diff --git a/toolbox/toolbox.cpp b/toolbox/toolbox.cpp
index 08bc833..e23d3ed 100644
--- a/toolbox/toolbox.cpp
+++ b/toolbox/toolbox.cpp
@@ -352,6 +352,10 @@ namespace ToolBox {
 				d0 = Utility::GetString(trap);
 				break;
 
+			case 0xa85d:
+				d0 = Utility::BitTst(trap);
+				break;
+
 			default:
 				fprintf(stderr, "Unsupported tool trap: %04x\n", trap);
 				fprintf(stderr, "pc: %08x\n", cpuGetPC());
diff --git a/toolbox/utility.cpp b/toolbox/utility.cpp
index 0277808..dd32278 100644
--- a/toolbox/utility.cpp
+++ b/toolbox/utility.cpp
@@ -78,5 +78,34 @@ namespace Utility {
 		return d0;
 	}
 
+	// FUNCTION BitTst (bytePtr: Ptr; bitNum: LONGINT) : BOOLEAN;
+	uint16_t BitTst(uint16_t trap)
+	{
+		/* 
+		 * BitTst tests whether a given bit is set and returns TRUE if so or 
+		 * FALSE if not. The bit is specified by bitNum, an offset from the 
+		 * high-order bit of the byte pointed to by bytePtr.
+		 */
+
+		uint32_t bytePtr;
+		uint32_t bitNum;
+		uint16_t rv;
+		uint32_t sp;
+
+		sp = StackFrame<8>(bytePtr, bitNum);
+		Log("%04x BitTst($%08x, $%08x)\n", trap, bytePtr, bitNum);
+
+
+		 uint32_t offset = bitNum >> 3;
+		 int mask = 0x80 >> (bitNum & 0x07);
+
+		 uint32_t value = memoryReadByte(bytePtr + offset);
+
+		 rv = value & mask ? 1 : 0;
+		 ToolReturn<2>(sp, rv);
+
+		 return 0;
+	}
+
 
 }
diff --git a/toolbox/utility.h b/toolbox/utility.h
index 926d7f4..6635c91 100644
--- a/toolbox/utility.h
+++ b/toolbox/utility.h
@@ -8,6 +8,8 @@ namespace Utility
 
 	uint16_t NewString(uint16_t trap);
 	uint16_t GetString(uint16_t trap);
+
+	uint16_t BitTst(uint16_t trap);
 }
 
 #endif