diff --git a/Makefile.target b/Makefile.target
index bab5998..fd46e09 100644
--- a/Makefile.target
+++ b/Makefile.target
@@ -12,7 +12,7 @@ HOSTCC := gcc
 HOSTCFLAGS+= -O2 -g -DFCOMPILER -DBOOTSTRAP $(CROSSCFLAGS)
 HOSTCFLAGS+= -Wall -Wredundant-decls -Wshadow -Wpointer-arith
 HOSTCFLAGS+= -Wstrict-prototypes -Wmissing-declarations -Wundef -Wendif-labels
-HOSTCFLAGS+= -Wstrict-aliasing -Wwrite-strings -Wmissing-prototypes
+HOSTCFLAGS+= -Wstrict-aliasing -Wwrite-strings -Wmissing-prototypes -Wnested-externs
 HOSTCFLAGS+= -W
 HOSTINCLUDES := -I$(SRCDIR)/include -I$(SRCDIR)/kernel -I$(SRCDIR)/kernel/include -I$(ODIR)/target/include
 
@@ -27,7 +27,7 @@ RANLIB := $(TARGET)ranlib
 CFLAGS+= -Os -g -DNATIVE_BITWIDTH_EQUALS_HOST_BITWIDTH -USWAP_ENDIANNESS
 CFLAGS+= -Wall -Wredundant-decls -Wshadow -Wpointer-arith
 CFLAGS+= -Wstrict-prototypes -Wmissing-declarations -Wundef -Wendif-labels
-CFLAGS+= -Wstrict-aliasing -Wwrite-strings -Wmissing-prototypes
+CFLAGS+= -Wstrict-aliasing -Wwrite-strings -Wmissing-prototypes -Wnested-externs
 CFLAGS+= -Werror
 INCLUDES := -I$(SRCDIR)/include -I$(SRCDIR)/kernel/include -I$(ODIR)/target/include
 AS_FLAGS+= -g
diff --git a/arch/ppc/qemu/init.c b/arch/ppc/qemu/init.c
index 7ec67b8..0d0db3a 100644
--- a/arch/ppc/qemu/init.c
+++ b/arch/ppc/qemu/init.c
@@ -50,7 +50,6 @@ struct cpudef {
 static uint16_t machine_id = 0;
 
 extern void unexpected_excep( int vector );
-extern void setup_timers( void );
 
 void
 unexpected_excep( int vector )
diff --git a/arch/sparc32/context.c b/arch/sparc32/context.c
index 9f1dac4..52d48ed 100644
--- a/arch/sparc32/context.c
+++ b/arch/sparc32/context.c
@@ -8,6 +8,7 @@
 #include "context.h"
 #include "libopenbios/sys_info.h"
 #include "boot.h"
+#include "openbios.h"
 
 #define MAIN_STACK_SIZE 16384
 #define IMAGE_STACK_SIZE 4096
@@ -46,7 +47,6 @@ unsigned long __boot_ctx;
 static void start_main(void)
 {
     int retval;
-    extern int openbios(void);
 
     /* Save startup context, so we can refer to it later.
      * We have to keep it in physical address since we will relocate. */
diff --git a/arch/sparc64/context.c b/arch/sparc64/context.c
index 7377fc5..8e2befb 100644
--- a/arch/sparc64/context.c
+++ b/arch/sparc64/context.c
@@ -8,6 +8,7 @@
 #include "context.h"
 #include "libopenbios/sys_info.h"
 #include "boot.h"
+#include "openbios.h"
 
 #define MAIN_STACK_SIZE 16384
 #define IMAGE_STACK_SIZE 4096*2
@@ -46,7 +47,6 @@ unsigned long __boot_ctx;
 static void start_main(void)
 {
     int retval;
-    extern int openbios(void);
 
     /* Save startup context, so we can refer to it later.
      * We have to keep it in physical address since we will relocate. */
diff --git a/arch/x86/context.c b/arch/x86/context.c
index c1fd904..d543f74 100644
--- a/arch/x86/context.c
+++ b/arch/x86/context.c
@@ -12,6 +12,7 @@
 #include "context.h"
 #include "libopenbios/sys_info.h"
 #include "boot.h"
+#include "openbios.h"
 
 #define MAIN_STACK_SIZE 16384
 #define IMAGE_STACK_SIZE 4096
@@ -57,8 +58,6 @@ unsigned long __boot_ctx;
 static void start_main(void)
 {
     int retval;
-    extern int openbios(void);
-    extern void init_exceptions(void);
 
     /* Save startup context, so we can refer to it later.
      * We have to keep it in physical address since we will relocate. */
diff --git a/arch/x86/openbios.c b/arch/x86/openbios.c
index bc388c1..62ef587 100644
--- a/arch/x86/openbios.c
+++ b/arch/x86/openbios.c
@@ -50,8 +50,6 @@ static void init_memory(void)
 static void
 arch_init( void )
 {
-	void setup_timers(void);
-
 	openbios_init();
 	modules_init();
 #ifdef CONFIG_DRIVER_PCI
diff --git a/arch/x86/openbios.h b/arch/x86/openbios.h
index 2d49dbf..a130822 100644
--- a/arch/x86/openbios.h
+++ b/arch/x86/openbios.h
@@ -19,6 +19,9 @@
 
 int openbios(void);
 
+/* entry.S */
+void init_exceptions(void);
+
 /* console.c */
 extern void	cls(void);
 #ifdef CONFIG_DEBUG_CONSOLE
diff --git a/drivers/timer.c b/drivers/timer.c
index 32d5947..863888b 100644
--- a/drivers/timer.c
+++ b/drivers/timer.c
@@ -10,6 +10,7 @@
  */
 
 #include "config.h"
+#include "drivers/drivers.h"
 #include "timer.h"
 #include "asm/io.h"
 
@@ -86,7 +87,6 @@ unsigned long get_timer_freq(void)
 
 void udelay(unsigned int usecs)
 {
-	extern void _wait_ticks(unsigned long);
 	unsigned long ticksperusec = get_timer_freq() / 1000000;
 	_wait_ticks(ticksperusec * usecs);
 }
diff --git a/drivers/timer.h b/drivers/timer.h
index c822665..7e86db3 100644
--- a/drivers/timer.h
+++ b/drivers/timer.h
@@ -48,13 +48,15 @@
 #define	PPCB_SPKR	0x02	/* Bit 1 */
 #define	PPCB_T2GATE	0x01	/* Bit 0 */
 
-extern void setup_timers(void);
 extern void ndelay(unsigned int nsecs);
 extern void udelay(unsigned int usecs);
 extern void mdelay(unsigned int msecs);
 extern unsigned long currticks(void);
 extern unsigned long get_timer_freq(void);
 
+/* arch/ppc/timebase.S */
+void _wait_ticks(unsigned long nticks);
+
 #define TICKS_PER_SEC 1000
 
 #endif	/* TIMER_H */
diff --git a/include/drivers/drivers.h b/include/drivers/drivers.h
index 10cd7ba..6829572 100644
--- a/include/drivers/drivers.h
+++ b/include/drivers/drivers.h
@@ -123,4 +123,8 @@ int keyboard_dataready(void);
 unsigned char keyboard_readdata(void);
 #endif
 #endif
+
+/* drivers/timer.c */
+void setup_timers(void);
+
 #endif /* OPENBIOS_DRIVERS_H */