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Destination = Source - w
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THE MID-RANGE INSTRUCTION SET
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w
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Figure 7.25 The subwf instruction subtracts the contents of the w register from the contents of the parameter register.
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which is what you would probably expect, subtraction in the PIC microcontroller is actually:
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Destination = Source + (-w)
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The negative w term of the equation above is found by substituting in the 2 s complement negation formula:
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Negative = (Positive ^ 0x0FF) + 1
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for -w, which makes the true operation of the instruction:
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Destination = Source + (w ^ 0x0FF) + 1
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I nd that when I am using the instruction it helps to remember this formula, because I can easily understand what subwf is doing and predict how it will behave. Remembering this formula also helps me to understand how the carry ags work. Looking at the instruction above, the carry and digit carry ags probably run counterintuitively to what you expect (and may have experienced with other processors). For example, if you were to subtract 2 from 1 in the PIC:
Source = 1 w = 2 Instruction = subwf Source, w
the formula:
Destination = Source + (w ^ 0xFF) + 1
USING THE PIC MCU INSTRUCTION SET
is used, which yields (for the subwf Source, w instruction):
w = = = = = Source + (w ^ 0xFF) + 1 1 + (2 ^ 0xFF) + 1 1 + 0xFD + 1 0xFF -1
This result matches what you would expect. Note that in this case the carry ag is reset, which is not what we expect in a typical processor. If a negative result was produced in a traditional processor, we would expect the carry (or an explicit borrow) ag to be set. Working through the same instruction (subwf Source, w) and the registers were loaded with:
Source = 2 w = 1
The subwf formula values become:
w = = = = Source + (w ^ 0xFF) + 1 2 + (1 ^ 0xFF) + 1 2 + 0xFE + 1 0x101
The value 0x1 (0x101 AND 0xFF) is the value actually stored in w. But in this case (subtracting a lower value from a higher value), the carry ag (and, possibly, the digit carry ag) are set. If you work through an example where the contents of w were equal to Source, you will nd that the result is 0x100 and the carry ag will also be set in this situation. The carry ag can be used as a negative active borrow ag in your PIC microcontroller applications. When the result is less than zero, the carry ag is reset, indicating that if there were higher order bits, one would have to be taken away (or borrowed ) from them. When the result of a subtraction in the PIC microcontroller is zero or a value greater than zero, the carry ag is reset, indicating that the result is not negative and there is no need to take away a value from any higher order bits. For the next program to run in MPLAB IDE, go through the subtraction examples above and before the subwf instruction is executed for each case, try to predict the operation of the carry ag, followed by the digit carry ag.
movlw 1 movwf 0x20 movlw 2 subwf 0x20, w movlw 2 movwf 0x20 movlw 1 subwf 0x20, w ; First test case, 1 2
; Predict the value of C after instruction ; Second test case, 2 1
; Predict the value of C after instruction
THE MID-RANGE INSTRUCTION SET
Figure 7.26 The literal value subtraction instruction, sublw, operation.
Before going on, I recommend that you change the program so the values in le register 0x20 as well as WREG are arbitrary and before the subwf instruction executes, predict the value that will be stored in WREG as well as the value of the carry ag. I still nd myself thinking through to the operation of the instruction to make sure I can understand what the value of carry will be after a subtraction instruction for different values. The ability to accurately predict the result of the subtraction instruction is something that comes through practice and seeing the behavior of the instruction over different circumstances. The sublw subtracts the value in w from the literal value of the instruction as shown in Fig. 7.26. The concept of having the contents of WREG being subtracted from a constant probably doesn t seem to make sense, but it is a very useful instruction once you become comfortable with it. The invocation of the sublw instruction is quite straightforward and is:
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