diff --git a/src/libsys/Makefile b/src/libsys/Makefile
index 23e10fb..c356f9b 100644
--- a/src/libsys/Makefile
+++ b/src/libsys/Makefile
@@ -5,6 +5,7 @@
 BOARD ?= ml402
 TLB ?= no
 ENDIANESS ?= eb
+DEBUGGER ?= mips
 
 		
 ifeq ($(ENDIANESS),el)
@@ -12,8 +13,21 @@ ifeq ($(ENDIANESS),el)
 else
   ENDIAN_FLAGS=-EB
 endif
-		  
-CFLAGS=$(ENDIAN_FLAGS) -g -msoft-float -O2 -march=r3000 -I. -Iboards/$(BOARD) -D$(BOARD) -DWITH_ROM_GDBSTUB
+
+ifeq ($(DEBUGGER),gdb)
+	DBG_FLAGS := -DWITH_GDB_STUB
+	DBG_OBJ := gdb_stub.o
+endif
+ifeq ($(DEBUGGER),mips)
+	DBG_FLAGS := 
+	DBG_OBJ := mips_dbg.o mips_dis.o
+endif
+ifeq ($(DEBUGGER),rom)
+	DBG_FLAGS := -DWITH_ROM_DEBUGGER
+endif
+
+  
+CFLAGS=$(ENDIAN_FLAGS) -g -msoft-float -O2 -march=r3000 -I. -Iboards/$(BOARD) -D$(BOARD) $(DBG_FLAGS)
 
 ifeq ($(TLB),yes)
   CFLAGS+=-DSYS_IO_BASE=0xAC000000 -DSDRAM_BASE=0x80000000 -DFLASH_BASE_IO=0xA4000000 -DFLASH_BASE_MEM=0x88000000
@@ -31,8 +45,8 @@ OBJDUMP=mips-elf-objdump
 OBJCOPY=mips-elf-objcopy
 FLASHGEN=flashgen
 
-OBJS-ml402=$(addprefix $(BUILD_DIR)/, libsys.o xcpt.o syscalls.o cop0.o irq.o mips_dbg.o mips_dis.o mips_ps2.o mips_gfx.o console.o screen.o uart.o board.o gpio.o gdb_stub.o)
-OBJS-denano=$(addprefix $(BUILD_DIR)/, libsys.o xcpt.o syscalls.o cop0.o irq.o mips_dbg.o mips_dis.o console.o uart.o board.o gpio.o gdb_stub.o)
+OBJS-ml402=$(addprefix $(BUILD_DIR)/, libsys.o xcpt.o syscalls.o cop0.o irq.o $(DBG_OBJ) console.o uart.o board.o gpio.o mips_ps2.o mips_gfx.o screen.o)
+OBJS-denano=$(addprefix $(BUILD_DIR)/, libsys.o xcpt.o syscalls.o cop0.o irq.o $(DBG_OBJ) console.o uart.o board.o gpio.o)
 
 all: $(BUILD_DIR) $(BUILD_DIR)/libsys.a
 	
diff --git a/src/libsys/boards/denano/board.c b/src/libsys/boards/denano/board.c
index 3eb7a19..6a80d69 100644
--- a/src/libsys/boards/denano/board.c
+++ b/src/libsys/boards/denano/board.c
@@ -12,17 +12,29 @@
 #include "../../regdef.h"
 
 extern uart_if_t uart_if[];
-extern void handle_exception (unsigned long *registers);
 
-int gdb_stub(struct xcptcontext * xcp)
+#ifdef WITH_GDB_STUB
+extern void handle_exception (unsigned long *registers);
+#else
+extern int dbg_handler(struct xcptcontext * xcp);
+#endif
+
+int dbg_stub(struct xcptcontext * xcp) __attribute__ ((section (".dbg_stub"))) __attribute__ ((used));
+int dbg_stub(struct xcptcontext * xcp)
 {
+#ifdef WITH_GDB_STUB
 	handle_exception((unsigned long *)xcp);
 	return 0;
+#else
+	return dbg_handler(xcp);
+#endif
+	
 }
-#ifdef WITH_ROM_GDBSTUB
+
+#ifdef WITH_ROM_DEBUGGER
 const fp_xcpt_t dbg_func = (fp_xcpt_t)0xbfc01fc0;
 #else
-const fp_xcpt_t dbg_func = (fp_xcpt_t)gdb_stub;
+const fp_xcpt_t dbg_func = (fp_xcpt_t)dbg_stub;
 #endif
 
 #define ARRAYSIZE(a)	(sizeof(a)/sizeof(a[0]))
diff --git a/src/libsys/cop0.c b/src/libsys/cop0.c
index 50c845b..beca1ad 100644
--- a/src/libsys/cop0.c
+++ b/src/libsys/cop0.c
@@ -144,7 +144,6 @@ void DCACHE_invalidate_at(uint32_t* pPtr)
 	(
 		"mtc0   %[pPtr], $31\n"
 		"cop0	35\n"
-		"nop"				// fix needed: don't work without nop on de-nano
 		:
 		: [pPtr] "r" (pPtr)
 	);
diff --git a/src/libsys/screen.c b/src/libsys/screen.c
index 2de688f..37d46d4 100755
--- a/src/libsys/screen.c
+++ b/src/libsys/screen.c
@@ -100,7 +100,7 @@ void cg_writechar(void const *pInst, char c)
 	
 }
 
-void cg_set_csr_x(vga_if_t *pIF, uint32_t coord)
+void cg_set_csr_x(vga_if_t const *pIF, uint32_t coord)
 {
 	assert (pIF);
 
@@ -109,7 +109,7 @@ void cg_set_csr_x(vga_if_t *pIF, uint32_t coord)
 		*pIF->pCSRX = coord;
 }
 
-void cg_set_csr_y(vga_if_t *pIF, uint32_t coord)
+void cg_set_csr_y(vga_if_t const *pIF, uint32_t coord)
 {
 	assert (pIF);
 
@@ -118,7 +118,7 @@ void cg_set_csr_y(vga_if_t *pIF, uint32_t coord)
 		*pIF->pCSRY = coord;
 }
 
-void cg_clr_line(vga_if_t *pIF)
+void cg_clr_line(vga_if_t const *pIF)
 {
 	assert (pIF);
 
@@ -126,7 +126,7 @@ void cg_clr_line(vga_if_t *pIF)
 		*pIF->pCTRL |= SYS_VGA_BIT_CLRLINE;
 }
 
-void cg_clr_screen(vga_if_t *pIF)
+void cg_clr_screen(vga_if_t const *pIF)
 {
 	assert (pIF);
 
diff --git a/src/libsys/screen.h b/src/libsys/screen.h
index 2a357da..1c78719 100644
--- a/src/libsys/screen.h
+++ b/src/libsys/screen.h
@@ -26,10 +26,10 @@ typedef struct _svga_if_t
 extern "C" {
 #endif
 
-void cg_set_csr_x(vga_if_t *pIF, uint32_t coord);
-void cg_set_csr_y(vga_if_t *pIF, uint32_t coord);
-void cg_clr_line(vga_if_t *pIF);
-void cg_clr_screen(vga_if_t *pIF);
+void cg_set_csr_x(vga_if_t const *pIF, uint32_t coord);
+void cg_set_csr_y(vga_if_t const *pIF, uint32_t coord);
+void cg_clr_line(vga_if_t const *pIF);
+void cg_clr_screen(vga_if_t const *pIF);
 
 uint32_t Screen_get_resx(void);
 uint32_t Screen_get_resy(void);
diff --git a/src/testbench_denano.c b/src/testbench_denano.c
index 1d3e3cd..86228c3 100644
--- a/src/testbench_denano.c
+++ b/src/testbench_denano.c
@@ -833,6 +833,7 @@ int main (void)
 
 	g_cnt = 1;
 	gpio_init(&gpio_if, SYS_GPIO_0_BASE);
+	gpio_write(&gpio_if, GPIO_0_MASK_LED, GPIO_0_ALIGN_LED, GPIO_DIR_OFFSET, GPIO_0_MASK_LED);
 
 	*pTim0_cnt = 0;
 	*pTim1_cnt = 0;