PC Relative
  Effective address is built by adding the operand to the program counter.
    (How far from here?)

  Usually implemented in conditional branches.
  But sometimes used as an unconditional jump or branch over a block of memory.

  Often a signed value.

  6502: BEQ HERE - HERE is 8 bit value found by compiler by subtracting
    the @ of BEQ's operand (current contents of program counter) from the 
    address of HERE.  This value may be a +/- offset.

  Range of adjustment is often smaller than possible @ range.
    Signed 1 or 2 byte offset value even if system has 20-32 bit address range.

  Advantage is that code is relocatable in memory.

  And smaller command, so fewer or no additional operand fetches.

  Some systems revert to absolute addresses for very large distances
    (supported by CPU and compiler).

  SKIP 
    Skips the next instruction. Like branch, targets the PC, can also be
      classified as using a scalar effective address. 

    Skips are commonly conditional, 
      they test a flag to decide if skip should occur.