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// ============================================================================
//        __
//   \\__/ o\    (C) 2006-2016  Robert Finch, Stratford
//    \  __ /    All rights reserved.
//     \/_//     robfinch<remove>@finitron.ca
//       ||
//
// This source file is free software: you can redistribute it and/or modify 
// it under the terms of the GNU Lesser General Public License as published 
// by the Free Software Foundation, either version 3 of the License, or     
// (at your option) any later version.                                      
//                                                                          
// This source file is distributed in the hope that it will be useful,      
// but WITHOUT ANY WARRANTY; without even the implied warranty of           
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the            
// GNU General Public License for more details.                             
//                                                                          
// You should have received a copy of the GNU General Public License        
// along with this program.  If not, see <http://www.gnu.org/licenses/>.    
//
//	fpAddsub.v
//  - floating point adder/subtracter
//  - two cycle latency
//  - can issue every clock cycle
//  - parameterized width
//  - IEEE 754 representation
//
//	This adder/subtractor handles denormalized numbers.
// It has a two cycle latency.
// The output format is of an internal expanded representation
// in preparation to be fed into a normalization unit, then
// rounding. Basically, it's the same as the regular format
// except the mantissa is doubled in size, the leading two
// bits of which are assumed to be whole bits.
// ============================================================================
//
module fpAddsub(clk, ce, rm, op, a, b, o);
parameter WID = 32;
localparam MSB = WID-1;
localparam EMSB = WID==80 ? 14 :
                  WID==64 ? 10 :
				  WID==52 ? 10 :
				  WID==48 ? 10 :
				  WID==44 ? 10 :
				  WID==42 ? 10 :
				  WID==40 ?  9 :
				  WID==32 ?  7 :
				  WID==24 ?  6 : 4;
localparam FMSB = WID==80 ? 63 :
                  WID==64 ? 51 :
				  WID==52 ? 39 :
				  WID==48 ? 35 :
				  WID==44 ? 31 :
				  WID==42 ? 29 :
				  WID==40 ? 28 :
				  WID==32 ? 22 :
				  WID==24 ? 15 : 9;
 
localparam WX = 3;
localparam FX = (FMSB+1)*2-1;	// the MSB of the expanded fraction
localparam EX = FX + WX + EMSB + 1;
 
input clk;		// system clock
input ce;		// core clock enable
input [2:0] rm;	// rounding mode
input op;		// operation 0 = add, 1 = subtract
input [WID-1:0] a;	// operand a
input [WID-1:0] b;	// operand b
output [EX+1:0] o;	// output
 
 
// variables
wire so;			// sign output
wire [EMSB:0] xo;	// de normalized exponent output
reg [EMSB:0] xo1;	// de normalized exponent output
wire [FX+WX:0] mo;	// mantissa output
reg [FX+WX:0] mo1;	// mantissa output
 
// There's an extra bit output in the mantissa to allow for three whole
// digits which the normalizer uses.
assign o = {so,xo,mo};
 
// operands sign,exponent,mantissa
wire sa, sb;
wire [EMSB:0] xa, xb;
wire [FMSB:0] ma, mb;
wire [FMSB+1:0] fracta, fractb;
wire [FMSB+1:0] fracta1, fractb1;
 
// which has greater magnitude ? Used for sign calc
wire xa_gt_xb = xa > xb;
wire xa_gt_xb1;
wire a_gt_b = xa_gt_xb || (xa==xb && ma > mb);
wire a_gt_b1;
wire az, bz;	// operand a,b is zero
 
wire adn, bdn;		// a,b denormalized ?
wire xaInf, xbInf;
wire aInf, bInf, aInf1, bInf1;
wire aNan, bNan, aNan1, bNan1;
 
wire [EMSB:0] xad = xa|adn;	// operand a exponent, compensated for denormalized numbers
wire [EMSB:0] xbd = xb|bdn; // operand b exponent, compensated for denormalized numbers
 
fpDecompose #(WID) u1a (.i(a), .sgn(sa), .exp(xa), .man(ma), .fract(fracta), .xz(adn), .vz(az), .xinf(xaInf), .inf(aInf), .nan(aNan) );
fpDecompose #(WID) u1b (.i(b), .sgn(sb), .exp(xb), .man(mb), .fract(fractb), .xz(bdn), .vz(bz), .xinf(xbInf), .inf(bInf), .nan(bNan) );
 
// Figure out which operation is really needed an add or
// subtract ?
// If the signs are the same, use the orignal op,
// otherwise flip the operation
//  a +  b = add,+
//  a + -b = sub, so of larger
// -a +  b = sub, so of larger
// -a + -b = add,-
//  a -  b = sub, so of larger
//  a - -b = add,+
// -a -  b = add,-
// -a - -b = sub, so of larger
wire realOp = op ^ sa ^ sb;
wire realOp1;
wire op1;
 
// Find out if the result will be zero.
wire resZero = (realOp && xa==xb && ma==mb) ||	// subtract, same magnitude
			   (az & bz);		// both a,b zero
 
// Compute output exponent
//
// The output exponent is the larger of the two exponents,
// unless a subtract operation is in progress and the two
// numbers are equal, in which case the exponent should be
// zero.
 
always @(xaInf,xbInf,resZero,xa,xb,xa_gt_xb)
	xo1 = (xaInf&xbInf) ? xa : resZero ? 0 : xa_gt_xb ? xa : xb;
 
// Compute output sign
reg so1;
always @*
	case ({resZero,sa,op,sb})	// synopsys full_case parallel_case
	4'b0000: so1 <= 0;			// + + + = +
	4'b0001: so1 <= !a_gt_b;	// + + - = sign of larger
	4'b0010: so1 <= !a_gt_b;	// + - + = sign of larger
	4'b0011: so1 <= 0;			// + - - = +
	4'b0100: so1 <= a_gt_b;		// - + + = sign of larger
	4'b0101: so1 <= 1;			// - + - = -
	4'b0110: so1 <= 1;			// - - + = -
	4'b0111: so1 <= a_gt_b;		// - - - = sign of larger
	4'b1000: so1 <= 0;			//  A +  B, sign = +
	4'b1001: so1 <= rm==3;		//  A + -B, sign = + unless rounding down
	4'b1010: so1 <= rm==3;		//  A -  B, sign = + unless rounding down
	4'b1011: so1 <= 0;			// +A - -B, sign = +
	4'b1100: so1 <= rm==3;		// -A +  B, sign = + unless rounding down
	4'b1101: so1 <= 1;			// -A + -B, sign = -
	4'b1110: so1 <= 1;			// -A - +B, sign = -
	4'b1111: so1 <= rm==3;		// -A - -B, sign = + unless rounding down
	endcase
 
delay2 #(EMSB+1) d1(.clk(clk), .ce(ce), .i(xo1), .o(xo) );
delay2 #(1)      d2(.clk(clk), .ce(ce), .i(so1), .o(so) );
 
// Compute the difference in exponents, provides shift amount
wire [EMSB:0] xdiff = xa_gt_xb ? xad - xbd : xbd - xad;
wire [6:0] xdif = xdiff > FMSB+3 ? FMSB+3 : xdiff;
wire [6:0] xdif1;
 
// determine which fraction to denormalize
wire [FMSB+1:0] mfs = xa_gt_xb ? fractb : fracta;
wire [FMSB+1:0] mfs1;
 
// Determine the sticky bit
wire sticky, sticky1;
redor64 u1 (.a(xdif), .b({mfs,2'b0}), .o(sticky) );
 
// register inputs to shifter and shift
delay1 #(1)      d16(.clk(clk), .ce(ce), .i(sticky), .o(sticky1) );
delay1 #(7)      d15(.clk(clk), .ce(ce), .i(xdif),   .o(xdif1) );
delay1 #(FMSB+2) d14(.clk(clk), .ce(ce), .i(mfs),    .o(mfs1) );
 
wire [FMSB+3:0] md1 = ({mfs1,2'b0} >> xdif1)|sticky1;
 
// sync control signals
delay1 #(1) d4 (.clk(clk), .ce(ce), .i(xa_gt_xb), .o(xa_gt_xb1) );
delay1 #(1) d17(.clk(clk), .ce(ce), .i(a_gt_b), .o(a_gt_b1) );
delay1 #(1) d5 (.clk(clk), .ce(ce), .i(realOp), .o(realOp1) );
delay1 #(FMSB+2) d5a(.clk(clk), .ce(ce), .i(fracta), .o(fracta1) );
delay1 #(FMSB+2) d6a(.clk(clk), .ce(ce), .i(fractb), .o(fractb1) );
delay1 #(1) d7 (.clk(clk), .ce(ce), .i(aInf), .o(aInf1) );
delay1 #(1) d8 (.clk(clk), .ce(ce), .i(bInf), .o(bInf1) );
delay1 #(1) d9 (.clk(clk), .ce(ce), .i(aNan), .o(aNan1) );
delay1 #(1) d10(.clk(clk), .ce(ce), .i(bNan), .o(bNan1) );
delay1 #(1) d11(.clk(clk), .ce(ce), .i(op), .o(op1) );
 
// Sort operands and perform add/subtract
// addition can generate an extra bit, subtract can't go negative
wire [FMSB+3:0] oa = xa_gt_xb1 ? {fracta1,2'b0} : md1;
wire [FMSB+3:0] ob = xa_gt_xb1 ? md1 : {fractb1,2'b0};
wire [FMSB+3:0] oaa = a_gt_b1 ? oa : ob;
wire [FMSB+3:0] obb = a_gt_b1 ? ob : oa;
wire [FMSB+4:0] mab = realOp1 ? oaa - obb : oaa + obb;
 
always @*
	casex({aInf1&bInf1,aNan1,bNan1})
	3'b1xx:		mo1 = {1'b0,op1,{FMSB-1{1'b0}},op1,{FMSB{1'b0}}};	// inf +/- inf - generate QNaN on subtract, inf on add
	3'bx1x:		mo1 = {1'b0,fracta1[FMSB+1:0],{FMSB{1'b0}}};
	3'bxx1: 	mo1 = {1'b0,fractb1[FMSB+1:0],{FMSB{1'b0}}};
	default:	mo1 = {mab,{FMSB-1{1'b0}}};	// mab has an extra lead bit
	endcase
 
delay1 #(FX+WX+1) d3(.clk(clk), .ce(ce), .i(mo1), .o(mo) );
 
endmodule
 
module fpAddsub_tb();
reg clk;
wire ce = 1'b1;
wire [2:0] rm = 3'b0;
wire [57:0] o1,o2,o3,o4,o5,o6;
wire [35:0] o11,o12,o13;
wire [31:0] o21,o22,o23;
 
initial begin
  clk = 1'b0;
end
always #10 clk = ~clk;
 
fpAddsub u1 (clk, ce, rm, 1'b0, 32'h0, 32'h0, o1);  // zero plus zero
fpAddsub u2 (clk, ce, rm, 1'b1, 32'h0, 32'h0, o2);  // zero minus zero
fpAddsub u3 (clk, ce, rm, 1'b0, 32'h3F000000, 32'h3F000000, o3);  // .5 + .5
fpAddsub u4 (clk, ce, rm, 1'b0, 32'h43520000, 32'h41700000, o4);  // 210+15
fpAddsub u5 (clk, ce, rm, 1'b1, 32'hC3520000, 32'hC1700000, o5);  // -210- -15
 
fpNormalize u11 (clk, ce, 1'b0, o3, o11);
fpNormalize u12 (clk, ce, 1'b0, o4, o12);
fpNormalize u13 (clk, ce, 1'b0, o5, o13);
 
fpRound u21 (3'd1, o11, o21);         // zero for zero
fpRound u22 (3'd1, o12, o22); // 
fpRound u23 (3'd1, o13, o23); // 
 
endmodule