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When you rst see the results of a compiler converting high level source code into assembly language, you will probably feel like the process of changing high level language source code into processor assembly code is more a result of magic than a series of mathematical operations executed by a computer. As well as converting the source code into assembly language, modern compilers also look for opportunities to simplify the code that is output, resulting in smaller and more ef cient applications. If you are a beginner with PIC microcontroller assembly language development, you should not be surprised to discover that modern compilers can produce more ef cient assembly code than you can. The low-end and mid-range PIC microcontrollers may seem like they are poorly designed for compilers to develop ef cient code for them, but ef cient compilers can be created for them reasonably easily if the compiler is well thought out beforehand. Throughout this book, you will see statements like:
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A = B + (C * D);
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TABLE 3.4 THE FIVE BASIC TYPES OF HIGH LEVEL PROGRAMMING STATEMENTS STATEMENT FORMAT TYPE
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variable = . . . if ( . . . ) label ( . . . ) type variable [= constant] type label(variable, . . . )
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Assignment statement Conditional test statement Subroutine/function call Variable declaration Subroutine/function declaration
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To convert this statement into assembly language, a number of steps have to be carried out. Most compilers take advantage of a processor data stack for working through these types of instructions and keeping track of temporary values. When processing the statements, the values are pushed onto the stack in the reverse order that they are required. When the statement is executed, this data is popped off the stack and processed. Because most of the PIC microcontroller architectures do not have a built-in data stack, the compiler developer will have to decide how to push and pop the data. To explain how compilers work, I will introduce you to their operation using a stack and then discuss the options available in the PIC microcontroller. The rst thing a compiler does is determine the type of statement it has to work on next. In most high level languages, there are the ve types of statements listed in Table 3.4. In the rst types (assignment statement, conditional execution, and subroutine/function call), the part or the statement shown as . . . can be considered as the statement. This is the data that will be put on the data stack and then executed. Putting the statement in a heap stored in a post x order (the least signi cant operations given the highest priority) does this. For the operation in the assignment statement:
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A = B + (C * D);
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The post x heap is shown in Fig. 3.4. Next, the order of operations is determined by pulling the data from the heap in a pre x (left to right) order. In this operation, the lowest operation on the left is pushed onto the stack followed by the lowest on the right and then the values required for the operations
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A = B + (C * D) + B C * D
Figure 3.4 Compiler post x heap for a high level language statement.
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are pushed onto the stack. When the operator executes, it pops the previous two (or one) stack elements and then pushes the result onto the stack. The stack operations for:
A = B + (C * D);
are:
Push Push Execute Push Execute Pop D C * B + A
In this sequence of stack operations, D, followed by C, is pushed onto the stack. Next, they are popped off the stack and multiplied together and the product is pushed onto the stack. The product of C * D is popped off the stack along with B and added together with the result pushed onto the stack. To nish off the instruction, the nal result B+ (C*D) is popped from the stack and stored in A. Depending on your age, you may remember Hewlett-Packard calculators that worked this way. Data entry took the form of the stack instructions listed above and were known as reverse polish notation (RPN). It took a bit of getting used to, but once you were able to think in RPN it actually was easier working through complex problems because you didn t have to remember how deep the parentheses of the expression were. In universities and colleges all over the world, the truly cool people could think in RPN and not use a pencil and paper to plan out how they were going to enter statements into their calculators. Leaving a result on the stack is important for the other two types of statements. In the if statement, if the value on the top of the stack is not equal to zero, then the condition is determined to be true. As will be discussed below, for the subroutine/function call statement, the parameters passed to the subroutine/function are accessed from the stack by the subroutine/function code according to their position relative to the top of the stack. Array elements are also stack values that are popped off when an element is to be accessed. In the PIC microcontroller, a data stack can be implemented using the FSR register. To push an element onto the stack of the mid-range processors, you could use the code:
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