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Page 9 Table 1.2 Direct or hardwired control vs. microprogrammed control: comparative decoder features. Attribute Hardwired Decoders Microprogrammed Decoders Speed Fast Slower Chip area efficiency Uses least area Uses more area Ease of change, additions Somewhat difficult Easier Ability to handle large/complex instruction sets Somewhat difficult Easier Ability to support operating systems and diagnostic features Very difficult (unless anticipated during design) Easy Where used Mostly RISC microprocessors Mainframes, some microprocessors Instruction set size Usually under 100 instructions Usually over 100 instructions ROM size 2K10K by 40200 bit microinstructions figuration management, etc.) or machine diagnostics. Microprogrammed decoders are usually used in these environments. Note that a slower decoder does not necessarily mean a slower machine, as other paths in the machine may be the determining speed factor. The cycle time of the machine is determined by the time it takes to move data from one register to another through intervening combinatorial logic. For machines with particularly long or elaborate combinatorial logic in the data paths, it is possible to distinguish short paths from long and to define multiples of the original clock to manage these long paths, as discussed later. 1.3 The Instruction Set Computers deal with many different kinds of data in many different ways. There are probably as many variations of data representations as there are processor implementations. Additionally, the operations available to perform the requisite data manipulations are often subtly different on different processors and frequently use different assembly syntax. All of these factors make choosing a data and instruction representation system that is generally acceptable difficult at best. This section aims at providing a basic understanding of the key concepts involved. Both data types and instruction set designs are presented from a ''birds-eye view," and comparisons between some of the major alternatives currently in use are considered to place the choices in perspective.
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