Conditional branching and working with bits br One of the most important features of any microprocessor or microcontroller program is its br ability to make ‘decisions’, i.e. to act differently according to the state of logical variables. br Microprocessors generally have within their instruction sets a number of instructions which br allow them to test a particular bit, and either continue program execution if a condition is not br met or branch to another part of the program if it is. This is illustrated in Figure 5.3. These br variables are often bit values in condition code or Status registers. br ThePIC16Seriesmicrocontrollersarealittleunusualwhenitcomestoconditionalbranching br as they do not have branch instructions as such. They have instead four conditional ‘skip’ br instructions.Thesetestforacertaincondition,skippingjustoneinstructioniftheconditionis br met and continuing normal program execution if it is not. The most versatile and general- br purpose of these are the instructions: br br br br Branching on Status register bits br br Aswedevelopbiggerprograms,wequicklyfindthatthereareprogramsectionsthataresouseful br thatwewouldliketousethemindifferentplaces.Yetitistedious,andspace-andmemory- br consuming,towriteouttheprogramsectionwheneveritisneeded.Enterthesubroutine. br The subroutine is a program section structured in such a way that it can be called from br anywhere in the program. Once it has been executed the program continues to execute from br wherever it was before. The idea is illustrated in Figure 5.4. At some point in the main br program there is an instruction ‘Call SR1’. Program execution then switches to Subroutine 1, br identified by its label. The subroutine must end with a ‘Return from Subroutine’ instruction. br Program execution then continues from the instruction after the Call instruction. A little later br in the program another subroutine is called, followed a little later by another call to the br first routine. br TheactionoftheCallinstructionistwo-fold.ItsavesthecontentsoftheProgramCounteronto br theStacksothattheCPUwillknowwheretocomebacktoafterithasfinishedthesubroutine. br It then loads the subroutine start address into the Program Counter. Program execution thus br continuesatthesubroutine.ThereturninstructioncomplementstheactionoftheCall.Itloads br theProgramCounterwiththedataheldatthetopoftheStack,whichwillbetheaddressofthe br instruction following the Call instruction. Program execution then continues at this address. br SubroutineCallandReturninstructionsmustalwaysworkinpairs.GothroughProgramming br Exercise 5.3 to find out what happens if they don’t.