For Format 3 instructions -- 24 bits long
   For base addressing disp is a 12 bit unsigned integer
   For program counter addressing disp is a 12 bit signed
      integer using 2's complement notation
For Format 3 instructions e = 0:
   relative addressing
   b = 1, p = 0 --- TA = (B) + disp   0<=disp<=4095
   b = 0, p = 1 --- TA = (PC) + disp  -2048<=disp<=2047
   direct addressing
   b = 0, p = 0 --- TA = disp (fmt 3,  e = 0 ) or 
                    TA = address (fmt 4, e = 1)

   Any of the above addressing can be combined with indexed
register.
   x = 1 add (X) to TA.

Bits i and n determine how TA is used.
   immediate addressing
   n = 0, i = 1 --- TA is used as operand, no memory is referenced
   indirect addressing
   n = 1, i = 0 --- word at TA is fetched as and value there
      is used as address of operand
   simple addressing
   n = 0, i = 0 --- TA is address of operand; p, b, and e are
part of the address
   n = 1, i = 1 --- TA is address of operand