diff --git a/src/chipset/scat.c b/src/chipset/scat.c
index 3197285cc..4028cba70 100644
--- a/src/chipset/scat.c
+++ b/src/chipset/scat.c
@@ -1538,7 +1538,7 @@ scat_init(const device_t *info)
 	for (i = 0; i < 16; i++) {
 		mem_mapping_add(&dev->high_mapping[i], (i << 14) + 0xfc0000, 0x04000,
 				bios_read, bios_readw, bios_readl,
-				mem_write_null, mem_write_nullw, mem_write_nulll,
+				NULL, NULL, NULL,
 				rom + ((i << 14) & biosmask), 0, NULL);
 		mem_mapping_enable(&dev->high_mapping[i]);
 	}
@@ -1558,7 +1558,7 @@ scat_init(const device_t *info)
 	for (i = (dev->regs[SCAT_VERSION] < 4 ? 0 : 8); i < 16; i++) {
 		mem_mapping_add(&dev->high_mapping[i], (i << 14) + 0xfc0000, 0x04000,
 				bios_read, bios_readw, bios_readl,
-				mem_write_null, mem_write_nullw, mem_write_nulll,
+				NULL, NULL, NULL,
 				rom + ((i << 14) & biosmask), 0, NULL);
 		mem_mapping_enable(&dev->high_mapping[i]);
 	}
diff --git a/src/dma.c b/src/dma.c
index fca5a4796..54e97f9e6 100644
--- a/src/dma.c
+++ b/src/dma.c
@@ -1284,7 +1284,8 @@ _dma_write(uint32_t addr, uint8_t val, dma_t *dma_c)
 		dma_bm_write(addr, &val, 1, dma_transfer_size(dma_c));
     } else {
 	mem_writeb_phys(addr, val);
-	mem_invalidate_range(addr, addr);
+	if (AT)
+		mem_invalidate_range(addr, addr);
     }
 }
 
@@ -1662,5 +1663,6 @@ dma_bm_write(uint32_t PhysAddress, const uint8_t *DataWrite, uint32_t TotalSize,
 	mem_write_phys((void *) bytes, PhysAddress + n, TransferSize);
     }
 
-    mem_invalidate_range(PhysAddress, PhysAddress + TotalSize - 1);
+    if (AT)
+	mem_invalidate_range(PhysAddress, PhysAddress + TotalSize - 1);
 }
diff --git a/src/include/86box/mem.h b/src/include/86box/mem.h
index d6c93028a..6410d6d30 100644
--- a/src/include/86box/mem.h
+++ b/src/include/86box/mem.h
@@ -318,10 +318,6 @@ extern void	mem_write_ram_2gb(uint32_t addr, uint8_t val, void *priv);
 extern void	mem_write_ram_2gbw(uint32_t addr, uint16_t val, void *priv);
 extern void	mem_write_ram_2gbl(uint32_t addr, uint32_t val, void *priv);
 
-extern void	mem_write_null(uint32_t addr, uint8_t val, void *p);
-extern void	mem_write_nullw(uint32_t addr, uint16_t val, void *p);
-extern void	mem_write_nulll(uint32_t addr, uint32_t val, void *p);
-
 extern int	mem_addr_is_ram(uint32_t addr);
 
 extern uint64_t	mmutranslate_noabrt(uint32_t addr, int rw);
diff --git a/src/mem/mem.c b/src/mem/mem.c
index fcb6ccde2..b895cfb5b 100644
--- a/src/mem/mem.c
+++ b/src/mem/mem.c
@@ -1589,15 +1589,16 @@ uint8_t
 mem_readb_phys(uint32_t addr)
 {
     mem_mapping_t *map = read_mapping[addr >> MEM_GRANULARITY_BITS];
+    uint8_t ret = 0xff;
 
     mem_logical_addr = 0xffffffff;
 
     if (_mem_exec[addr >> MEM_GRANULARITY_BITS])
-	return _mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK];
+	ret = _mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK];
     else if (map && map->read_b)
-       	return map->read_b(addr, map->p);
-    else
-	return 0xff;
+       	ret = map->read_b(addr, map->p);
+
+    return ret;
 }
 
 
@@ -1605,21 +1606,21 @@ uint16_t
 mem_readw_phys(uint32_t addr)
 {
     mem_mapping_t *map = read_mapping[addr >> MEM_GRANULARITY_BITS];
-    uint16_t temp, *p;
+    uint16_t ret, *p;
 
     mem_logical_addr = 0xffffffff;
 
     if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_HBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS])) {
 	p = (uint16_t *) &(_mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK]);
-	return *p;
+	ret = *p;
     } else if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_HBOUND) && (map && map->read_w))
-       	return map->read_w(addr, map->p);
+       	ret = map->read_w(addr, map->p);
     else {
-	temp = mem_readb_phys(addr + 1) << 8;
-	temp |=  mem_readb_phys(addr);
+	ret = mem_readb_phys(addr + 1) << 8;
+	ret |=  mem_readb_phys(addr);
     }
 
-    return temp;
+    return ret;
 }
 
 
@@ -1627,21 +1628,21 @@ uint32_t
 mem_readl_phys(uint32_t addr)
 {
     mem_mapping_t *map = read_mapping[addr >> MEM_GRANULARITY_BITS];
-    uint32_t temp, *p;
+    uint32_t ret, *p;
 
     mem_logical_addr = 0xffffffff;
 
     if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_QBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS])) {
 	p = (uint32_t *) &(_mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK]);
-	return *p;
+	ret = *p;
     } else if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_QBOUND) && (map && map->read_l))
-       	return map->read_l(addr, map->p);
+       	ret = map->read_l(addr, map->p);
     else {
-	temp = mem_readw_phys(addr + 2) << 16;
-	temp |=  mem_readw_phys(addr);
+	ret = mem_readw_phys(addr + 2) << 16;
+	ret |=  mem_readw_phys(addr);
     }
 
-    return temp;
+    return ret;
 }
 
 
@@ -1672,7 +1673,7 @@ mem_writeb_phys(uint32_t addr, uint8_t val)
 
     mem_logical_addr = 0xffffffff;
 
-    if (_mem_exec[addr >> MEM_GRANULARITY_BITS])
+    if ((_mem_exec[addr >> MEM_GRANULARITY_BITS]) && (map && map->write_b))
 	_mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK] = val;
     else if (map && map->write_b)
        	map->write_b(addr, val, map->p);
@@ -1687,7 +1688,8 @@ mem_writew_phys(uint32_t addr, uint16_t val)
 
     mem_logical_addr = 0xffffffff;
 
-    if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_HBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS])) {
+    if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_HBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS]) &&
+	(map && map->write_w)) {
 	p = (uint16_t *) &(_mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK]);
 	*p = val;
     } else if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_HBOUND) && (map && map->write_w))
@@ -1707,7 +1709,8 @@ mem_writel_phys(uint32_t addr, uint32_t val)
 
     mem_logical_addr = 0xffffffff;
 
-    if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_QBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS])) {
+    if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_QBOUND) && (_mem_exec[addr >> MEM_GRANULARITY_BITS]) &&
+	(map && map->write_l)) {
 	p = (uint32_t *) &(_mem_exec[addr >> MEM_GRANULARITY_BITS][addr & MEM_GRANULARITY_MASK]);
 	*p = val;
     } else if (((addr & MEM_GRANULARITY_MASK) <= MEM_GRANULARITY_QBOUND) && (map && map->write_l))
@@ -2128,24 +2131,6 @@ mem_write_remappedl(uint32_t addr, uint32_t val, void *priv)
 }
 
 
-void
-mem_write_null(uint32_t addr, uint8_t val, void *p)
-{
-}
-
-
-void
-mem_write_nullw(uint32_t addr, uint16_t val, void *p)
-{
-}
-
-
-void
-mem_write_nulll(uint32_t addr, uint32_t val, void *p)
-{
-}
-
-
 void
 mem_invalidate_range(uint32_t start_addr, uint32_t end_addr)
 {
diff --git a/src/mem/rom.c b/src/mem/rom.c
index 8e4eda437..342eaf705 100644
--- a/src/mem/rom.c
+++ b/src/mem/rom.c
@@ -409,7 +409,7 @@ bios_add(void)
 	/* 256k+ BIOS'es only have low mappings at E0000-FFFFF. */
 	mem_mapping_add(&bios_mapping, 0xe0000, 0x20000,
 			bios_read,bios_readw,bios_readl,
-			mem_write_null,mem_write_nullw,mem_write_nulll,
+			NULL,NULL,NULL,
 			&rom[0x20000], MEM_MAPPING_EXTERNAL|MEM_MAPPING_ROM|MEM_MAPPING_ROMCS, 0);
 
 	mem_set_mem_state_both(0x0e0000, 0x20000,
@@ -417,7 +417,7 @@ bios_add(void)
     } else {
 	mem_mapping_add(&bios_mapping, biosaddr, biosmask + 1,
 			bios_read,bios_readw,bios_readl,
-			mem_write_null,mem_write_nullw,mem_write_nulll,
+			NULL,NULL,NULL,
 			rom, MEM_MAPPING_EXTERNAL|MEM_MAPPING_ROM|MEM_MAPPING_ROMCS, 0);
 
 	mem_set_mem_state_both(biosaddr, biosmask + 1,
@@ -427,7 +427,7 @@ bios_add(void)
     if (AT) {
 	mem_mapping_add(&bios_high_mapping, biosaddr | (temp_cpu_16bitbus ? 0x00f00000 : 0xfff00000), biosmask + 1,
 			bios_read,bios_readw,bios_readl,
-			mem_write_null,mem_write_nullw,mem_write_nulll,
+			NULL,NULL,NULL,
 			rom, MEM_MAPPING_EXTERNAL|MEM_MAPPING_ROM|MEM_MAPPING_ROMCS, 0);
 
 	mem_set_mem_state_both(biosaddr | (temp_cpu_16bitbus ? 0x00f00000 : 0xfff00000), biosmask + 1,
@@ -536,7 +536,7 @@ rom_init(rom_t *rom, wchar_t *fn, uint32_t addr, int sz, int mask, int off, uint
     mem_mapping_add(&rom->mapping,
 		    addr, sz,
 		    rom_read, rom_readw, rom_readl,
-		    mem_write_null, mem_write_nullw, mem_write_nulll,
+		    NULL, NULL, NULL,
 		    rom->rom, flags | MEM_MAPPING_ROM, rom);
 
     return(0);
@@ -566,7 +566,7 @@ rom_init_oddeven(rom_t *rom, wchar_t *fn, uint32_t addr, int sz, int mask, int o
     mem_mapping_add(&rom->mapping,
 		    addr, sz,
 		    rom_read, rom_readw, rom_readl,
-		    mem_write_null, mem_write_nullw, mem_write_nulll,
+		    NULL, NULL, NULL,
 		    rom->rom, flags | MEM_MAPPING_ROM, rom);
 
     return(0);
@@ -594,7 +594,7 @@ rom_init_interleaved(rom_t *rom, wchar_t *fnl, wchar_t *fnh, uint32_t addr, int
     mem_mapping_add(&rom->mapping,
 		    addr, sz,
 		    rom_read, rom_readw, rom_readl,
-		    mem_write_null, mem_write_nullw, mem_write_nulll,
+		    NULL, NULL, NULL,
 		    rom->rom, flags | MEM_MAPPING_ROM, rom);
 
     return(0);