3 files changed, 9 insertions, 5 deletions
diff --git a/techlibs/xilinx/cells_sim.v b/techlibs/xilinx/cells_sim.v
index 5410983ae..1262fc8c1 100644
--- a/techlibs/xilinx/cells_sim.v
+++ b/techlibs/xilinx/cells_sim.v
@@ -516,7 +516,7 @@ module DSP48E1 (
         if (PCIN != 48'b0)          $fatal(1, "Unsupported PCIN value");
         if (CARRYIN != 1'b0)        $fatal(1, "Unsupported CARRYIN value");
 `endif
-        Pr[42:0] <= Ar[24:0] * Br;
+        Pr[42:0] <= $signed(Ar[24:0]) * $signed(Br);
     end
 
     generate
diff --git a/techlibs/xilinx/dsp_map.v b/techlibs/xilinx/dsp_map.v
index da1d6f3a9..2063c45e2 100644
--- a/techlibs/xilinx/dsp_map.v
+++ b/techlibs/xilinx/dsp_map.v
@@ -1,4 +1,4 @@
-module \$__MUL25X18 (input [24:0] A, input [17:0] B, output [42:0] Y);
+module \$__MUL25X18 (input [23:0] A, input [16:0] B, output [40:0] Y);
 	wire [47:0] P_48;
 	DSP48E1 #(
 		// Disable all registers
@@ -20,8 +20,8 @@ module \$__MUL25X18 (input [24:0] A, input [17:0] B, output [42:0] Y);
 		.PREG(0)
 	) _TECHMAP_REPLACE_ (
 		//Data path
-		.A({5'b0, A}),
-		.B(B),
+		.A({6'b0, A}),
+		.B({1'b0, B}),
 		.C(48'b0),
 		.D(24'b0),
 		.P(P_48),
diff --git a/techlibs/xilinx/synth_xilinx.cc b/techlibs/xilinx/synth_xilinx.cc
index 796615211..815bf0848 100644
--- a/techlibs/xilinx/synth_xilinx.cc
+++ b/techlibs/xilinx/synth_xilinx.cc
@@ -284,8 +284,12 @@ struct SynthXilinxPass : public ScriptPass
 
 			run("techmap -map +/cmp2lut.v -D LUT_WIDTH=6");
 
+			// The actual behaviour of the Xilinx DSP is a signed 25x18 multiply
+			// Due to current limitations of mul2dsp, we are actually mapping as a 24x17
+			// unsigned multiply with MSBs set to 1'b0
+
 			if (!nodsp || help_mode)
-				run("techmap -map +/mul2dsp.v -D DSP_A_MAXWIDTH=25 -D DSP_B_MAXWIDTH=18 -D DSP_NAME=$__MUL25X18");
+				run("techmap -map +/mul2dsp.v -D DSP_A_MAXWIDTH=24 -D DSP_B_MAXWIDTH=17 -D DSP_NAME=$__MUL25X18");
 
 			run("alumacc");
 			run("share");