diff --git a/c4emu.cpp b/c4emu.cpp
index a7c009cb..65e0f553 100644
--- a/c4emu.cpp
+++ b/c4emu.cpp
@@ -1013,9 +1013,16 @@ void S9xSetC4 (uint8 byte, uint16 Address)
 						printf("$7f4d=%02x, expected 02 for command 10 %02x\n", Memory.C4RAM[0x1f4d], Memory.C4RAM[0x1f4d]);
 				#endif
 					int32	tmp;
-					tmp = SAR((int32) READ_WORD(Memory.C4RAM + 0x1f83) * C4CosTable[READ_WORD(Memory.C4RAM + 0x1f80) & 0x1ff] * 2, 16);
+					int32   r1;
+					r1 = READ_WORD(Memory.C4RAM + 0x1f83);
+					if (r1 & 0x8000)
+					        r1 |= ~0x7fff;
+					else
+					        r1 &= 0x7fff;				
+										
+					tmp = SAR(r1 * C4CosTable[READ_WORD(Memory.C4RAM + 0x1f80) & 0x1ff] * 2, 16);
 					WRITE_3WORD(Memory.C4RAM + 0x1f86, tmp);
-					tmp = SAR((int32) READ_WORD(Memory.C4RAM + 0x1f83) * C4SinTable[READ_WORD(Memory.C4RAM + 0x1f80) & 0x1ff] * 2, 16);
+					tmp = SAR(r1 * C4SinTable[READ_WORD(Memory.C4RAM + 0x1f80) & 0x1ff] * 2, 16);
 					WRITE_3WORD(Memory.C4RAM + 0x1f89, (tmp - SAR(tmp, 6)));
 					break;
 				}