barcode reader in asp.net The Program Counter and Stack in Software

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The Program Counter and Stack
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Understanding how the program counter (PC) works and taking advantage of its design to provide conditional execution in your application code are two of the more dif cult things you will have to learn with the PIC microcontroller. Looking across the different families of PIC microcontroller devices, implementing gotos, calls, and table writes (writing to the program counter registers directly) will seem inconsistent and dif cult to understand. Actually, these operations work according to a similar philosophy in the different architectures, and once you understand it, they really won t seem all that scary. In this section I explain how the program counter works in mid-range devices, and later in this chapter I will discuss the minor differences in the program counters used in the other PIC microcontroller families. The mid-range s program counter can be represented by the block diagram in Fig. 6.14. When you see this diagram for the rst time, it probably will seem very complex. As I work through this section, I will explain how the different parts of the program counter work and how they interrelate. When I discuss the low-end and PIC18 program counters, I will present similar block diagrams for you to work through. In all PIC microcontroller devices, instructions take one word or address. This is part of the Reduced Instruction Set Computing (RISC) philosophy that is used for the design. This may mean that there is not suf cient space in a goto or call instruction for the
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THE PROGRAM COUNTER AND STACK
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Figure 6.14 Mid-range PIC microcontroller program counter and stack subsystem block diagram.
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entire address of the new location of the program counter. A certain number of the address s least signi cant bits are put in the instruction. These bits in the instruction are directly related to the page size of the PIC microcontroller. Tables are an important feature of the PIC microcontroller that allow for conditional jumping or data access. Many of the applications presented in this book use tables for user interfaces or conditional execution. Tables are code artifacts in which the program counter is written to force a jump to a speci c location in program memory. The least signi cant 8 bits of the program counter can be accessed by application software via the PCL register. Writing to these bits will change the program counter to a new value. When the 8 least signi cant bits are written to the PCL, the remaining, more signi cant, bits are taken from the PCLATH register and concatenated to the 8 bits written to PCL. The value in the PCLATH register is written into the program counter any time PCL is changed. This is also true for goto and call instructions, but it works somewhat differently in these cases. To demonstrate how this works, you could consider the example of wanting to jump to address 0x01234 within a mid-range PIC microcontroller s program memory using a direct write to the program counter. First, the value 0x012 is written into the PCLATH register. Next, the value 0x034 is written into the PCL register. When the write to the PCL register is made, the upper bits of the program counter are loaded from the PCLATH register. This operation could be modeled as
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PCLATH = 0x012; PCL = 0x034; // // // // // // // Set the PCLATH Value Change the Program Counter Program Counter = (PCLATH << 8) + PCL = (0x012 << 8) + 0x034 = 0x01200 + 0x034 = 0x01234
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THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
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addwf PCL, f instruction operation.
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Another way of approaching how the write to the PIC microcontroller s program counter is to look at the block diagram of the PIC microcontroller program counter hardware to see how the data ows from the processor into the program counter. In Fig. 6.15, the addwf PCL, f instruction, which adds the current value in PCL to the contents of the w register and puts the result back into the program counter, is shown. In the diagram, you can see that the PCLATH bits are combined with the data coming out of the ALU after the addition operation and then are passed back to the 13-bit counter (the actual PIC microcontroller program counter) through the 3-to-1 mux (multiplexor). When the addwf PCL, f instruction is executed, 8 bits of data are added to the program counter. This means that only 256 unique addresses can be accessed (they can be anywhere in the PIC microcontroller s program memory because the PCLATH register will provide the upper address bits). While a table size of 255 seems to be the maximum, there are some tricks that you can do that will increase the size signi cantly. In each PIC microcontroller, a page is the number to the power of 2 instructions that can be conveniently jumped within using the available bits in the instruction. The page size for the low-end PIC microcontroller is determined by the 9-bit address that is embedded in the 12-bit instruction. These 9 bits can address 512 (0x0200) instructions, which is the low-end PIC microcontroller s page size. In mid-range devices, 11 bits are used for the address within an instruction, which gives the devices a 2,048 (0x0800) instruction page size. Any address within a page can be accessed directly by a goto or call instruction. The addresses speci ed by gotos and calls are zero-based within the page and are not relative to the location of the goto or call instruction. This is an important point and one that can be confusing because in the assembly-language instructions; goto and call instructions can jump to instructions that are relative to the goto and call instructions without regard to the start of the page.
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