Test Cases:

  load/hold accumulators

  accumulate through zero, one cycle mode, left/right major
  accumulate through max, one cycle mode, left/right major
  accumulate through zero, two cycle mode, left/right major
  accumulate through max, two cycle mode, left/right major
  
  dAdx adders
  
  x_offset multiply

  y_offset multiply

  clamp, underflow/overflow




Misc Info:

  There are 9 accumulators in the st module for R,G,B,A,Z,S,T,W,L

  for the RGBA accumulators, the number formats are:
    dadx: 		s,10.11 (s,10.2 for offsets)
    dady: 		s,10.2
    start point: 	10.11
    offsets:		0.2 
    result:       	8.0 (clamped)

  for the Z accumulator, the number formats are:
    dadx: 		s,15.16 (s,15.6 for offsets)
    dady: 		s,15.6
    start point: 	s,15.16
    offsets:		0.2 
    result:       	15.3 (clamped)

  for the STWL accumulators, the number formats are:
    dadx: 		s,15.16 
    start point: 	s,15.16
    result:       	s,15.0 (not clamped)

  The clamps are the Akeley variety, which means:
    Carry one extra bit of precision through all computations
    and never clamp until the last computation is done (e.g. never clamp
    intermediate values, especially those that continue to be interpolated).
    When done, clamp the result if the MSB (extra bit) is one.  In this case,
    clamp to all zeros if the next most significant bit is 1, and to all ones
    if the next most significant bit is zero.  (To undo the wrap.)