Mumbai University > Information Technology> sem 4> computer organization and architecture

Marks: 10M

Year: Dec16

Hardwired and Microprogrammed Control

• For each instruction, the control unit causes the CPU to execute a sequence of steps correctly. In reality, there must be control signals to assert lines on various digital components to make things happen. For example, when we perform an Add instruction in assembly language, we assume the addition takes place because the control signals for the ALU are set to "add" and the result is put into the AC. The ALU has various control lines that determine which operation to perform.

• We can take one of two approaches to ensure control lines are set properly. The first approach is to physically connect all of the control lines to the actual machine instructions. The instructions are divided up into fields, and different bits in the instruction are combined through various digital logic components to drive the control lines. This is called hardwired control, and is illustrated in figure (1)

• The control unit is implemented using hardware (for example: NAND gates, flip-flops, and counters).We need a special digital circuit that uses , as inputs, the bits from the Opcode field in our instructions, bits from the flag (or status) register, signals from the bus, and signals from the clock. It should produce, as outputs, the control signals to drive the various components in the computer.

• The advantage of hardwired control is that is very fast. The disadvantage is that the instruction set and the control logic are directly tied together by special circuits that are complex and difficult to design or modify. If someone designs a hardwired computer and later decides to extend the instruction set, the physical components in the computer must be changed. This is prohibitively expensive, because not only must new chips be fabricated but also the old ones must be located and replaced.

• Microprogramming is a second alternative for designing control unit of digital computer (uses software for control). A control unit whose binary control variables are stored in memory is called a microprogrammed control unit. The control variables at any given time can be represented by a string of 1's and 0's called a control word (which can be programmed to perform various operations on the component of the system). Each word in control memory contains within it a microinstruction. The microinstruction specifies one or more microoperatiotins for the system. A sequence of microinstructions constitutes a microprogram. A memory that is part of a control unit is referred to as a control memory.

• A more advanced development known as dynamic microprogramming permits a microprogram to be loaded initially from an auxiliary memory such as a magnetic disk. Control units that use dynamic microprogramming employ a writable control memory; this type of memory can be used for writing (to change the microprogram) but is used mostly for reading.

• The general configuration of a microprogrammed control unit is demonstrated in the block diagram of Figure (2). The control memory is assumed to be a ROM, within which all control information is permanently stored