Microprogram sequencing
Microprogram sequencing is a fundamental aspect of computer architecture that defines the order in which microinstructions are executed by the control unit. Its purpose is to provide a mechanism to guide the system through the proper steps of an operation, ensuring correct execution of the microprogram, which is the set of microinstructions that make up a single machine language instruction.
Microprogram sequencing involves several techniques to carry out the sequence of microinstructions, including:
1. Straight-Line Sequencing:
This is the simplest sequencing technique. In straight-line sequencing, the control unit fetches and executes microinstructions one after another from the control memory, in the same order as they are stored. This approach is used when there are no branches in the microprogram.
2. Conditional Branching:
Sometimes, it is necessary to change the sequence of microinstruction execution based on specific conditions (like a result of zero or an overflow in an arithmetic operation). In this case, conditional branching is used. The sequence may branch to a different location in the control memory depending on the condition.
3. Subroutine Call and Return:
For certain complex operations or instructions that are used frequently, a subroutine of microinstructions can be defined. This subroutine can be called from any location in the control memory. Once the subroutine is completed, control returns to the instruction following the subroutine call. This helps to avoid duplication of microinstructions in the control memory and makes the microprogramming more efficient.
4. Mapping and Indirect Addressing:
Mapping and indirect addressing can be used to implement complex sequencing mechanisms, like those required for handling interrupts. A map in control memory can be used to redirect the sequencing mechanism to the appropriate routine based on the type of interrupt, and indirect addressing can be used to facilitate jumping to the appropriate location in control memory.
The microprogram sequencing process is controlled by the microprogram counter and the control logic in the control unit. These are responsible for determining the next address in the control memory to fetch the next microinstruction based on the current microinstruction and the status of the system.
