Jazz Jackrabbit uses odd/even mode with 256-color graphics.  This is
probably so that it can do very fast blitting with a decent resolution
(two pixels, compared to four pixels for "regular" mode X).

Accesses still use all planes (reads go to the latches and the game uses
read mode 1 so that the CPU always gets 0xFF; writes use the plane mask
register because the game sets bit 2 of the sequencer's memory mode
register).  For this to work, QEMU needs to use the code for latched
memory accesses in odd/even mode.  The only difference between odd/even
mode and "regular" planar mode is how the plane is computed in read mode
0, and how the planes are masked if the aforementioned bit 2 is reset.

It is almost enough to fix the game.  You also need to honor byte/word
mode selection, which is done in the next patch.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 hw/display/vga.c | 40 +++++++++++++---------------------------
 1 file changed, 13 insertions(+), 27 deletions(-)

diff --git a/hw/display/vga.c b/hw/display/vga.c
index 08ba5bb1118..731501cb7af 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -842,15 +842,12 @@ uint32_t vga_mem_readb(VGACommonState *s, hwaddr addr)
     if (s->gr[VGA_GFX_MODE] & 0x10) {
         /* odd/even mode (aka text mode mapping) */
         plane = (s->gr[VGA_GFX_PLANE_READ] & 2) | (addr & 1);
-        addr = ((addr & ~1) << 1) | plane;
-        if (addr >= s->vram_size) {
-            return 0xff;
-        }
-        return s->vram_ptr[addr];
+        addr >>= 1;
+    } else {
+        /* standard VGA latched access */
+        plane = s->gr[VGA_GFX_PLANE_READ];
     }
 
-    /* standard VGA latched access */
-    plane = s->gr[VGA_GFX_PLANE_READ];
     if (addr * sizeof(uint32_t) >= s->vram_size) {
         return 0xff;
     }
@@ -903,11 +900,12 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val)
         break;
     }
 
+    mask = sr(s, VGA_SEQ_PLANE_WRITE);
     if (sr(s, VGA_SEQ_MEMORY_MODE) & VGA_SR04_CHN_4M) {
         /* chain 4 mode : simplest access */
         plane = addr & 3;
-        mask = (1 << plane);
-        if (sr(s, VGA_SEQ_PLANE_WRITE) & mask) {
+        mask &= (1 << plane);
+        if (mask) {
             assert(addr < s->vram_size);
             s->vram_ptr[addr] = val;
 #ifdef DEBUG_VGA_MEM
@@ -919,26 +917,14 @@ void vga_mem_writeb(VGACommonState *s, hwaddr addr, uint32_t val)
         return;
     }
 
-    if (s->gr[VGA_GFX_MODE] & 0x10) {
-        /* odd/even mode (aka text mode mapping) */
-        plane = (s->gr[VGA_GFX_PLANE_READ] & 2) | (addr & 1);
-        mask = (1 << plane);
-        if (sr(s, 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" HWADDR_FMT_plx "]\n", addr);
-#endif
-            s->plane_updated |= mask; /* only used to detect font change */
-            memory_region_set_dirty(&s->vram, addr, 1);
-        }
-        return;
+    if ((sr(s, VGA_SEQ_MEMORY_MODE) & VGA_SR04_SEQ_MODE) == 0) {
+        mask &= (addr & 1) ? 0x0a : 0x05;
     }
 
-    mask = sr(s, VGA_SEQ_PLANE_WRITE);
+    if (s->gr[VGA_GFX_MODE] & 0x10) {
+        /* odd/even mode (aka text mode mapping) */
+        addr >>= 1;
+    }
 
     /* standard VGA latched access */
     write_mode = s->gr[VGA_GFX_MODE] & 3;
-- 
2.43.0