diff --git a/debug.h b/debug.h
index 932bbf9..e10ce33 100644
--- a/debug.h
+++ b/debug.h
@@ -26,13 +26,13 @@
 
 // -------------- TOS debugging --------------
 
-#if 1
+#if 0
 #define tos_debugf(...) iprintf(__VA_ARGS__)
 #else
 #define tos_debugf(...)
 #endif
 
-#if 1
+#if 0
 // ikbd debug output in red
 #define ikbd_debugf(a, ...) iprintf("\033[1;31mIKBD: " a "\033[0m\n", ##__VA_ARGS__)
 #else
diff --git a/tos.c b/tos.c
index 1d329a7..8407b56 100644
--- a/tos.c
+++ b/tos.c
@@ -93,7 +93,15 @@ static void hexdump(void *data, unsigned long size, unsigned long offset) {
   }
 }
 
+static void mist_bus_request(char req) {
+  EnableFpga();
+  SPI(req?MIST_BUS_REQ:MIST_BUS_REL);
+  DisableFpga();
+}
+
 static void mist_memory_read(char *data, unsigned long words) {
+  mist_bus_request(1);
+
   EnableFpga();
   SPI(MIST_READ_MEMORY);
 
@@ -105,6 +113,7 @@ static void mist_memory_read(char *data, unsigned long words) {
 
   DisableFpga();
 
+  mist_bus_request(0);
 }
 
 static void mist_memory_read_block(char *data) {
@@ -117,6 +126,8 @@ static void mist_memory_read_block(char *data) {
 }
 
 static void mist_memory_write(char *data, unsigned long words) {
+  mist_bus_request(1);
+
   EnableFpga();
   SPI(MIST_WRITE_MEMORY);
 
@@ -126,6 +137,8 @@ static void mist_memory_write(char *data, unsigned long words) {
   }
 
   DisableFpga();
+
+  mist_bus_request(0);
 }
 
 static void mist_memory_write_block(char *data) {
@@ -284,10 +297,6 @@ static void handle_fdc(unsigned char *buffer) {
       SPI(MIST_ACK_DMA);
       DisableFpga(); 
     }
-  } else {
-    EnableFpga();
-    SPI(MIST_NAK_DMA);
-    DisableFpga(); 
   }
 }  
 
diff --git a/tos.h b/tos.h
index 92a5829..59d72d4 100644
--- a/tos.h
+++ b/tos.h
@@ -13,7 +13,8 @@
 #define MIST_SET_CONTROL  0x04
 #define MIST_GET_DMASTATE 0x05   // reads state of dma and floppy controller
 #define MIST_ACK_DMA      0x06   // acknowledges a dma command
-#define MIST_NAK_DMA      0x07   // don't acknowledge a dma command, but release bus
+#define MIST_BUS_REQ      0x07   // request bus
+#define MIST_BUS_REL      0x08   // release bus
 
 // tos sysconfig bits:
 // 0     - RESET