diff --git a/hw/display/sm501.c b/hw/display/sm501.c
index e47be99451..ca0840f6fa 100644
--- a/hw/display/sm501.c
+++ b/hw/display/sm501.c
@@ -836,27 +836,30 @@ static void sm501_system_config_write(void *opaque, hwaddr addr,
 
     switch (addr) {
     case SM501_SYSTEM_CONTROL:
-        s->system_control = value & 0xE300B8F7;
+        s->system_control &= 0x10DB0000;
+        s->system_control |= value & 0xEF00B8F7;
         break;
     case SM501_MISC_CONTROL:
-        s->misc_control = value & 0xFF7FFF20;
+        s->misc_control &= 0xEF;
+        s->misc_control |= value & 0xFF7FFF10;
         break;
     case SM501_GPIO31_0_CONTROL:
         s->gpio_31_0_control = value;
         break;
     case SM501_GPIO63_32_CONTROL:
-        s->gpio_63_32_control = value;
+        s->gpio_63_32_control = value & 0xFF80FFFF;
         break;
     case SM501_DRAM_CONTROL:
         s->local_mem_size_index = (value >> 13) & 0x7;
         /* TODO : check validity of size change */
-        s->dram_control |=  value & 0x7FFFFFC3;
+        s->dram_control &= 0x80000000;
+        s->dram_control |= value & 0x7FFFFFC3;
         break;
     case SM501_ARBTRTN_CONTROL:
-        s->arbitration_control =  value & 0x37777777;
+        s->arbitration_control = value & 0x37777777;
         break;
     case SM501_IRQ_MASK:
-        s->irq_mask = value;
+        s->irq_mask = value & 0xFFDF3F5F;
         break;
     case SM501_MISC_TIMING:
         s->misc_timing = value & 0xF31F1FFF;