2.5 Module 6: control unit operation  (Page 4/6)

00: Fetch

01: Indirect

10: Execute

11: Interrupt

At the end of each of the four cycles, the ICC is set appropriately. The indirect cycle is always followed by the execute cycle. The interrupt cycle is always followed by the fetch cycle. For both the execute and fetch cycles, the next cycle depends on the state of the system.

Thus, the flowchart of Figure 6.3 defines the complete sequence of micro-operations, depending only on the instruction sequence and the interrupt pattern. Of course, this is a simplified example. The flowchart for an actual processor would be more complex. In any case, we have reached the point in our discussion in which the operation of the processor is defined as the performance of a sequence of micro-operations. We can now consider how the control unit causes this sequence to occur.

2. control of the processor

2.1 functional requirements

As a result of our analysis in the preceding section, we have decomposed the behavior or functioning of the processor into elementary operations, called micro-operations. By reducing the operation of the processor to its most fundamental level, we are able to define exactly what it is that the control unit must cause to happen. Thus, we can define the functional requirements for the control unit those functions that the control unit must perform. A definition of these functional requirements is the basis for the design and implementation of the control unit.

With the information at hand, the following three-step process leads to a characterization of the control unit:

1. Define the basic elements of the processor.
2. Describe the micro-operations that the processor performs.
3. Determine the functions that the control unit must perform to cause the micro-operations to be performed.

We have already performed steps 1 and 2. Let us summarize the results. First, the basic functional elements of the processor are the following:

• ALU
• Registers
• Internal data paths
• External data paths
• Control unit

Some thought should convince you that this is a complete list. The ALU is the functional essence of the computer. Registers are used to stoic data internal to the processor. Some registers contain status information needed to manage instruction sequencing (e.g., a program status word). Others contain data that go to or come from the ALU, memory, and I/O modules. Internal data paths are used to move data between registers and between register and ALU. External data paths link registers to memory and I/O modules, often by means of a system bus. The control unit causes operations to happen within the processor.

The execution of a program consists of operations involving these processor elements. As we have seen, these operations consist of a sequence of micro-operations. All micro-operations fall into one of the following categories:

• Transfer data from one register to another.
• Transfer data from a register to an external interface (e.g., system bus).
• Transfer data from an external interface lo a register.
• Perform an arithmetic or logic operation, using registers for input and output.