diff --git a/SheepShaver/src/kpx_cpu/src/cpu/ppc/ppc-execute.cpp b/SheepShaver/src/kpx_cpu/src/cpu/ppc/ppc-execute.cpp
index fc0e6e4d..e678dd89 100644
--- a/SheepShaver/src/kpx_cpu/src/cpu/ppc/ppc-execute.cpp
+++ b/SheepShaver/src/kpx_cpu/src/cpu/ppc/ppc-execute.cpp
@@ -749,6 +749,35 @@ void powerpc_cpu::execute_stwcx(uint32 opcode)
 	increment_pc(4);
 }
 
+/**
+ *	Basic (and probably incorrect) implementation for missing functions
+ *	in older C libraries
+ **/
+
+#ifndef signbit
+#define signbit(X) my_signbit(X)
+#endif
+
+static inline bool my_signbit(double X) {
+	return X < 0.0;
+}
+
+#ifndef isless
+#define isless(X, Y) my_isless(X, Y)
+#endif
+
+static inline bool my_isless(double X, double Y) {
+	return X < Y;
+}
+
+#ifndef isgreater
+#define isgreater(X, Y) my_isgreater(X, Y)
+#endif
+
+static inline bool my_isgreater(double X, double Y) {
+	return X > Y;
+}
+
 /**
  *	Floating-point compare instruction
  *