4.7 A Discussion on Decoding — Hardwired vs. Microprogrammed Control
How does the control unit really function? We have done some hand waving and simply assumed everything works as described, with a basic understanding that, 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 as described (recall the various digital components from Chapter 3). For example, when we perform an Add instruction in MARIE 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. The question we need to answer is, "How do these control lines actually become asserted?"
You 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 4.14.
The control unit is implemented using hardware (with simple NAND gates, flip-flops, and counters, for example). 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. For example, a 4-to-16 decoder could be used to decode the opcode. By using the contents of the IR register and the status of the ALU, this unit controls the registers, the ALU operations, all shifters, and bus access.
The advantage of hardwired control is that it 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 (as we did with MARIE), 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 e located and replaced.
The other approach, called microprogramming, uses software for control, and is illustrated in Figure 4.15.
All machine instructions are input into a special program, the microprogram, to convert the instruction into the appropriate control signals. The microprogram is essentially an interpreter, written in microcode, that is stored in firmware (ROM, PROM, or EPROM), which is often referred to as the control store. This program converts machine instructions of zeros and ones into control signals. Essentially there is one subroutine in this program for each machine instruction. The advantage of this approach is that if the instruction set requires modification, the microprogram is simply updated to match—no change is required in the actual hardware.
Microprogramming is flexible, simple in design, and lends itself to very powerful instruction sets. Microprogramming allows for convenient hardware/software tradeoffs: If what you want is not implemented in hardware (for example, your machine has no multiplication statement), it can be implemented in the microcode. The disadvantage to this approach is that all instructions must go through an additional level of interpretation, slowing down the program execution. In addition to this cost in time, there is a cost associated with the actual development, because appropriate tools are required. We discuss hardwired control versus microprogramming in more detail in Chapter 9.
It is important to note that whether we are using hardwired control or microprogrammed control, timing is critical. The control unit is responsible for the actual timing signals that direct all data transfers and actions. These signals are generated in sequence with a simple binary counter. For example, the timing signals for an architecture might include T1, T2, T3, T4, T5, T6, T7, and T8. These signals control when actions can occur. A fetch for an instruction might occur only when T1 is activated, whereas the fetch for an operand may occur only when T4 is activated. We know that registers can change states only when the clock pulses, but they are also limited to changing in conjunction with a given timing signal. We saw an example of memory in Chapter 3 that included a Write Enable control line. This control line could be ANDed with a timing signal to ensure that memory only changed during specific intervals.
