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Instruction-level parallel processing can be informally defined as the concurrent processing of multiple instructions Traditional sequential processors execute one instruction at a time A leading instruction is completed before the next instruction is processed To a certain extent, pipelined processors achieve a form of instructionlevel parallel processing by overlapping the processing of multiple instructions As many instructions as there are pipeline stages can be concurrently in flight at any onetime Traditional sequential (CISC) processors can require an average of about 10 machine cycles for processing each instruction, that is CPI = 10 With pipelined (RIS Xprpcessors,^yejiJhQugh each instruction may stiU require multiple cyclesTo" complete, by overlapping Hje processing of multiple instructions in the pipeuneTtfie effective average CPI can be reducedtrJcToleTo ongtfVne^Tnkrucnon can be initio ated everxrnachinexycle With scalar pipelined processors, there is still the limitation of fetching and initiating at most one instruction into the pipeline every machine cycle With this hmitation, the best possible CPI that can be achieved is one; or inversely, the best possible throughput of a scalar processor is one instruction per cycle (IPC) A more aggressive form of instruction-level parallel processing is possible that involves fetching and initiating multiple instructions into a wider pipelined processor every machine cycle While the decade of the 1980s adopted CPI = 1 as its design objective for single-chip microprocessors, the goal for the decade of the 1990s was to reduce CPI to below one, or to achieve a throughput of IPC greater than one Processors capable of IPC greater than oneare termed superscalar pro; cessors This section presents the overview of mstruction-level parallel processing and provides the bridge between scalar pipelined processors and their natural descendants, the superscalar processors 141 From Scaiar to Superscalar

Figure 7-10. This console window shows the results of the program addThese, which demonstrates a function that adds two numbers together and returns the sum to the calling function.

It is worth noting that printf() is actually declared to return an int that reflects the number of characters generated or a negative value if an error occurs. The vast majority of programmers ignore this return value, but it is an important programming practice to be aware of the return value of every single function that you call.

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Danger! Avoid Uninitialized Return Values!

Before we leave the topic of function return values, there s one pitfall worth mentioning. If you re going to use a function in an expression, make sure the function provides a return value. For example, this code will produce unpredictable results:

Lab Exercise 1101: Installing/Upgrading Considerations 183 Lab Exercise 1102: Using Windows Easy Transfer 186 Lab Exercise 1103: Upgrading a Windows Operating System 192 Lab Exercise 1104: Performing a Clean Installation 196 Lab Exercise 1105: Post-Installation Tasks: Drivers and Updates 199 Lab Analysis Test 201 Key Term Quiz 201

Sca/a *processors are pipelined processors that a|e-degigned to fetch and issue at most one instruction every machine cycle <SiuperscalapprocessoTS are those that _are designed to fetch and issue multiple inslrurtians'every machine cycle This subsection presents the basis and motivation for evolving from scalar to superscalar processor implementations

#include <stdio.h> int AddTheseNumbers( int num1, int num2 );

int main (int argc, const char * argv[]) { int sum; sum = AddTheseNumbers( 5, 6 ); printf( "The sum is %d.", sum ); return 0; } int AddTheseNumbers( int num1, int num2 ) {

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return; }

/* Yikes! We forgot to set the return value */

When AddTheseNumbers() returns, what will its value be No one knows! When I ran the preceding on my computer, Xcode reported two warnings (as it should) and ran the program, generating a sum of 0. Unpredictable results! Don t forget to set a return value if you intend to use a function in an expression. And be sure to pay attention to warnings.

Lab Exercise 1201: Paging Dr Keyboard! 204 Lab Exercise 1202: Exploring Scanners 207 Lab Exercise 1203: Testing Digital Cameras 209 Lab Analysis Test 211 Key Term Quiz 212

.

Should you use a return value or a passed-by-address parameter Which is correct This is basically a question of functional fit. Use whichever works best for you. Just remember that a function can have only one return value but an unlimited number of parameters. If you need to get more than one piece of data back to the calling function, your best bet is to use parameters. The function AddTheseNumbers() was a natural fit for the return statement. It took in a pair of numbers (the input parameters) and needed to return the sum of those numbers. Since it needed to return only a single value, the return statement worked perfectly. Another nice thing about using the return statement is that it frequently allows us to avoid declaring an extra variable. In addThese, we declared sum to receive the value returned by AddTheseNumbers(). Since all we did with sum was print its value, we could have accomplished the same thing with this version of main():

int main (int argc, const char * argv[]) { printf( "The sum is %d.", AddTheseNumbers( 5, 6 ) ); return 0; }

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See the difference We included the call to AddTheseNumbers() in the printf(), bypassing sum entirely. When AddTheseNumbers() returns its int, that value is passed on to printf().

Are you ready for some more code The next few sample programs make use of pointers, function parameters, global variables, and function returns. Fire up your Mac, crank up your iPod, and break out the pizza. Let s code!