Control Memory

The function of the control unit in a digital computer is to initiate sequences of micro-operations. The number of different types of micro-operations available in a given system is finite. The complexity of the digital system is derived from the number of sequences of micro-operations that are performed. When control signals are generated by hardware using conventional logic design techniques, the control unit is said to be hardwired. Microprogramming is an alternative method for designing the control unit of a digital computer. The principle of microprogramming is an elegant and systematic method for controlling the micro-operation sequences in a digital computer.

Control Variables

The control function that specifies a micro-operation is a binary variable. When it is in one binary state, the corresponding micro-operation is executed. A control variable in the opposite binary state does not change the state of the registers in the system. The active state of a control variable may be either the 1 state or the 0 state, depending on the application. In a bus-organized system, the control signals that specify micro-operations are groups of bits that select the paths in multiplexers, decoders, and arithmetic logic units.

Control Words and Microinstructions

The control unit initiates a series of sequential steps of micro-operations. During any given time, certain micro-operations are initiated, while others remain idle. The control variables at any given time can be represented by a string of 1's and 0's called a control word. Control words can be programmed to perform various operations on the components of the system. A control unit whose binary control variables are stored in memory is called a microprogrammed control unit. Each word in control memory contains a microinstruction. The microinstruction specifies one or more micro-operations for the system. A sequence of microinstructions constitutes a microprogram.

Types of Control Memory

Read-Only Memory (ROM)

Since alterations of the microprogram are not needed once the control unit is in operation, the control memory can be a read-only memory (ROM). The content of the words in ROM are fixed and cannot be altered by simple programming since no writing capability is available in the ROM. ROM words are made permanent during the hardware production of the unit. The use of a microprogram involves placing all control variables in words of ROM for use by the control unit through successive read operations. The content of the word in ROM at a given address specifies a microinstruction.

Writable 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. A memory that is part of a control unit is referred to as control memory.

Control Memory Configuration

A computer that employs a microprogrammed control unit will have two separate memories: a main memory and a control memory. The main memory is available to the user for storing programs. The contents of main memory may alter when the data are manipulated and every time the program is changed. The user's program in main memory consists of machine instructions and data. In contrast, the control memory holds a fixed microprogram that cannot be altered by the occasional user. The microprogram consists of microinstructions that specify various internal control signals for execution of register micro-operations.

Each machine instruction initiates a series of microinstructions in control memory. These microinstructions generate the micro-operations to fetch the instruction from main memory, to evaluate the effective address, to execute the operation specified by the instruction, and to return control to the fetch phase to repeat the cycle for the next instruction.

Microprogrammed Control Unit Organization

Control Memory

The control memory is assumed to be a ROM, within which all control information is permanently stored. The control memory address register specifies the address of the microinstruction, and the control data register holds the microinstruction read from memory. The microinstruction contains a control word that specifies one or more micro-operations for the data processor. Once these operations are executed, the control must determine the next address. The location of the next microinstruction may be the one next in sequence, or it may be located somewhere else in the control memory.

Next Address Generation

For this reason, it is necessary to use some bits of the present microinstruction to control the generation of the address of the next microinstruction. The next address may also be a function of external input conditions. While the micro-operations are being executed, the next address is computed in the next address generator circuit and then transferred into the control address register to read the next microinstruction. Thus, a microinstruction contains bits for initiating micro-operations in the data processor part and bits that determine the address sequence for the control memory.

Microprogram Sequencer

The next address generator is sometimes called a microprogram sequencer, as it determines the address sequence that is read from control memory. The address of the next microinstruction can be specified in several ways, depending on the sequencer inputs. Typical functions of a microprogram sequencer are incrementing the control address register by one, loading into the control address register an address from control memory, transferring an external address, or loading an initial address to start the control operations.

Control Data Register

The control data register holds the present microinstruction while the next address is computed and read from memory. The data register is sometimes called a pipeline register. It allows the execution of the micro-operations specified by the control word simultaneously with the generation of the next microinstruction. This configuration requires a two-phase clock, with one clock applied to the address register and the other to the data register.

Single-Phase Clock Operation

The system can operate without the control data register by applying a single-phase clock to the address register. The control word and next-address information are taken directly from the control memory. It must be realized that a ROM operates as a combinational circuit, with the address value as the input and the corresponding word as the output. The content of the specified word in ROM remains in the output wires as long as its address value remains in the address register. No read signal is needed as in a random-access memory. Each clock pulse will execute the micro-operations specified by the control word and also transfer a new address to the control address register. In this example, we assume a single-phase clock, and therefore we do not use a control data register. In this way, the address register is the only component in the control system that receives clock pulses. The other two components: the sequencer and the control memory are combinational circuits and do not need a clock.

Advantages of Microprogrammed Control

The main advantage of microprogrammed control is that once the hardware configuration is established, there should be no need for further hardware or wiring changes. If we want to establish a different control sequence for the system, all we need to do is specify a different set of microinstructions for control memory. The hardware configuration should not be changed for different operations; the only thing that must be changed is the microprogram residing in control memory.

It should be mentioned that most computers based on the reduced instruction set computer (RISC) architecture concept use hardwired control rather than a control memory with a microprogram.