Explain the operation of a micro programmed control unit with the help of a diagram. 8m Jun2008
THE MICRO-PROGRAMMED CONTROL
The logic of the control unit is specified by a micro-program. A micro-program is also called firmware (midway between the hardware and the software). It consists of:
(a) One or more micro-operations to be executed; and
(b) The information about the micro-instruction to be executed next.
The general configuration of a micro-programmed control unit is demonstrated in Figure 5 below:
The micro-instructions are stored in the control memory. The address register for the control memory contains the address of the next instruction that is to be read. The control memory Buffer Register receives the micro-instruction that has been read. A micro-instruction execution primarily involves the generation of desired control signals and signals used to determine the next micro-instruction to be executed. The sequencing logic section loads the control memory address register. It also issues a read command to control memory. The following functions are performed by the micro-programmed control unit:
1. The sequence logic unit specifies the address of the control memory word that is to be read, in the Address Register of the Control Memory. It also issues the READ signal.
2. The desired control memory word is read into control memory Buffer Register.
3. The content of the control memory buffer register is decoded to create control signals and next-address information for the sequencing logic unit.
4. The sequencing logic unit finds the address of the next control word on the basis of the next-address information from the decoder and the ALU flags.
This decoder translates the opcode of the IR into a control memory address.