diff --git a/src/video/vid_ati_mach64.c b/src/video/vid_ati_mach64.c
index da1b5d556..587341538 100644
--- a/src/video/vid_ati_mach64.c
+++ b/src/video/vid_ati_mach64.c
@@ -510,6 +510,7 @@ mach64_recalctimings(svga_t *svga)
     const mach64_t *mach64 = (mach64_t *) svga->priv;
 
     if (((mach64->crtc_gen_cntl >> 24) & 3) == 3) {
+        svga->hoverride  = 1;
         svga->vtotal     = (mach64->crtc_v_total_disp & 2047) + 1;
         svga->dispend    = ((mach64->crtc_v_total_disp >> 16) & 2047) + 1;
         svga->htotal     = (mach64->crtc_h_total_disp & 255) + 1;
@@ -566,6 +567,7 @@ mach64_recalctimings(svga_t *svga)
 
         svga->vram_display_mask = mach64->vram_mask;
     } else {
+        svga->hoverride         = 0;
         svga->vram_display_mask = (mach64->regs[0x36] & 0x01) ? mach64->vram_mask : 0x3ffff;
     }
 }
diff --git a/src/video/vid_svga.c b/src/video/vid_svga.c
index a350b6d28..ff8dbbbea 100644
--- a/src/video/vid_svga.c
+++ b/src/video/vid_svga.c
@@ -766,17 +766,19 @@ svga_recalctimings(svga_t *svga)
     if (xga_active && (svga->xga != NULL))
         xga_recalctimings(svga);
 
-    svga->hblankend = (svga->hblankstart & ~(svga->hblank_end_len - 1)) | svga->hblank_end_val;
-    if (svga->hblankend <= svga->hblankstart)
-        svga->hblankend += svga->hblank_end_len;
-    svga->hblankend += svga->hblank_ext;
+    if (!svga->hoverride) {
+        svga->hblankend = (svga->hblankstart & ~(svga->hblank_end_len - 1)) | svga->hblank_end_val;
+        if (svga->hblankend <= svga->hblankstart)
+            svga->hblankend += svga->hblank_end_len;
+        svga->hblankend += svga->hblank_ext;
 
-    svga->hblank_sub = 0;
-    if (svga->hblankend > svga->htotal) {
-        svga->hblankend &= (svga->hblank_end_len - 1);
-        svga->hblank_sub = svga->hblankend + svga->hblank_overscan;
+        svga->hblank_sub = 0;
+        if (svga->hblankend > svga->htotal) {
+            svga->hblankend &= (svga->hblank_end_len - 1);
+            svga->hblank_sub = svga->hblankend + svga->hblank_overscan;
 
-        svga->hdisp -= (svga->hblank_sub * svga->dots_per_clock);
+            svga->hdisp -= (svga->hblank_sub * svga->dots_per_clock);
+        }
     }
 
     if (svga->hdisp >= 2048)