Use an HLSL accessor function for reading constant registers, allowing Xbox-native negative indices and out-of-bounds handling. This makes the generated HLSL look closed to the original Xbox vertex shader assembly.

src/core/hle/D3D8/Direct3D9/Xb.hlsl

@@ -21,16 +21,19 @@ struct VS_OUTPUT
 	float4 oT2  : TEXCOORD2; // Texture coordinate set 2
 	float4 oT3  : TEXCOORD3; // Texture coordinate set 3
 };
-
-// Xbox constant registers
-extern uniform float4 c[192] : register(c0);
+
+#define X_D3DSCM_CORRECTION                 96 // Add 96 to arrive at the range 0..191 (instead of -96..95)
+#define X_D3DVS_CONSTREG_COUNT              192
+
+// Xbox constant registers
+uniform float4 C[X_D3DVS_CONSTREG_COUNT] : register(c0);
 
 // Vertex input overrides for SetVertexData4f support
-extern float4 vOverrideValue[16]  : register(c192);
-extern float4 vOverridePacked[4]  : register(c208);
+uniform float4 vOverrideValue[16]  : register(c192);
+uniform float4 vOverridePacked[4]  : register(c208);
 
-extern float4  xboxViewportScale   : register(c212);
-extern float4  xboxViewportOffset  : register(c213);
+uniform float4 xboxViewportScale   : register(c212);
+uniform float4 xboxViewportOffset  : register(c213);
 
 // Overloaded casts, assuring all inputs are treated as float4
 float4 _tof4(float  src) { return float4(src, src, src, src); }
@@ -40,6 +43,20 @@ float4 _tof4(float4 src) { return src; }
 float4 _ssss(float s)    { return float4(s, s, s, s); } // a scalar output replicated across a 4-component vector
 #define _scalar(src) _tof4(src).x /* a scalar input */
 
+float4 c(int register_number)
+{
+	// Map Xbox [-96, 95] to Host [0, 191]
+	// Account for Xbox's negative constant indexes
+    register_number += X_D3DSCM_CORRECTION;
+    if (register_number < 0)
+        return 0;
+    
+    if (register_number >= X_D3DVS_CONSTREG_COUNT) // X_D3DVS_CONSTREG_COUNT
+        return 0;
+
+    return C[register_number];
+}
+
 // http://xboxdevwiki.net/NV2A/Vertex_Shader
 // https://www.khronos.org/registry/OpenGL/extensions/NV/NV_vertex_program.txt
 // https://www.khronos.org/registry/OpenGL/extensions/NV/NV_vertex_program1_1.txt

src/core/hle/D3D8/XbD3D8Types.h

@@ -986,7 +986,7 @@ typedef DWORD X_VERTEXSHADERCONSTANTMODE;
 #define X_D3DSCM_RESERVED_CONSTANT_SCALE -38 // Becomes 58 after correction, contains Scale v
 #define X_D3DSCM_RESERVED_CONSTANT_OFFSET -37 // Becomes 59 after correction, contains Offset
 
-#define X_D3DSCM_CORRECTION                 96 // Add 96 to arrive at the range 0..191 (instead of 96..95)
+#define X_D3DSCM_CORRECTION                 96 // Add 96 to arrive at the range 0..191 (instead of -96..95)
 #define X_D3DVS_CONSTREG_COUNT              192
 
 // Special Registers, used to pass additional information to the shaders

src/core/hle/D3D8/XbVertexShader.cpp

@@ -1601,7 +1601,9 @@ static void OutputHlsl(std::stringstream& hlsl, VSH_IMD_OUTPUT& dest)
 
 	switch (dest.Type) {
 	case IMD_OUTPUT_C:
-		hlsl << "c[" << dest.Address << "]";
+		// Access the HLSL capital C[] constants array, with the index bias applied :
+		// TODO : Avoid out-of-bound writes (perhaps writing to a reserverd index?)
+		hlsl << "C[" << dest.Address + X_D3DSCM_CORRECTION << "]";
 		LOG_TEST_CASE("Vertex shader writes to constant table");
 		break;
 	case IMD_OUTPUT_R:
@@ -1647,18 +1649,19 @@ static void ParameterHlsl(std::stringstream& hlsl, VSH_IMD_PARAMETER& paramMeta)
 
 	int register_number = param.Address;
 	if (param.ParameterType == PARAM_C) {
-		// Map Xbox [-96, 95] to Host [0, 191]
-		// Account for Xbox's negative constant indexes
-		register_number += 96;
+		// Access constant registers through our HLSL c() function,
+		// which allows dumping negative indices (like Xbox shaders),
+		// and which returns zero when out-of-bounds indices are passed in:
 		if (paramMeta.IndexesWithA0_X) {
-			// Only display the offset if it's not 0.
-			if (register_number != 0) {
-				hlsl << "c[a0.x+" << register_number << "]";
+			if (register_number == 0) {
+				hlsl << "c(a0.x)"; // Hide the offset if it's 0
+			} else if (register_number < 0) {
+				hlsl << "c(a0.x" << register_number << ")"; // minus is part of the offset
 			} else {
-				hlsl << "c[a0.x]";
+				hlsl << "c(a0.x+" << register_number << ")"; // show addition character
 			}
 		} else {
-			hlsl << "c[" << register_number << "]";
+			hlsl << "c(" << register_number << ")";
 		}
 	} else {
 		hlsl << RegisterName[param.ParameterType] << register_number;