read barcode in asp net web application THE MID-RANGE INSTRUCTION SET in Software

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THE MID-RANGE INSTRUCTION SET
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Figure 7.34 Before the program counter is loaded with the 13-bit address from the instruction and PCLATH register in the call instruction, it is stored on the program counter stack.
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instruction available in the low-end devices. This instruction is useful for implementing tables that return constant values. The operation of the retlw instruction:
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retlw Constant
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could be simulated in the mid-range PIC microcontroller by the use of the two instructions:
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movlw Constant return
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Figure 7.35 The return instruction will be pulled from the stack and stored into the program counter.
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USING THE PIC MCU INSTRUCTION SET
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Figure 7.36 The retlw instruction loads literal value into w and returns from subroutine instruction operation.
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The instruction is useful for returning table information (which is explained in the next section) or returning condition information in w that can be tested by the calling program. The instruction loads w with an immediate value before executing a return from interrupt. The ret e instruction is similar to the return instruction except that it is used to return from an interrupt; in fact, the only difference between ret e and return is that the GIE bit in the interrupt control register (INTCON) is set during the ret e instruction. This allows interrupt requests to be acknowledged immediately following the execution of the instruction and the interrupt handler. As I have indicated elsewhere in this book, the PIC microcontroller architecture doesn t use jump on condition instructions. Instead, there are a number of instructions that allow skipping the next instruction in line based on speci c conditions. The basic instructions that carry out this function are the skip on bit condition instructions, btfsc and btfss. These two instructions use the same format as the bit set and reset instructions (bcf and bsf) although they test the condition of a bit rather than specify its state. The btfsc instruction skips over the next instruction if the bit condition is reset (0). The format of the instruction is:
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btfsc Register, Bit
btfss skips over the next instruction if the bit condition is set (1). The format for the instruction is:
btfss Register, Bit
If the bit condition is not true and the skip is not executed, then the btfsc and btfss instructions execute in one instruction cycle. If the condition is true, then the instruction executes in two cycles, essentially treating the following instruction like a nop. These two instructions are used as the basic method of conditionally changing the execution of an application. For example, to jump to an address if the zero ag is set, a goto is added to the btfsc instruction:
THE MID-RANGE INSTRUCTION SET
btfsc STATUS, Z goto Label
; Test Zero, Skip over Next if Zero Flag is Reset ; Zero Flag Set Jump to Label
Notice that in the snippet above I test for the negative condition and skip over the next instruction if it is true. If you learned programming in an environment that didn t have structured programming instructions, you should be familiar for this. To explain what I mean, consider the structured if/else code in C:
if (a == b) { // Instructions to execute if a equals b
} else { // } Instructions to execute if a does NOT equal b
To execute this in an unstructured language, you would code it as:
if (a != b) then goto aNotEqualb; // Instructions that execute if a equals b
goto ConditionEnd aNotEqualb: // Instructions that execute if a does NOT equal b
ConditionEnd:
To implement the nonstructured case, you have to invert the test. To implement the structured if/else C code in PIC assembler, you would write it as:
movf subwf btfss goto // a, w b, w STATUS, Z ANotEqualb ; if (a != b) then goto ANotEqualb
Instructions that execute if a equals b
goto ConditionEnd aNotEqualb: // Instructions that execute if a does NOT equal b
ConditionEnd:
USING THE PIC MCU INSTRUCTION SET
The actual conditional execution code can be written in such a way to simulate structured code, and in fact, when you rst start writing PIC microcontroller assembly language code you can block it out using pseudo C code, like the example above, and then convert it to assembly code using the two steps I show here. Doing the correct bit condition test is something to be cognizant of when you rst start working with the PIC microcontroller. Conditional jumps based on a comparison of two values follow a very de nite format that you can use when you rst start programming. From a high level, the code implements:
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