vgabios: Don't declare custom internal BDA storage in std/bda.h

The vgabios uses storage in the BDA at offset 0xb9 for internal custom
storage (the contents do not appear to be part of any bios standard).
Move the description of this custom vgabios area from std/bda.h to
vgasrc/vgabios.h.  Add two new macros (GET_BDA_EXT and SET_BDA_EXT).
This should make it more clear that the area is for custom internal
storage.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
diff --git a/src/std/bda.h b/src/std/bda.h
index 948bdbf..c321266 100644
--- a/src/std/bda.h
+++ b/src/std/bda.h
@@ -95,12 +95,7 @@
     struct segoff_s video_savetable;
     u8 other_ac[4];
     // 40:B0
-    u8 other_b0[9];
-    u8 vbe_flag;
-    u16 vbe_mode;
-    u8 other_bc[4];
-    // 40:C0
-    u8 other_c0[4*16];
+    u8 other_b0[5*16];
 } PACKED;
 
 // BDA floppy_recalibration_status bitdefs
diff --git a/vgasrc/vbe.c b/vgasrc/vbe.c
index 12bd981..06ec22e 100644
--- a/vgasrc/vbe.c
+++ b/vgasrc/vbe.c
@@ -217,7 +217,7 @@
 static void
 vbe_104f03(struct bregs *regs)
 {
-    regs->bx = GET_BDA(vbe_mode);
+    regs->bx = GET_BDA_EXT(vbe_mode);
     dprintf(1, "VBE current mode=%x\n", regs->bx);
     regs->ax = 0x004f;
 }
@@ -247,7 +247,7 @@
 {
     if (regs->bh > 1 || regs->bl > 1)
         goto fail;
-    if (GET_BDA(vbe_mode) & MF_LINEARFB) {
+    if (GET_BDA_EXT(vbe_mode) & MF_LINEARFB) {
         regs->ah = VBE_RETURN_STATUS_INVALID;
         return;
     }
@@ -382,10 +382,10 @@
         regs->bx = 0x0f30;
         break;
     case 0x01:
-        SET_BDA(vbe_flag, regs->bh);
+        SET_BDA_EXT(vbe_flag, regs->bh);
         break;
     case 0x02:
-        regs->bh = GET_BDA(vbe_flag);
+        regs->bh = GET_BDA_EXT(vbe_flag);
         break;
     default:
         regs->ax = 0x014f;
diff --git a/vgasrc/vgabios.c b/vgasrc/vgabios.c
index e87b7eb..d36b62a 100644
--- a/vgasrc/vgabios.c
+++ b/vgasrc/vgabios.c
@@ -252,7 +252,7 @@
                    , sizeof(info->bda_0x49));
         memcpy_far(seg, info->bda_0x84, SEG_BDA, (void*)0x84
                    , sizeof(info->bda_0x84));
-        SET_FARVAR(seg, info->vbe_mode, GET_BDA(vbe_mode));
+        SET_FARVAR(seg, info->vbe_mode, GET_BDA_EXT(vbe_mode));
         SET_FARVAR(seg, info->font0, GET_IVT(0x1f));
         SET_FARVAR(seg, info->font1, GET_IVT(0x43));
     }
@@ -261,7 +261,7 @@
                    , sizeof(info->bda_0x49));
         memcpy_far(SEG_BDA, (void*)0x84, seg, info->bda_0x84
                    , sizeof(info->bda_0x84));
-        SET_BDA(vbe_mode, GET_FARVAR(seg, info->vbe_mode));
+        SET_BDA_EXT(vbe_mode, GET_FARVAR(seg, info->vbe_mode));
         SET_IVT(0x1f, GET_FARVAR(seg, info->font0));
         SET_IVT(0x43, GET_FARVAR(seg, info->font1));
     }
@@ -276,7 +276,7 @@
 struct vgamode_s *
 get_current_mode(void)
 {
-    return vgahw_find_mode(GET_BDA(vbe_mode) & ~MF_VBEFLAGS);
+    return vgahw_find_mode(GET_BDA_EXT(vbe_mode) & ~MF_VBEFLAGS);
 }
 
 // Setup BDA after a mode switch.
@@ -301,7 +301,7 @@
         SET_BDA(video_mode, mode);
     else
         SET_BDA(video_mode, 0xff);
-    SET_BDA(vbe_mode, mode | (flags & MF_VBEFLAGS));
+    SET_BDA_EXT(vbe_mode, mode | (flags & MF_VBEFLAGS));
     if (memmodel == MM_TEXT) {
         SET_BDA(video_cols, width);
         SET_BDA(video_rows, height-1);
diff --git a/vgasrc/vgabios.h b/vgasrc/vgabios.h
index 02cf3e3..70682c9 100644
--- a/vgasrc/vgabios.h
+++ b/vgasrc/vgabios.h
@@ -75,6 +75,19 @@
 #define GO_MEMSET  3
 #define GO_MEMMOVE 4
 
+// Custom internal storage in BDA
+#define VGA_CUSTOM_BDA 0xb9
+
+struct vga_bda_s {
+    u8 vbe_flag;
+    u16 vbe_mode;
+} PACKED;
+
+#define GET_BDA_EXT(var) \
+    GET_FARVAR(SEG_BDA, ((struct vga_bda_s *)VGA_CUSTOM_BDA)->var)
+#define SET_BDA_EXT(var, val) \
+    SET_FARVAR(SEG_BDA, ((struct vga_bda_s *)VGA_CUSTOM_BDA)->var, (val))
+
 // Debug settings
 #define DEBUG_VGA_POST 1
 #define DEBUG_VGA_10 3