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Microinstruction sequencing new
The document describes the components and functioning of a microprogram sequencer. The microprogram sequencer selects the next address from various sources like the current microinstruction address field, an incremented address, or an external source. It uses multiplexers and registers to select the appropriate next address and load it into the control address register to fetch the next microinstruction from memory. The input logic determines the types of operations the sequencer can perform, such as branching, subroutine calls and returns, and other address sequencing functions.
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Microinstruction sequencing new
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- 2. Microprogram Sequencer M.M. Ch-7, Page-232 The basic components of microprogrammed control unit are the control memory and the circuits that select the next address. The task of Microinstruction sequencing is done by Microprogram sequencer. The address selection part is called as microprogram sequencer. Microprogram sequencer can be constructed with digital functions to suit a particular application. Two imp. factors that must be considered while designing the microinstruction sequencer: The size of the microinstruction The address generation time
- 3. MUX 1 SBRIncrementer CAR Input Logic MUX 2 Select Control Memory Microprogram sequencer for a control Mmemory
- 4. CAR – ControlAddress Register SBR – Subroutine Register CD – Condition BR - Branch
- 5. The purpose of microprogram sequencer is to present an address to the control memory so that a microinstruction may be read and executed. The next address logic of the sequencer determines the specific address source to be loaded into the CAR. The choice of the address source is guided by the next address information bits that the sequencer receives from the present microinstruction. The control memory is included in the diagram to show the interaction between the sequencer and the memory attached to it. There are two multiplexers in the circuit. The first multiplexer selects an address from one of the four sources and routes it into the CAR.
- 6. The second multiplexertests the value of a selected status bit and the result of the test is applied to an input logic circuit. The output from CAR provides the address for the control memory. The contents of CAR is incremented and applied to one of the multiplexer inputs and to the SBR. The other three input come from the address field of the present microinstruction, from the output of SBR and from an external source that maps the instruction. Variables S 0 and S 1 select one of the source addresses for CAR . Variable L enables the load input in SBR .
- 7. The CD (condition)field of the microinstruction selects one of the status bits in the second multiplexer. If the bit selected is 1, the T (test) variable is equal to 1, otherwise it is 0. The T value together with the two bits from the BR (branch) field go to an input logic circuit. The Input logic in a particular sequencer determines the type of operations that are available in the unit. Typical sequencer operations are: increment, branch and jump, call and return from subroutine, load an external address, push or pop the stack, and other address sequencing operations. The binary values of the two selection variables determine the path in the multiplexer. E.g. S1S0 = 10, mulltiplexer input number 2 is selected and establishes a transfer path from SBR to CAR.