Micro programmed control

3. Microprogram sequencing

• Microprogram is simple because std software technique can be used.

• However, two major disadvantages may be answered by this method.

• Separate microroutine for each machine instruction cause large no of microinstruction and large control store.

Contd.,

• Most machine instruction involve several addressing mode.

• Many instruction of a processor have different combinations of addressing modes.

• A separate microroutine for each of these combinations would produce considerable duplication of common parts.

• We should organize the microroutine so that many common parts would share as possible.

• [when it arise execution time is longer because it takes more time to carry out the required branches]

Example

–Add src, dst

the source operand can be specified in the following addressing modes1. Register

2. Auto increment

3. Auto decrement

4. indexed

4. Microinstruction with next-Address field

5. Prefetching microinstructions

One major drawback of microprogrammed control is slower operating speed.bcoz of fetching every micoinstruction from CS.

Prefetching of microinstruction facing some operational difficulties that it must consider the status flags and result of currently being executed microinstruction.

Some time straight forward prefetches a wrong microinstruction. Thus, the fetch must be repeated with the correct address and it will require more complex hardware.

6.Emulation - Duplicate

• Additional instruction set.-Emulate the function of one system to another so the emulated machine reassembles the behavior of real system.

• Programs written in the machine language of M2 machine can then be run on computer M1.