diff --git a/src/scat.c b/src/scat.c
index 4846501a9..78f6b52d3 100644
--- a/src/scat.c
+++ b/src/scat.c
@@ -56,7 +56,7 @@ void scat_set_xms_bound(uint8_t val)
         uint32_t max_xms_size = (mem_size >= 16384) ? 0xFC0000 : mem_size << 10;
 	int i;
 
-        switch (val)
+        switch (val & 0x0F)
         {
                 case 1:
                 scat_xms_bound = 0x100000;
@@ -96,19 +96,21 @@ void scat_set_xms_bound(uint8_t val)
                 break;
         }
 
-        if ((scat_regs[SCAT_EXTENDED_BOUNDARY] & 0x40) == 0 && (scat_regs[SCAT_DRAM_CONFIGURATION] & 0x0F) == 3)
+        if ((val & 0x40) == 0 && (scat_regs[SCAT_DRAM_CONFIGURATION] & 0x0F) == 3)
         {
                 if (val != 1)
                 {
+                        if(mem_size > 1024) mem_mapping_disable(&ram_high_mapping);
                         for(i=0;i<6;i++)
                                 mem_mapping_enable(&scat_shadowram_mapping[i]);
-                        if (val == 0)
+                        if ((val & 0x0F) == 0)
                                 scat_xms_bound = 0x160000;
                 }
                 else
                 {
                         for(i=0;i<6;i++)
                                 mem_mapping_disable(&scat_shadowram_mapping[i]);
+                        if(mem_size > 1024) mem_mapping_enable(&ram_high_mapping);
                 }
                 pclog("Set XMS bound(%02X) = %06X(%dKbytes for EMS access)\n", val, scat_xms_bound, (0x160000 - scat_xms_bound) >> 10);
                 if (scat_xms_bound > 0x100000)
@@ -118,6 +120,10 @@ void scat_set_xms_bound(uint8_t val)
         }
         else
         {
+                for(i=0;i<6;i++)
+                        mem_mapping_disable(&scat_shadowram_mapping[i]);
+                if(mem_size > 1024) mem_mapping_enable(&ram_high_mapping);
+
                 if (scat_xms_bound > max_xms_size)
                         scat_xms_bound = max_xms_size;
                 pclog("Set XMS bound(%02X) = %06X(%dKbytes for EMS access)\n", val, scat_xms_bound, ((mem_size << 10) - scat_xms_bound) >> 10);
@@ -144,7 +150,7 @@ uint32_t get_scat_addr(uint32_t addr, scat_t *p)
                         addr |= (addr & 0x180000) << 2;
         }
 
-        if ((scat_regs[SCAT_EXTENDED_BOUNDARY] & 0x40) == 0 && (scat_regs[SCAT_DRAM_CONFIGURATION] & 0x0F) == 3 && addr >= 0x100000 && addr < 0x160000)
+        if ((scat_regs[SCAT_EXTENDED_BOUNDARY] & 0x40) == 0 && (scat_regs[SCAT_DRAM_CONFIGURATION] & 0x0F) == 3 && (addr & ~0x600000) >= 0x100000 && (addr & ~0x600000) < 0x160000)
                 addr ^= mem_size < 2048 ? 0x1F0000 : 0x670000;
         return addr;
 }
@@ -246,8 +252,7 @@ void scat_write(uint16_t port, uint8_t val, void *priv)
                         {
                                 if((val & 0x0F) == 3)
                                 {
-                                        if(mem_size > 640) mem_mapping_disable(&scat_A000_BFFF_mapping);
-                                        if(mem_size > 768) mem_mapping_disable(&ram_mid_mapping);
+                                        if(mem_size > 1024) mem_mapping_disable(&ram_high_mapping);
                                         for(index=0;index<6;index++)
                                                 mem_mapping_enable(&scat_shadowram_mapping[index]);
                                 }
@@ -255,13 +260,15 @@ void scat_write(uint16_t port, uint8_t val, void *priv)
                                 {
                                         for(index=0;index<6;index++)
                                                 mem_mapping_disable(&scat_shadowram_mapping[index]);
-                                        if(mem_size > 640 && (val & 0x0F) > 3)
-                                        {
-                                                mem_mapping_enable(&scat_A000_BFFF_mapping);
-                                                if(mem_size > 768) mem_mapping_enable(&ram_mid_mapping);
-                                        }
+                                        if(mem_size > 1024) mem_mapping_enable(&ram_high_mapping);
                                 }
                         }
+                        else
+                        {
+                                for(index=0;index<6;index++)
+                                        mem_mapping_disable(&scat_shadowram_mapping[index]);
+                                if(mem_size > 1024) mem_mapping_enable(&ram_high_mapping);
+                        }
                         scat_map_update = 1;
 
                         cpu_waitstates = (val & 0x70) == 0 ? 1 : 2;
@@ -270,9 +277,9 @@ void scat_write(uint16_t port, uint8_t val, void *priv)
                         scat_reg_valid = 1;
                         break;
                         case SCAT_EXTENDED_BOUNDARY:
-                        scat_set_xms_bound(val & 0x0f);
+                        scat_set_xms_bound(val & 0x4f);
                         mem_set_mem_state(0x40000, 0x60000, (val & 0x20) ? MEM_READ_EXTERNAL | MEM_WRITE_EXTERNAL : MEM_READ_INTERNAL | MEM_WRITE_INTERNAL);
-                        scat_map_update = 1;
+                        if((val ^ scat_regs[SCAT_EXTENDED_BOUNDARY]) & 0x40) scat_map_update = 1;
                         scat_reg_valid = 1;
                         break;
                         case SCAT_ROM_ENABLE: