Switch/case statements

The natural way to do this is to use an integer index (spin, say) in a loop and use the switch/case statements ... 

The possibility of executing some statements is permanently determined by the hardware configuration. For example, execution of some switch-case statement is based on a control variable indicating deviation of the centre hall sensor installation position from the actual centre position of the pump. Such deviation never occurs unless one performs physical reconstruction of the pneumatic pump. 

In a Moore finite state machine, the output of the circuit is dependent only on the state of the machine and not on its inputs. This is described i pictorially in Figure 3-5. Since the outputs are dependent only on the j state, a good way to describe a Moore machine is to use an always state- j ment with a case statement. The case statement is used to switch between j the various states and the output logic for each state is described in the appropriate branch. The always statement can have the clock event in its] event list to indicate that it is a clocked always statement. This models the] condition of a finite state machine going from state to state synchronously j on every clock edge. The machine state itself is modeled using a reg vari-] able (a variable of reg data type). 

ISPS provides two mechanisms for conditional execution. IF operations evaluate an expression, and execute an associated statement or block of statements if that expression evaluates true (non-zero). DECODE operations, similar to the SWITCH or CASE statements in other languages, evaluate an expression, and execute one of a set of alternate statements or statement blocks, depending on the value of the expression. 

This construction is scarcely optimal. It is easy to see that instead of m+1 new variables, log m will suffice and indeed even fewer switch variables are usually needed since one needs only if k = 1 or k is a statement addressed by a GOTO (is a transfer address in a test statement). By taking advantage of otter features of particular schemes, still fewer switch variables may suffice in particular cases. 

What happens in the above case if we switch the order of the statements around In this case, since the value of Temp is used before its assignment, its value needs to be retained across multiple clock cycles, thereby inferring flip-flops for Temp. Temp models the internal state of the always statement. This is shown in the following example, where Temp is used before its assignment. 

The spreadsheet representation of the titration will now involve switching from one equation to another, as soon as the product [Ag+] [Br ] exceeds the value of soAgBr- In this case, then, the titration curve really consists of separate pieces, not for reasons of mathematical convenience but as the direct consequence of the formation of a new precipitant phase. In the spreadsheet we can accomplish this change-over between the two formalisms by using IF statements. Note that it is still a completely straightforward calculation, without any circular reasoning. 

The switch statement is less general than the cascaded if statement, because in a cascaded if statement each alternative can be associated with a complex expression, while in a switch statement each alternative is associated with a constant value (or with several constant values multiple case labels are allowed). 

The switch statement has two advantages over the more flexible cascaded if statement. The first is clarity when a solution can be expressed by a switch statement, then that solution is probably the clearest solution. The second is efficiency. In a cascaded if statement, each test expression must be evaluated in turn until one of the expressions is true. In a switch statement, it is often possible for the C compiler to generate code that branches directly to the target case. 

C has two additional flow of control statements, break and continue, that augment the capabilities of the loops that have just been described. The break statement is used to break out of (to immediately terminate) the enclosing do, while, for, or switch statement. Its use is routine in switch statements, to terminate the switch statement at the conclusion of each individual case. In do, while, and for loops, break is often used to terminate the loop prematurely when an error or special condition occurs. Using a break in the body of a loop often simplifies the control expression of the loop, because it allows special case code to be placed elsewhere. 

Some statements are logieally unreaehable. For example, the switch statement had trae and false paths to follow, but the programmer, for some unknown reason, had implemented yet another default path to follow in case neither path is taken. Statements in tire default clause could never be executed in the absence of abnormal control errors. 

The VHDL description must accoxmt for non-synthesizable states. This relates particularly to the use of Case and If statements, and methods of dealing with these states were discussed in section 7.1. The most extreme scenario is to switch the synthesizer off and on in different sections of the description. 

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