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PIC18 INSTRUCTION SET
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i = ArrayVar(3) movlw 2 addwf FSR, f movf INDF, w movwf i movlw -2 addwf FSR, f
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// ; ; ; ;
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Simulate an array read Calculate Offset to 3rd Element Get the 3rd Element and store it Restore FSR to point to rst element
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This code has to rst add 2 to the current address in the FSR, followed by loading and storing the third element and then returning the index pointer to the rst element in the array. Now, compare this to the same code for the PIC18 in which FSR0 (which means that the PLUSW0 register will be used to access the data) points to the start of ArrayVar.
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; i = ArrayVar(3) movlw 2 movff PLUSW0, i // ; ; Simulate an array read Want Offset to the 3rd Element Move Contents of ArrayVar(3) into i
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The PREINC and POSTDEC INDF registers can be used for popping and pushing, respectively, data onto a stack pointed to by an FSR register. The POSTDEC INDF register is used for the push operation because it will allow the access of pushed data using the PLUSW INDF register as shown in the previous example. Using FSR0 for the stack, the byte push function could be as simple as:
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BytePush: movff i, POSTDEC0 return ; Push the contents of i onto the stack
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and the byte pop could be:
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BytePop: movff PREINC0, i return ; Pop the top of the stack
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The PLUSW INDF register comes in useful for high level functions in which data has been pushed onto the stack to implement temporary variables. In the example below, I have speci ed a function that uses a data stack and with the parameters and local variables (the same thing) being pushed onto a stack implemented with FSR0:
; int StackDemo(char i, char j) // i is stack top, j is one less ; { ; char k = 0; // k is at two less than stack top movlw 0 ; Initialize k to zero movwf POSTDEC, 0 ; ; i = j + k; // Perform a basic calculation movlw 2 ; Get offset to j
USING THE PIC MCU INSTRUCTION SET
movff movlw movf addwf movlw movff
Temp, PLUSW0 1 PLUSW0 Temp, f, 0 3 PLUSW0, Temp
; ; ; ; ;
Store value in Temp Get offset to k Add k to j in Temp Get offset to i Store result
While this code may look very complex, it is actually simple and, once you are comfortable with it, very easy to implement. This capability is also critical for ef cient implementation of compilers that implement local variables as shown here.
DATA PROCESSING INSTRUCTIONS
The PIC18 has some added exibility and conventional capabilities compared to the other PIC microcontroller processors. As you look through the PIC18 instruction set, you will see that the additions and modi cations to its instruction set make it more similar to that of other processors while retaining the PIC18 s ability to create very ef cient code. The most signi cant addition to the PIC18 s data processing instructions is the subfwb (Fig. 7.49) instruction. This instruction carries out a subtract operation with borrow in the order most people are familiar with if they have worked with other processors. Instead of the typical PIC microcontroller subtraction instruction:
Result = (Source Value) WREG [- !C]
the subfwb instruction executes as:
Result = WREG (Source Value) - !C
Program Memory
PC Program Counter Stack ALU
Register Space
File Registers
Instruction Bit Pattern: 12345678 12345678f f f f 010101da ffff Register Address Bus
STATUS WREG BSR
Instruction Operation: Destination = WREG - Reg - !C
Fast Stack FSR
Instruction Register/ Decode Second Instruction Register
Notes: This instruction behaves like a Traditional Subtract and is different from the Standard subtraction Instructions Available in the other PICmicro architectures
Flags Affected: N, OV, C, DC, Z Instruction Cycles: 1
Figure 7.49 The subfwb instruction provides the expected subtract operation instead of the addition of the negated value of WREG used by the other subtract instructions.
PIC18 INSTRUCTION SET
This instruction frees you from the need of thinking backwards when subtraction instructions are used in an application. To use the subfwb instruction, WREG is loaded with the value to be subtracted from (the subtend) and the value to take away (the subtractor) is speci ed in the instruction. This means that if you have the statement:
A = B C
the values of the expression can be loaded in the same left to right order as the PIC microcontroller instructions and use the sequence:
bcf movf subfwb movwf STATUS, C, 0 B, w, 0 C, w, 0 A, 0
This is the same order as would be used in most other processors. Note that I reset the carry ag before the instruction sequence to avoid any possibilities of the carry being reset unexpectedly and taking away an extra 1, which will be very hard to nd in application code. A PIC18 16-bit subtraction operation could be:
bcf movf subfwb movwf movf subfwb movwf STATUS, C B, w, 0 C, w, 0 A, 0 B + 1, w, 0 C + 1, w, 0 A + 1, 0
Or if you want to save on the instruction used to clear the carry ag at the start of the sequence:
movf subwf movwf movf subfwb movwf C, w, 0 B, w, 0 A, 0 B + 1, w, 0 C + 1, w, 0 A + 1, 0
Another difference between the PIC18 and the other PIC microcontroller processors is the inclusion of the negf (Fig. 7.50) instruction, which can negate any register in the PIC18 s register space. The single instruction cycle multiply instructions multiply the contents of WREG against the contents of another register (mulfw) or a constant (mullw) and store the 16-bit product in the PRODH:PRODL register combination (Fig. 7.51). These instructions are very well behaved and will work for 2 s complement numbers and can provide you with some basic digital signal processing (DSP) capabilities in the PIC18.
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