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If you are new to microcontroller programming, you might have asked yourself how exactly are constants loaded into an application. In a PC program, you could load memory addresses with the value to be used in an operation and then read them back during program execution. It is also a possible to use this method in some microcontrollers in which the program memory can be accessed by the ALU during execution. The PIC microcontroller s processor does not have the ability to read directly from its program memory, which means that the only method for using constant values in a program is to include them as part of an arithmetic or Boolean operation instruction. Providing a constant value in an instruction is known as immediate addressing. To provide immediate addressing in the PIC microcontroller architecture, a multiplexor is placed before the ALU to select the data source from either the 8 least signi cant bits of the instruction or the registers of the PIC microcontroller (Fig. 6.11).
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THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
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Figure 6.11 PIC microcontroller processor block diagram modi ed to allow immediate data values that are part of the instruction.
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There will be instances in applications where the ability to address a register directly or to specify a constant value immediately will not be suf cient, and some method of arithmetically specifying an address will be required. To do this, the processor has to calculate the address of the register to be accessed; in the PIC microcontroller, this indexed addressed is carried out by loading the FSR register with the address you want to access. This 8-bit register has some bank considerations for data movement. The contents of the FSR register are multiplexed with the 7 immediate address bits, as shown in Figure 6.12. The format for using indexed addressing is somewhat
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Figure 6.12 Allowing the FSR register to be used in selecting the address of the le register is known as indexed addressing.
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different from that in other processors with which you may be familiar. In other processors, accessing the address pointed to by the index register is implemented by modifying the index register (the register that contains the address to be accessed) such as enclosing it in parenthesis or brackets. For example, you will see instructions like
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Move Accumulator, (Index) with the ; data at the address pointed to ; by Index ; Load the Accumulator
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Specifying indexed addressing in the PIC microcontroller is accomplished by accessing the INDF register, which is a phantom register and does not have any physical hardware. Instead, when the INDF address is accessed, the index or FSR register is selected to provide the address into the register space, as shown in Fig. 6.13. The INDF/FSR mechanism is used in all the different PIC microcontroller processor architectures, even though the memory spaces they are accessing are different. Indexed addressing typically is described in high level languages as specifying the index to an array variable. This method of addressing may be called aray addressing because the array variable simply may be known as an Array. Adding 1 to an array variable could be written out as
Array[ Index ] = Array[ Index ] + 1;
Reading
Instruction Address FSR FSR
Writing
Instruction Address
Access to INDF Specifies FSR as Address Source
INDF INDF
Access to INDF Specifies FSR as Address Source
Register Space
Register Space
Figure 6.13 To access indexed data pointed to by the FSR register, the INDF register is used to select the FSR register to provide the address instead of the least signi cant 7 bits of the instruction as in direct addressing.
THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
The start of the array variable is the label Array, whereas the byte (or element) within it is the Index. When specifying the array variable and element in the PIC microcontroller, the offset to the start of the array variable has to be added to the element number to get the register address within the PIC microcontroller. Thus, to carry out the array increment operation shown above in the PIC microcontroller, the following steps would have to be taken:
w = Index; w = w + Array; // The Element Address is the Index // into the // array variable added to the start of the // array variable FSR = w; // Load the Index register with the // Element Address // Address INDF = INDF + 1; // Increment the Element Address
This example is fairly simple. Accessing array variables that have elements that are larger than 1 byte or cases where the destination is not the same as the source (and a constant isn t added to them) make the operations of the PIC microcontroller somewhat more complex. Single-byte, single-dimensional arrays can be implemented quite easily, as can multidimensional arrays. Multidimensional arrays are treated like single-dimensional arrays, but the index is calculated arithmetically from each parameter (i.e., the index for element 3, 5 in an 8 8 array would be 2 * 8 + 5).
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