This allows setting the start address to a high value, and reading the
bottom of the screen from the beginning of VRAM.  Commander Keen 4
("Goodbye, Galaxy!") relies on this behavior.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/display/vga-helpers.h | 9 +++++----
 hw/display/vga.c         | 3 +++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/hw/display/vga-helpers.h b/hw/display/vga-helpers.h
index 10e9cfd40a0..83b9a15604a 100644
--- a/hw/display/vga-helpers.h
+++ b/hw/display/vga-helpers.h
@@ -108,7 +108,7 @@ static void vga_draw_line2(VGACommonState *vga, uint8_t *d,
     plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
     width >>= 3;
     for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
         data &= plane_mask;
         v = expand2[GET_PLANE(data, 0)];
         v |= expand2[GET_PLANE(data, 2)] << 2;
@@ -144,7 +144,7 @@ static void vga_draw_line2d2(VGACommonState *vga, uint8_t *d,
     plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
     width >>= 3;
     for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
         data &= plane_mask;
         v = expand2[GET_PLANE(data, 0)];
         v |= expand2[GET_PLANE(data, 2)] << 2;
@@ -177,7 +177,7 @@ static void vga_draw_line4(VGACommonState *vga, uint8_t *d,
     plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
     width >>= 3;
     for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
         data &= plane_mask;
         v = expand4[GET_PLANE(data, 0)];
         v |= expand4[GET_PLANE(data, 1)] << 1;
@@ -209,7 +209,7 @@ static void vga_draw_line4d2(VGACommonState *vga, uint8_t *d,
     plane_mask = mask16[vga->ar[VGA_ATC_PLANE_ENABLE] & 0xf];
     width >>= 3;
     for(x = 0; x < width; x++) {
-        data = vga_read_dword_le(vga, addr);
+        data = vga_read_dword_le(vga, addr & (VGA_VRAM_SIZE - 1));
         data &= plane_mask;
         v = expand4[GET_PLANE(data, 0)];
         v |= expand4[GET_PLANE(data, 1)] << 1;
@@ -242,6 +242,7 @@ static void vga_draw_line8d2(VGACommonState *vga, uint8_t *d,
     palette = vga->last_palette;
     width >>= 3;
     for(x = 0; x < width; x++) {
+        addr &= VGA_VRAM_SIZE - 1;
         PUT_PIXEL2(d, 0, palette[vga_read_byte(vga, addr + 0)]);
         PUT_PIXEL2(d, 1, palette[vga_read_byte(vga, addr + 1)]);
         PUT_PIXEL2(d, 2, palette[vga_read_byte(vga, addr + 2)]);
diff --git a/hw/display/vga.c b/hw/display/vga.c
index fea26f91232..5bf4d14f342 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -47,6 +47,9 @@ bool have_vga = true;
 /* 16 state changes per vertical frame @60 Hz */
 #define VGA_TEXT_CURSOR_PERIOD_MS       (1000 * 2 * 16 / 60)
 
+/* Address mask for non-VESA modes.  */
+#define VGA_VRAM_SIZE                   (256 * KiB)
+
 /*
  * Video Graphics Array (VGA)
  *
-- 
2.43.0