9
Microprogramming
and Microarchitecture
This chapter discusses about microprogramming and microarchitecture adopted in proces-
sors. After completion of this chapter, the reader would know about:
R How to design the data path and necessary control signals
R Functional details of execution of instruction cycle of a processor
R Hardwired and software control of processors and their micro-instructions
C HAPTER OBJECTIVES
9.1 INTRODUCTION
In Chapter 5, during the discussions on instruction decoding of processors, it was mentioned that this
instruction decoding may be implemented through hardware as well as software. In this chapter, we
shall have a detailed discussions about the techniques used for instruction decoding by processors. We
shall discuss about the hardware control rst, followed by software control.
In this context, we have to be familiar with two major operations of computer microarchitecture. First
one is the control of data ow from one register to another. Note that in most of the cases, the data reside
within some registers of the processor. The second one is the method of latching data presently available
with a speci ed register. Almost 95% of control operations of the computer are related with these two
methods. In Chapter 3, we have discussed some details about these operations. However, we shall have a
quick look into these techniques, before dealing with other details based on these techniques.
9.1.1 Latching Data Available Within a Register
In Chapter 3, we have discussed about operations of D ip- ops (Section 3.6.5). It was also indicated
in that section on how a 4-bit register may be constructed using four D ip- ops (Figure 3.22). Note
that it was a parallel-in–parallel-out register, and the data present at the input may be latched with the
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