ID: CP-Dave_P3

Proposer: David Bishop

Status:	  Proposed (03/05/2003)
          Seconded by Rob Anderson.
	  To be analyzed
	  To be resolved

• fixed_pkg.vhd package for the fixed point standard
• fixed_pkg_body.vhd synthesizable body
• test_fixed.vhd, test_fixed2.vhd tests for the above code

Sub proposals:
• There are two ways to implement this:
  1. Modify the "signed" and "unsigned" types in 1076.3 to be "array (integer range <>)" instead of the current "array (natural range <>)" and use anything that is a negative index to be the part of the number less than zero. For positive indices the functionality will not change. (Problem, what to do with "signed (-3 to 3)"?).
  2. Created a new type called "fixed" which will be a "array (integer range <>) of std_logic" that will be used for fixed point arithmetic. (signed or unsigned?)
  Please tell me what you think. Note that this discussion will parallel one on the Verilog 200x side.
  Resolution: A new type needs to be created. Changing the type of "signed" and "unsigned" would cause funcitonal changes in how these types work and cause existing code problems.
• Create two fixed point representations, one for "signed" and one for "unsigned".
  Resolution: These will be called "sfixed" (signed fixed point) and "ufixed" (unsigned fixed point). Their behavior will be similar to that of the signed and unsigned arithmetic in numeric_std.
• Allow for "little endian" representations of fixed point numbers.
  Normally the fixed point representation will be "ufixed (7 downto -8)" however may people expressed the need for "ufixed (-8 to 7)" to be supported and to be treated as having the same direction as the downto example.
  Resolution: Will be supported
• Use a record type to store the fixed point number.
  Resolution: Can't be done because arrays in records must have fixed lenghts.
• Bit widths: The following bit widths will be the result of the following operations:

  fixed + fixed := fixed (max(l'high,r'high) downto -(max(-l'low,-r'low)))
     8:-3 + 9:-2 := 9:-3
     numeric_std rule:  + and - = size of largest input strung
  fixed - fixed := fixed (max(l'high,r'high) downto -(max(-l'low,-r'low)))
     8:-3 + 9:-2 := 9:-3
  fixed * fixed := fixed (l'high+r'high downto l'low+r'low)
     8:-3 * 9:-2 := 17:-5
     numeric_std rule:  * = add the sizes together to create result
  fixed / fixed := fixed (l'high downto l'low+r'low)
     8:-3 / 9:-2 := 8:-5
     numeric_std rule:  / = size of left operand.

  Resolution: Still to be decided.