Renamed qsel to uslc and simplified radix2 uslc

src/fpu/fdivsqrt/fdivsqrtstage2.sv

@@ -49,7 +49,7 @@ module fdivsqrtstage2 import cvw::*;  #(parameter cvw_t P) (
 
   // Quotient Selection logic
   // Given partial remainder, select digit of +1, 0, or -1 (up, uz, un)
-  fdivsqrtqsel2 qsel2(WS[P.DIVb+3:P.DIVb], WC[P.DIVb+3:P.DIVb], up, uz, un);
+  fdivsqrtuslc2 uslc2(.WS(WS[P.DIVb+3:P.DIVb]), .WC(WC[P.DIVb+3:P.DIVb]), .up, .uz, .un);
 
   // Sqrt F generation.  Extend C, U, UM to Q4.k
   fdivsqrtfgen2 #(P) fgen2(.up, .uz, .C({2'b11, CNext}), .U({3'b000, U}), .UM({3'b000, UM}), .F);
@@ -60,7 +60,7 @@ module fdivsqrtstage2 import cvw::*;  #(parameter cvw_t P) (
     else if (uz) Dsel = '0;
     else         Dsel = D; // un
 
-  // Residual Generation
+  // Residual Update
   //  WSA, WCA = WS + WC - qD
   mux2 #(P.DIVb+4) addinmux(Dsel, F, SqrtE, AddIn);
   csa #(P.DIVb+4) csa(WS, WC, AddIn, up&~SqrtE, WSA, WCA);

src/fpu/fdivsqrt/fdivsqrtstage4.sv

@@ -31,36 +31,29 @@ module fdivsqrtstage4 import cvw::*;  #(parameter cvw_t P) (
   input  logic [P.DIVb:0]   U,UM,               // U1.DIVb
   input  logic [P.DIVb+3:0] WS, WC,             // Q4.DIVb
   input  logic [P.DIVb+1:0] C,                  // Q2.DIVb
-  input  logic             SqrtE, j1,
+  input  logic              SqrtE, j1,
   output logic [P.DIVb+1:0] CNext,              // Q2.DIVb
-  output logic             un,
+  output logic              un,
   output logic [P.DIVb:0]   UNext, UMNext,      // U1.DIVb
   output logic [P.DIVb+3:0] WSNext, WCNext      // Q4.DIVb
 );
 
   logic [P.DIVb+3:0]        Dsel;               // Q4.DIVb
-  logic [3:0]              udigit;
+  logic [3:0]               udigit;             // {+2, +1, -1, -2} or 0000 for 0
   logic [P.DIVb+3:0]        F;                  // Q4.DIVb
   logic [P.DIVb+3:0]        AddIn;              // Q4.DIVb
-  logic [4:0]              Smsbs;
+  logic [4:0]               Smsbs;              // U1.4
-  logic [2:0]              Dmsbs;
+  logic [2:0]               Dmsbs;              // U0.3   drop leading 1 from D
-  logic [7:0]              WCmsbs, WSmsbs;
+  logic [7:0]               WCmsbs, WSmsbs;     // U4.4
-  logic                    CarryIn;
+  logic                     CarryIn;
   logic [P.DIVb+3:0]        WSA, WCA;           // Q4.DIVb
 
   // Digit Selection logic
-  // u encoding:
-  // 1000 = +2
-  // 0100 = +1
-  // 0000 =  0
-  // 0010 = -1
-  // 0001 = -2
   assign Smsbs  = U[P.DIVb:P.DIVb-4];       // U1.4 most significant bits of square root
   assign Dmsbs  = D[P.DIVb-1:P.DIVb-3];     // U0.3 most significant fractional bits of divisor after leading 1
   assign WCmsbs = WC[P.DIVb+3:P.DIVb-4];    // Q4.4 most significant bits of residual
   assign WSmsbs = WS[P.DIVb+3:P.DIVb-4];    // Q4.4 most significant bits of residual
-
+  fdivsqrtuslc4cmp uslc4(.Dmsbs, .Smsbs, .WSmsbs, .WCmsbs, .SqrtE, .j1, .udigit);
-  fdivsqrtqsel4cmp qsel4(.Dmsbs, .Smsbs, .WSmsbs, .WCmsbs, .SqrtE, .j1, .udigit);
   assign un = 1'b0; // unused for radix 4
 
   // F generation logic

src/fpu/fdivsqrt/fdivsqrtuslc2.sv

@@ -1,10 +1,10 @@
 ///////////////////////////////////////////
-// fdivsqrtqsel2.sv
+// fdivsqrtuslc2.sv
 //
 // Written: David_Harris@hmc.edu, me@KatherineParry.com, cturek@hmc.edu 
 // Modified:13 January 2022
 //
-// Purpose: Radix 2 Quotient Digit Selection
+// Purpose: Radix 2 Unified Quotient/Square Root Digit Selection
 // 
 // Documentation: RISC-V System on Chip Design Chapter 13
 //
@@ -26,22 +26,26 @@
 // and limitations under the License.
 ////////////////////////////////////////////////////////////////////////////////////////////////
 
-module fdivsqrtqsel2 ( 
+module fdivsqrtuslc2 ( 
-  input  logic [3:0] WS, WC, 
+  input  logic [3:0] WS, WC,      // Q4.0 most significant bits of redundant residual
-  output logic       up, uz, un
+  output logic       up, uz, un   // {+1, 0, -1}
 );
  
-  logic        magnitude, sign;
+  logic        sign;
+
+  // Carry chain logic determines if W = WS + WC = -1, < -1, > -1 to choose 0, -1, 1 respectively
  
-  assign magnitude = ~((WS[2]^WC[2]) & (WS[1]^WC[1]) & 
+  //if p2 * p1 * p0, W = -1 and choose digit of 0
+  assign uz = ((WS[2]^WC[2]) & (WS[1]^WC[1]) & 
         (WS[0]^WC[0]));
+
+  // Otherwise determine sign using carry chain: sign = p3 ^ g_2:0
   assign sign = (WS[3]^WC[3])^
       (WS[2] & WC[2] | ((WS[2]^WC[2]) &
           (WS[1]&WC[1] | ((WS[1]^WC[1]) &
             (WS[0]&WC[0])))));
 
   // Produce digit = +1, 0, or -1
-  assign up = magnitude & ~sign;
+  assign up = ~uz & ~sign;
-  assign uz = ~magnitude;
+  assign un = ~uz & sign;
-  assign un = magnitude & sign;
 endmodule

src/fpu/fdivsqrt/fdivsqrtuslc4.sv

@@ -1,10 +1,10 @@
 ///////////////////////////////////////////
-// fdivsqrtqsel4.sv
+// fdivsqrtuslc4.sv
 //
 // Written: David_Harris@hmc.edu, me@KatherineParry.com, cturek@hmc.edu 
 // Modified:13 January 2022
 //
-// Purpose: Radix 4 Quotient Digit Selection
+// Purpose: Table-based Radix 4 Unified Quotient/Square Root Digit Selection
 // 
 // Documentation: RISC-V System on Chip Design Chapter 13
 //
@@ -26,25 +26,25 @@
 // and limitations under the License.
 ////////////////////////////////////////////////////////////////////////////////////////////////
 
-module fdivsqrtqsel4 (
+module fdivsqrtuslc4 (
-  input  logic [2:0] Dmsbs,
+  input  logic [2:0] Dmsbs,             // U0.3 fractional bits after implicit leading 1
-  input  logic [4:0] Smsbs,
+  input  logic [4:0] Smsbs,             // U1.4 leading bits of square root approximation
-  input  logic [7:0] WSmsbs, WCmsbs,
+  input  logic [7:0] WSmsbs, WCmsbs,    // Q4.4 redundant residual most significant bits
   input  logic       Sqrt, j1,
-  output logic [3:0] udigit
+  output logic [3:0] udigit             // {2, 1, -1, -2} digit is 0 if none are hot
 );
-  logic [6:0] Wmsbs;
+  logic [7:0] PreWmsbs;                 // Q4.4 nonredundant residual msbs
-  logic [7:0] PreWmsbs;
+  logic [6:0] Wmsbs;                    // Q4.3 truncated nonredundant residual
-  logic [2:0] A;
+  logic [2:0] A;                        // U0.3 upper bits of D or Smsbs, discarding integer bit
 
-  assign PreWmsbs = WCmsbs + WSmsbs;
+  assign PreWmsbs = WCmsbs + WSmsbs;    // add redundant residual to find msbs
-  assign Wmsbs = PreWmsbs[7:1];
+  assign Wmsbs = PreWmsbs[7:1];         // truncate least significant bit to Q4.3 to index table
   // D = 0001.xxx...
   // Dmsbs = |   |
   // W =      xxxx.xxx...
   // Wmsbs = |        |
 
-  logic [3:0] USel4[1023:0];
+  logic [3:0] USel4[1023:0];            // 1024-bit table indexed with 3 bits of A and 7 bits of Wmsbs
 
   // Prepopulate selection table; this is constant at compile time
   always_comb begin 
@@ -101,10 +101,10 @@ module fdivsqrtqsel4 (
   // Select A
   always_comb
     if (Sqrt) begin 
-      if (j1) A = 3'b101;
+      if (j1) A = 3'b101;                       // on first sqrt iteration        A = .101
-      else if (Smsbs == 5'b10000) A = 3'b111;
+      else if (Smsbs == 5'b10000) A = 3'b111;   // if S = 1.0, use                A = .111
-      else A = Smsbs[2:0];
+      else A = Smsbs[2:0];                      // otherwise use                  A = S (in U0.3 format)
-    end else A = Dmsbs;
+    end else A = Dmsbs;                         // division Unless                A = D (IN U0.3 format, dropping leading 1)
 
   // Select quotient digit from lookup table based on A and W
   assign udigit = USel4[{A,Wmsbs}];

src/fpu/fdivsqrt/fdivsqrtuslc4cmp.sv

@@ -1,10 +1,10 @@
 ///////////////////////////////////////////
-// fdivsqrtqsel4cmp.sv
+// fdivsqrtuslc4cmp.sv
 //
 // Written: David_Harris@hmc.edu, me@KatherineParry.com, cturek@hmc.edu 
 // Modified:13 January 2022
 //
-// Purpose: Comparator-based Radix 4 Quotient Digit Selection
+// Purpose: Comparator-based Radix 4 Unified Quotient/Square Root Digit Selection 
 // 
 // Documentation: RISC-V System on Chip Design Chapter 13
 //
@@ -26,12 +26,12 @@
 // and limitations under the License.
 ////////////////////////////////////////////////////////////////////////////////////////////////
 
-module fdivsqrtqsel4cmp (
+module fdivsqrtuslc4cmp (
   input  logic [2:0] Dmsbs,             // U0.3 fractional bits after implicit leading 1
   input  logic [4:0] Smsbs,             // U1.4 leading bits of square root approximation
-  input  logic [7:0] WSmsbs, WCmsbs,    // Q4.4
+  input  logic [7:0] WSmsbs, WCmsbs,    // Q4.4 residual most significant bits
   input  logic       SqrtE, j1,
-  output logic [3:0] udigit
+  output logic [3:0] udigit             // {2, 1, -1, -2} digit is 0 if none are hot
 );
   logic [6:0] Wmsbs;
   logic [7:0] PreWmsbs;