PIC and C operators

Addressing modes

The PIC architecture has several addressing modes that are needed to access C arrays, structures, and pointers.

• File register — sum
• Register direct — W5
• Register indirect — [W6]
• Immediate — #2012

Types of register indirect addressing

• No modification: [Wn]
• Pre-increment: [++Wn]
• Pre-decrement: [--Wn]
• Post-increment: [Wn++]
• Post-decrement: [Wn--]
• Register offset: [Wn+Wb]

The ++ and -- increment or decrement by two for word operations and by one for byte operations. Also, the following is not allowed because the offset register differs in the two fields.
    MOV  [W5+W6],[W7+W8]

Literal offset

MOV allows signed literal register offsets as either source or destination. Here are a couple of examples:

    MOV    W5, [W6+32]
    MOV.B  W5, [W6+15]
    MOV    [W7-1000], W13
    MOV    [W4+5], [W8+13]         ;; ILLEGAL -- only one literal offset
    MOV    [W7+2000], W13          ;; ILLEGAL -- offset too large
    MOV.B  [W7+1000], W13          ;; ILLEGAL -- offset too large for byte

Places to use these addressing modes

• Arrays — V[i]
• Structures — P.x
• Pointers — *P
• Local varaibles

Some C operators and their PIC equivalents

In these examples, Wb, Wn, Wns, and Wnd are registers; Ws and Wd are registers with indirect addressing modes; Wso and Wdo are registers with indirect and offset addressing modes; lit10 is a 10-bit literal (0 to 1023), and lit5 is a 5-bit literal (0 to 31).

• 16-bit +
  •     ADD    f
  •     ADD    f,WREG
  •     ADD    Wb,Ws,Wd
  •     ADD    Wb,#lit5,Wd
  •     ADD    #lit10,Wn
• 8-bit +
  • Replace ADD with ADD.B in the the 16-bit + examples.
• 16-bit -
  •     SUB    f
  •     SUB    f,WREG
  •     SUB    Wb,Ws,Wd
  •     SUB    Wb,#lit5,Wd
  •     SUB    #lit10,Wn
  •     SUBR   f
  •     SUBR   f,WREG
  •     SUBR   Wb,Ws,Wd
  •     SUBR   #lit10,Wn
• 8-bit -
  • Replace SUB with SUB.B and SUBR with SUBR.B in the the 16-bit - examples.
• 16-bit ++
  •     INC    f
  •     INC    f,WREG
  •     INC    Ws,Wd
• 8-bit ++
  • Replace INC with DEC in the the 16-bit ++ examples.
• 16-bit +=2
  • Replace INC with INC2 in the the 16-bit ++ examples.
• 8-bit +=2
  • Replace INC with INC2.B in the the 16-bit ++ examples.
• 16-bit --
  • Replace INC with DEC in the the 16-bit ++ examples.
• 8-bit --
  • Replace INC with DEC.B in the the 16-bit ++ examples.
• 16-bit -=2
  • Replace INC with DEC2 in the the 16-bit ++ examples.
• 8-bit -=2
  • Replace INC with DEC2.B in the the 16-bit ++ examples.
• 16-bit &
  • Replace ADD with AND in the the 16-bit + examples.
• 8-bit &
  • Replace ADD with AND.B in the the 16-bit + examples.
• 16-bit |
  Replace ADD with IOR in the the 16-bit + examples.
• 8-bit |
  • Replace ADD with IOR.B in the the 16-bit + examples.
• 16-bit ^
  • Replace ADD with XOR in the the 16-bit + examples.
• 8-bit ^
  Replace ADD with XOR.B in the the 8-bit + examples.
• 16-bit ~
  • Replace INC with COM in the the 16-bit ++ examples.
• 8-bit ~
  • Replace INC with COM.B in the the 16-bit ++ examples.
• 16-bit clear – set all bits to zero
  •     CLR    f
  •     CLR    WREG
  •     CLR    Wd
• 8-bit clear – set all bits to zero
  • Replace CLR with CLR.B in the the 16-bit clear examples.
• 16-bit set – set all bits to one
  • Replace CLR with SETM in the the 16-bit clear examples.
• 8-bit set – set all bits to one
  • Replace CLR with SETM.B in the the 16-bit clear examples.

Uses of these instructions

• x = y + a ;
• x = y + 10 ;
• x = y + 1000 ;
• x = ++y + a ;
• x = 3*x + 1 ;
• x = x && 0xBEEF ;
• x = 0 ;

C declarations

You cannot depend on short, int, and long in C. Use the fixed width types of C99 and C++.