Download DB038 - Van Ooijen Technische Informatica

DB038 : A PIC experiments board
version 2.00
placement of code and data in the PIC address ranges. The presence of a linker script in the list of project
files selects relocatable mode, the absence selects absolute mode.
3.2 Data load and store
There MOVLW instruction loads the W register with a literal. The MOVWF instruction saves the W register
to a memory location. These instructions do not affect the flags. The MOVFW instruction that loads the W
register from a memory location is technically an arithmetic instruction: it affects the zero flag, and its
destination can be either the W registers or (!) the memory location. The CLRW instructions clears the W
register, the CLRF instruction clears a memory location. Both set the zero flag.
Operation
W := n
[ a ] := W
W := [ a ]
W := 0
[ a ] := 0
Opcode
MOVLW n
MOVWF a
MOVF a, W
MOVFW a
CLRW
CLRF a
Notes
affects Z
sets Z
sets Z
Note that the assembler accepts “MOVFW a” as synonym for “MOVF a,W”.
3.3 Arithmetic and logic instructions
The monadic (one-operand) instructions operate on the content of a memory location specified in the
instruction. The result can be stored either in the memory location or in W. These instructions include
increment, decrement, rotate left, rotate right, swap nibbles, and (!) move.
Operation
Increment
Decrement
Rotate right
Rotate left
Swap nibbles
Move
Opcode for
W := f( [ a ] )
INCF a, W
DECF a, W
RRF a, W
RLF a, W
SWAPF a, W
MOVF a, W
notes
[ a ] = f( [ a ] )
INCF a, F
DECF a, F
RRF a, F
RLF a, F
SWAPF a, F
MOVF a, F
affects Z
affects Z
involves C
involves C
affects no flags
affects Z
Note the rather peculiar instruction “MOVF a, F”. This instruction copies the content of a memory location
to … that same memory location. Its only useable effect is that it sets or clears the Z flag depending on
whether the content is zero or not.
The dyadic (two-operand) arithmetic instructions have two forms. The first form is like the monadic
instructions. It operates on a memory location specified in the instruction, uses the W register as second
operand, and can store the result either in the memory location or in W. The second form operates on an 8bit constant, uses the W register as second operand, and stores the result in the W register. The arithmetic
operations that can be done in both forms are add, subtract, and, or and xor. Note that W is the second
operand, not the first. This makes a difference for subtraction only, but has confused many people.
Add and subtract affect all three flags. Rotate involves the carry bit. The others affect only the zero flag,
except swap nibbles which affects no flag at all. For subtracting the carry flag is set when no carry occurs
and cleared when a carry occurs, so it is often referred to as the carry - /borrow flag: carry refers to the carry
for addition, /borrow refers to the negated (the / is used to indicate negation) borrow for subtraction.
(c) Van Ooijen Technische Informatica - Wouter van Ooijen
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