Load Another Register with Zero in Software

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Load Another Register with Zero
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; Read in Non-Zero Constant ; Test Zero, Note Zero Flag getting Set
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This program rst loads a non-zero value into the le register at address 0x20 and then the simulated processor will test the contents of the le register at 0x21 to see if they are zero. When you rst started up MPLAB IDE and loaded in the InsTemplate.asm project, you probably noticed that most of the registers had zero in them. These values cannot be
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THE MID-RANGE INSTRUCTION SET
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considered to be an accurate representation of the initial values in a PIC microcontroller. As I will repeat throughout this book, you must always initialize all your variables and le registers to ensure you know exactly what the contents of the registers are. When you single-step through the movf 0x20, w instruction, the value in register 0x20 (0x47) will be loaded into WREG. When you execute movf 0x21, f even though WREG does not change, the zero ag of the STATUS register (bit 2) will become set; you can see this in the STATUS register displayed in the Watch window as well as on the bottom toolbar of the MPLAB IDE desktop. To the right of the WREG indicator, there are three ags, z, dc, and c, which represent the three arithmetic result ags of the STATUS register. After executing the movf 0x21, f instruction, you will see that the zero ag will become a capital Z, indicating that the bit is set and showing how the movf instruction can be used to test the contents of a register to see if it is zero. Another way of setting a register value is to use the clear instructions: clrw and clrf (see Fig. 7.13). clrw clears WREG and sets the zero ag, while clrf clears the speci ed register and also sets the zero ag. The clrw instruction does not have any parameters that are invoked by simply entering:
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clrw
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The clrf instruction only has one parameter and that is the register that is speci ed:
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clrf Register
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Storing the contents of w into a register is accomplished using the movwf instruction (Fig. 7.14). This instruction simply loads the speci ed register with the contents of WREG. No STATUS ags are affected by the operation. The format of movwf is:
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Figure 7.13 operation.
The clrf register clear instruction
USING THE PIC MCU INSTRUCTION SET
Figure 7.14 The movwf instruction which copies the contents of w into a register operation.
where Register is the destination register for the contents of WREG. The last test program (demonstrating the operation of the movf instruction) uses the movwf instruction to store values into registers for testing later. Along with the movwf instruction, the option and tris instructions will store the contents of w into speci c registers. None of the STATUS ags are changed (or required to be changed, as will be discussed below) for these instructions. The option instruction (which doesn t have any parameters and whose operation is shown in Fig. 7.15) is speci ed as:
option
The option instruction operation.
THE MID-RANGE INSTRUCTION SET
This instruction copies the contents of w into the OPTION_REG register (at address 0x081), bypassing the need (in the mid-range PIC microcontrollers) to set the RP0 bit of the STATUS register to set the contents of OPTION. As I indicated elsewhere, remember that in the Microchip PIC microcontroller manuals, you will most often see this register referred to as OPTION_REG. The reason for this is due to the same label being given to both the OPTION register and option instruction. In this book, I will use the OPTION_REG convention as much as possible to specify the register instead of the instruction, but I will capitalize the register (as I have done with all registers) and put the instruction in lowercase. tris (shown in Fig. 7.16) is used for loading an I/O port driver enable (TRIS) register with the contents of w in the same manner as the option instruction saves the contents of WREG into OPTION_REG. The TRISA, TRISB, and TRISC registers can be accessed using these instructions, which have the format:
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