part and bits that determine the address sequence for the control memoy

The next address generator is sometimes called a micro-program sequencer , as it determines the address sequence that is read from control memory The address of the next micro-instruction can be specified in several ways, depending i on the sequencer inputs. Typical functions of a micro-program sequencer are increment the control address register address register an address from control memory, transferring an external address or loading an initial address to start the control operation
The control data register holds the.present micro-instruction next address is Computed and read hold the present micro-instruction while the next address is computed and read from memory. The data register is some time called a pipeline register. It allows the execution of the micro-operation specified by the control word simultaneously with the generation of the next micro-instruction.This configuration requires a two-phase clock with one clock applied to the address register and the other to the data register.

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