diff --git a/hw/display/vga.c b/hw/display/vga.c
index 657e9f196d8d1eeee8731261abd69312b504ed09..4a55ec6dbbc378f0a1e3bf7a297f61cabb426b1c 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -142,6 +142,13 @@ static uint32_t expand4[256];
 static uint16_t expand2[256];
 static uint8_t expand4to8[16];
 
+static void vbe_update_vgaregs(VGACommonState *s);
+
+static inline bool vbe_enabled(VGACommonState *s)
+{
+    return s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED;
+}
+
 static void vga_update_memory_access(VGACommonState *s)
 {
     hwaddr base, offset, size;
@@ -179,6 +186,7 @@ static void vga_update_memory_access(VGACommonState *s)
             size = 0x8000;
             break;
         }
+        assert(offset + size <= s->vram_size);
         memory_region_init_alias(&s->chain4_alias, memory_region_owner(&s->vram),
                                  "vga.chain4", &s->vram, offset, size);
         memory_region_add_subregion_overlap(s->legacy_address_space, base,
@@ -478,6 +486,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         printf("vga: write SR%x = 0x%02x\n", s->sr_index, val);
 #endif
         s->sr[s->sr_index] = val & sr_mask[s->sr_index];
+        vbe_update_vgaregs(s);
         if (s->sr_index == VGA_SEQ_CLOCK_MODE) {
             s->update_retrace_info(s);
         }
@@ -509,6 +518,7 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         printf("vga: write GR%x = 0x%02x\n", s->gr_index, val);
 #endif
         s->gr[s->gr_index] = val & gr_mask[s->gr_index];
+        vbe_update_vgaregs(s);
         vga_update_memory_access(s);
         break;
     case VGA_CRT_IM:
@@ -527,10 +537,12 @@ void vga_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             if (s->cr_index == VGA_CRTC_OVERFLOW) {
                 s->cr[VGA_CRTC_OVERFLOW] = (s->cr[VGA_CRTC_OVERFLOW] & ~0x10) |
                     (val & 0x10);
+                vbe_update_vgaregs(s);
             }
             return;
         }
         s->cr[s->cr_index] = val;
+        vbe_update_vgaregs(s);
 
         switch(s->cr_index) {
         case VGA_CRTC_H_TOTAL:
@@ -563,7 +575,7 @@ static void vbe_fixup_regs(VGACommonState *s)
     uint16_t *r = s->vbe_regs;
     uint32_t bits, linelength, maxy, offset;
 
-    if (!(r[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED)) {
+    if (!vbe_enabled(s)) {
         /* vbe is turned off -- nothing to do */
         return;
     }
@@ -638,6 +650,49 @@ static void vbe_fixup_regs(VGACommonState *s)
     s->vbe_start_addr  = offset / 4;
 }
 
+/* we initialize the VGA graphic mode */
+static void vbe_update_vgaregs(VGACommonState *s)
+{
+    int h, shift_control;
+
+    if (!vbe_enabled(s)) {
+        /* vbe is turned off -- nothing to do */
+        return;
+    }
+
+    /* graphic mode + memory map 1 */
+    s->gr[VGA_GFX_MISC] = (s->gr[VGA_GFX_MISC] & ~0x0c) | 0x04 |
+        VGA_GR06_GRAPHICS_MODE;
+    s->cr[VGA_CRTC_MODE] |= 3; /* no CGA modes */
+    s->cr[VGA_CRTC_OFFSET] = s->vbe_line_offset >> 3;
+    /* width */
+    s->cr[VGA_CRTC_H_DISP] =
+        (s->vbe_regs[VBE_DISPI_INDEX_XRES] >> 3) - 1;
+    /* height (only meaningful if < 1024) */
+    h = s->vbe_regs[VBE_DISPI_INDEX_YRES] - 1;
+    s->cr[VGA_CRTC_V_DISP_END] = h;
+    s->cr[VGA_CRTC_OVERFLOW] = (s->cr[VGA_CRTC_OVERFLOW] & ~0x42) |
+        ((h >> 7) & 0x02) | ((h >> 3) & 0x40);
+    /* line compare to 1023 */
+    s->cr[VGA_CRTC_LINE_COMPARE] = 0xff;
+    s->cr[VGA_CRTC_OVERFLOW] |= 0x10;
+    s->cr[VGA_CRTC_MAX_SCAN] |= 0x40;
+
+    if (s->vbe_regs[VBE_DISPI_INDEX_BPP] == 4) {
+        shift_control = 0;
+        s->sr[VGA_SEQ_CLOCK_MODE] &= ~8; /* no double line */
+    } else {
+        shift_control = 2;
+        /* set chain 4 mode */
+        s->sr[VGA_SEQ_MEMORY_MODE] |= VGA_SR04_CHN_4M;
+        /* activate all planes */
+        s->sr[VGA_SEQ_PLANE_WRITE] |= VGA_SR02_ALL_PLANES;
+    }
+    s->gr[VGA_GFX_MODE] = (s->gr[VGA_GFX_MODE] & ~0x60) |
+        (shift_control << 5);
+    s->cr[VGA_CRTC_MAX_SCAN] &= ~0x9f; /* no double scan */
+}
+
 static uint32_t vbe_ioport_read_index(void *opaque, uint32_t addr)
 {
     VGACommonState *s = opaque;
@@ -714,13 +769,10 @@ void vbe_ioport_write_data(void *opaque, uint32_t addr, uint32_t val)
         case VBE_DISPI_INDEX_Y_OFFSET:
             s->vbe_regs[s->vbe_index] = val;
             vbe_fixup_regs(s);
+            vbe_update_vgaregs(s);
             break;
         case VBE_DISPI_INDEX_BANK:
-            if (s->vbe_regs[VBE_DISPI_INDEX_BPP] == 4) {
-              val &= (s->vbe_bank_mask >> 2);
-            } else {
-              val &= s->vbe_bank_mask;
-            }
+            val &= s->vbe_bank_mask;
             s->vbe_regs[s->vbe_index] = val;
             s->bank_offset = (val << 16);
             vga_update_memory_access(s);
@@ -728,52 +780,19 @@ void vbe_ioport_write_data(void *opaque, uint32_t addr, uint32_t val)
         case VBE_DISPI_INDEX_ENABLE:
             if ((val & VBE_DISPI_ENABLED) &&
                 !(s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED)) {
-                int h, shift_control;
 
                 s->vbe_regs[VBE_DISPI_INDEX_VIRT_WIDTH] = 0;
                 s->vbe_regs[VBE_DISPI_INDEX_X_OFFSET] = 0;
                 s->vbe_regs[VBE_DISPI_INDEX_Y_OFFSET] = 0;
                 s->vbe_regs[VBE_DISPI_INDEX_ENABLE] |= VBE_DISPI_ENABLED;
                 vbe_fixup_regs(s);
+                vbe_update_vgaregs(s);
 
                 /* clear the screen */
                 if (!(val & VBE_DISPI_NOCLEARMEM)) {
                     memset(s->vram_ptr, 0,
                            s->vbe_regs[VBE_DISPI_INDEX_YRES] * s->vbe_line_offset);
                 }
-
-                /* we initialize the VGA graphic mode */
-                /* graphic mode + memory map 1 */
-                s->gr[VGA_GFX_MISC] = (s->gr[VGA_GFX_MISC] & ~0x0c) | 0x04 |
-                    VGA_GR06_GRAPHICS_MODE;
-                s->cr[VGA_CRTC_MODE] |= 3; /* no CGA modes */
-                s->cr[VGA_CRTC_OFFSET] = s->vbe_line_offset >> 3;
-                /* width */
-                s->cr[VGA_CRTC_H_DISP] =
-                    (s->vbe_regs[VBE_DISPI_INDEX_XRES] >> 3) - 1;
-                /* height (only meaningful if < 1024) */
-                h = s->vbe_regs[VBE_DISPI_INDEX_YRES] - 1;
-                s->cr[VGA_CRTC_V_DISP_END] = h;
-                s->cr[VGA_CRTC_OVERFLOW] = (s->cr[VGA_CRTC_OVERFLOW] & ~0x42) |
-                    ((h >> 7) & 0x02) | ((h >> 3) & 0x40);
-                /* line compare to 1023 */
-                s->cr[VGA_CRTC_LINE_COMPARE] = 0xff;
-                s->cr[VGA_CRTC_OVERFLOW] |= 0x10;
-                s->cr[VGA_CRTC_MAX_SCAN] |= 0x40;
-
-                if (s->vbe_regs[VBE_DISPI_INDEX_BPP] == 4) {
-                    shift_control = 0;
-                    s->sr[VGA_SEQ_CLOCK_MODE] &= ~8; /* no double line */
-                } else {
-                    shift_control = 2;
-                    /* set chain 4 mode */
-                    s->sr[VGA_SEQ_MEMORY_MODE] |= VGA_SR04_CHN_4M;
-                    /* activate all planes */
-                    s->sr[VGA_SEQ_PLANE_WRITE] |= VGA_SR02_ALL_PLANES;
-                }
-                s->gr[VGA_GFX_MODE] = (s->gr[VGA_GFX_MODE] & ~0x60) |
-                    (shift_control << 5);
-                s->cr[VGA_CRTC_MAX_SCAN] &= ~0x9f; /* no double scan */
             } else {
                 s->bank_offset = 0;
             }
@@ -819,13 +838,21 @@ uint32_t vga_mem_readb(VGACommonState *s, hwaddr addr)
 
     if (s->sr[VGA_SEQ_MEMORY_MODE] & VGA_SR04_CHN_4M) {
         /* chain 4 mode : simplest access */
+        assert(addr < s->vram_size);
         ret = s->vram_ptr[addr];
     } else if (s->gr[VGA_GFX_MODE] & 0x10) {
         /* odd/even mode (aka text mode mapping) */
         plane = (s->gr[VGA_GFX_PLANE_READ] & 2) | (addr & 1);
-        ret = s->vram_ptr[((addr & ~1) << 1) | plane];
+        addr = ((addr & ~1) << 1) | plane;
+        if (addr >= s->vram_size) {
+            return 0xff;
+        }
+        ret = s->vram_ptr[addr];
     } else {
         /* standard VGA latched access */
+        if (addr * sizeof(uint32_t) >= s->vram_size) {
+            return 0xff;
+        }
         s->latch = ((uint32_t *)s->vram_ptr)[addr];
 
         if (!(s->gr[VGA_GFX_MODE] & 0x08)) {
@@ -882,6 +909,7 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val)
         plane = addr & 3;
         mask = (1 << plane);
         if (s->sr[VGA_SEQ_PLANE_WRITE] & mask) {
+            assert(addr < s->vram_size);
             s->vram_ptr[addr] = val;
 #ifdef DEBUG_VGA_MEM
             printf("vga: chain4: [0x" TARGET_FMT_plx "]\n", addr);
@@ -895,6 +923,9 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val)
         mask = (1 << plane);
         if (s->sr[VGA_SEQ_PLANE_WRITE] & mask) {
             addr = ((addr & ~1) << 1) | plane;
+            if (addr >= s->vram_size) {
+                return;
+            }
             s->vram_ptr[addr] = val;
 #ifdef DEBUG_VGA_MEM
             printf("vga: odd/even: [0x" TARGET_FMT_plx "]\n", addr);
@@ -968,6 +999,9 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val)
         mask = s->sr[VGA_SEQ_PLANE_WRITE];
         s->plane_updated |= mask; /* only used to detect font change */
         write_mask = mask16[mask];
+        if (addr * sizeof(uint32_t) >= s->vram_size) {
+            return;
+        }
         ((uint32_t *)s->vram_ptr)[addr] =
             (((uint32_t *)s->vram_ptr)[addr] & ~write_mask) |
             (val & write_mask);
@@ -1046,7 +1080,7 @@ static void vga_get_offsets(VGACommonState *s,
 {
     uint32_t start_addr, line_offset, line_compare;
 
-    if (s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED) {
+    if (vbe_enabled(s)) {
         line_offset = s->vbe_line_offset;
         start_addr = s->vbe_start_addr;
         line_compare = 65535;
@@ -1371,7 +1405,7 @@ static int vga_get_bpp(VGACommonState *s)
 {
     int ret;
 
-    if (s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED) {
+    if (vbe_enabled(s)) {
         ret = s->vbe_regs[VBE_DISPI_INDEX_BPP];
     } else {
         ret = 0;
@@ -1383,7 +1417,7 @@ static void vga_get_resolution(VGACommonState *s, int *pwidth, int *pheight)
 {
     int width, height;
 
-    if (s->vbe_regs[VBE_DISPI_INDEX_ENABLE] & VBE_DISPI_ENABLED) {
+    if (vbe_enabled(s)) {
         width = s->vbe_regs[VBE_DISPI_INDEX_XRES];
         height = s->vbe_regs[VBE_DISPI_INDEX_YRES];
     } else {