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want to discuss some advanced aspects of it, including how arrays can be used with structures. Arrays with pointers are simply too complex to implement in the PIC microcontroller assembler to even think about it. Multidimensional arrays have to be de ned according to the amount of space that is required. If you are going to implement a 5 by 5 array of bytes, the CBLOCK directive can be used as follows:
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CBLOCK 0x : Array:3*5 : ENDC
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; De ne a 5 by 5 array
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While the declaration of this array is simple, working with it is not. For example, accessing the byte at array element 2, 3 would require multiplying the rst dimension speci cation by 5 before the second dimension speci cation is added to it. For the C code
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Array[2][3] = 0x012; ; Load Element 2, 3 with 0x012
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the PIC microcontroller assembler code would be
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movf movwf bcf rlf bcf rlf addwf Parm1, w FSR STATUS, C FSR, f STATUS, C FSR, f FSR, f ; ; ; ; Multiply the rst dimension speci ed by 5 Multiply the rst dimension by 4 rst
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movf Parm2, w addwf FSR, f movlw 0x012 movwf INDF
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; Add rst dimension to 4x rst ; dimension to get 5x ; Add the second dimension to 5x rst ; Do assignment at array element 2,3
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Note that the multiply by 5 uses the function of multiplying by a power of 2 and then adding the value again to get the odd multiplier. Without using this trick, this array access code would be much more complex and probably require the use of a temporary variable. To simplify this operation, I would like suggest two improvements. The rst is to change the way the array is declared to 5 by 4 from 5 by 3. Four is a power of 2 and very easy to multiply by. By doing this, 5 bytes are added to the array, and hopefully, this is not a signi cant amount of memory in the application (it could be for something like a low-end PIC microcontroller, where only 16 unique le registers are available in each bank). Second, I would reverse the order in which data is stored in the array. Instead of the rst parameter being multiplied by 5, I want the second parameter to be multiplied by 4. Making these changes, the code becomes
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bcf rlf movwf bcf rlf movf addwf movlw movwf
STATUS, C Parm2, w FSR STATUS, C FSR, f Parm1, w FSR, f 0x012 INDF
; Multiply the second dimension ; speci ed by four
; Add rst dimension to second*4 ; Do assignment at array element 2,3
Making these two changes results in an over 20 percent decrease in application code size. It is debatable whether or not it is easier to read and understand than the rst example (it is for me). Arrays can be used with structures to allow quite complex data-tracking operations. For example, a 5 by 3 array of VarStruct could be created. This would be done in C using the code
struct VarStruct VarStructArray[3][5];
In the MPLAB assembler, this operation is also quite simple:
CBLOCK 0x0 : VarStructArray:3*5*SizeOfVarStruct ; De ne a 5 by 5 VarStruct array : ENDC
To access elements in the two-dimensional array, the same code as above is used, but once the result is calculated, it will have to be multiplied by the size of VarStruct (which is 5). For example, the C code statement
VarStructArray[2][3].varB = A ;
would be implemented in PIC microcontroller assembler as
movf Parm1, w ; Multiply the rst dimension speci ed movwf FSR ; by 5 bcf STATUS, C ; Multiply the rst dimension by 4 rlf FSR, f ; rst bcf STATUS, C rlf FSR, f addwf FSR, f ; Add rst dimension to 4x rst ; dimension to get 5x movf Parm2, w ; Add the second dimension to 5x rst addwf FSR, w ; Save rst * 5 + second and multiply movwf FSR, w ; it by the SizeOfVarStruct which is bcf STATUS, C ; 5
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rlf FSR, f bcf STATUS, C rlf FSR, f addwf FSR, f movlw A movwf INDF
; Do assignment at structure element ; in array element 2,3
Even when working with single-byte multidimensional arrays, make sure that you are always aware of what the ultimate array size is. The preceding example produces the same result as if a three-dimensional array of 3 by 5 by 5 was created. If the array were increased in size to 5 by 5 by 5, 125 le registers would be required, which is outside the capabilities of all the PIC microcontrollers except for the PIC18, unless you are playing around with bank registers (and the base address was desired). Implementing single arrays across multiple register banks is something that I highly recommend you avoid, and if there is no other way of implementing the application, then you should be looking at another microcontroller.
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