diff --git a/Components/DiskII/IWM.cpp b/Components/DiskII/IWM.cpp
index ac8143317..61d71ba76 100644
--- a/Components/DiskII/IWM.cpp
+++ b/Components/DiskII/IWM.cpp
@@ -175,7 +175,15 @@ void IWM::write(int address, uint8_t input) {
 				bit 6: 1 = MZ-reset.
 				bit 7: reserved for future expansion.
 			*/
+
 			mode_ = input;
+
+			switch(mode_ & 0x18) {
+				case 0x00:		bit_length_ = Cycles(24);		break;	// slow mode, 7Mhz
+				case 0x08:		bit_length_ = Cycles(12);		break;	// fast mode, 7Mhz
+				case 0x10:		bit_length_ = Cycles(32);		break;	// slow mode, 8Mhz
+				case 0x18:		bit_length_ = Cycles(16);		break;	// fast mode, 8Mhz
+			}
 			printf("IWM mode is now %02x\n", mode_);
 		break;
 
diff --git a/Components/DiskII/IWM.hpp b/Components/DiskII/IWM.hpp
index 1329d330e..c63dfe465 100644
--- a/Components/DiskII/IWM.hpp
+++ b/Components/DiskII/IWM.hpp
@@ -53,6 +53,8 @@ class IWM {
 		Cycles cycles_until_motor_off_;
 
 		void access(int address);
+
+		Cycles bit_length_;
 };