Image corruption when moving window in Windows 3.11 with CL-GD5446 and SVGA driver

[Vort]

During window movement in Windows 3.11 with CL-GD5446 card and default SVGA driver in 800x600x8 resolution, image corruption appear sometimes.

I noticed two scenarios when it happens:

1. Movement 1 pixel up;
2. Movement 4 pixels right.

Problem 1 is related to mode 1 writes in SVGA mode.
Problem 2 is related to Chain-4 toggling.

Here are (hack-)fixes for them:

diff --git a/bochs/iodev/display/svga_cirrus.cc b/bochs/iodev/display/svga_cirrus.cc
index 2644abaf9..3afa386f1 100644
--- a/bochs/iodev/display/svga_cirrus.cc
+++ b/bochs/iodev/display/svga_cirrus.cc
@@ -629,6 +629,25 @@ Bit8u bx_svga_cirrus_c::mem_read(bx_phy_address addr)
         offset <<= 3;
       }
       offset &= BX_CIRRUS_THIS memsize_mask;
+
+      if (BX_CIRRUS_THIS s.sequencer.chain_four) {
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[0] = *(BX_CIRRUS_THIS s.memory + offset);
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[1] = *(BX_CIRRUS_THIS s.memory + offset + 1);
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[2] = *(BX_CIRRUS_THIS s.memory + offset + 2);
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[3] = *(BX_CIRRUS_THIS s.memory + offset + 3);
+        if (BX_CIRRUS_THIS control.reg[0x0b] & 0x08) {
+          BX_CIRRUS_THIS ext_latch[0] = *(BX_CIRRUS_THIS s.memory + offset + 4);
+          BX_CIRRUS_THIS ext_latch[1] = *(BX_CIRRUS_THIS s.memory + offset + 5);
+          BX_CIRRUS_THIS ext_latch[2] = *(BX_CIRRUS_THIS s.memory + offset + 6);
+          BX_CIRRUS_THIS ext_latch[3] = *(BX_CIRRUS_THIS s.memory + offset + 7);
+        }
+      } else {
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[0] = *(BX_CIRRUS_THIS s.memory + (offset << 2));
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[1] = *(BX_CIRRUS_THIS s.memory + (offset << 2) + 1);
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[2] = *(BX_CIRRUS_THIS s.memory + (offset << 2) + 2);
+        BX_CIRRUS_THIS s.graphics_ctrl.latch[3] = *(BX_CIRRUS_THIS s.memory + (offset << 2) + 3);
+      }
+
       return *(BX_CIRRUS_THIS s.memory + offset);
     }
     else {
@@ -866,7 +885,34 @@ void bx_svga_cirrus_c::mem_write(bx_phy_address addr, Bit8u value)
       offset &= BX_CIRRUS_THIS memsize_mask;
       mode = BX_CIRRUS_THIS control.reg[0x05] & 0x07;
       if ((mode < 4) || (mode > 5) || ((BX_CIRRUS_THIS control.reg[0x0b] & 0x4) == 0)) {
-        *(BX_CIRRUS_THIS s.memory + offset) = value;
+        if (mode == 1) {
+          if (BX_CIRRUS_THIS s.sequencer.chain_four) {
+            unsigned i;
+            for (i = 0; i < 4; i++) {
+              *(BX_CIRRUS_THIS s.memory + offset + i) = BX_CIRRUS_THIS s.graphics_ctrl.latch[i];
+            }
+            if ((BX_CIRRUS_THIS control.reg[0x0b] & 0x0c) == 0x0c) {
+              for (i = 0; i < 4; i++) {
+                *(BX_CIRRUS_THIS s.memory + offset + i + 4) = BX_CIRRUS_THIS ext_latch[i];
+              }
+            }
+          }
+          else {
+            Bit8u sequ_map_mask = BX_CIRRUS_THIS s.sequencer.map_mask & 0x0f;
+            if (sequ_map_mask & 0x0f) {
+              if (sequ_map_mask & 0x01)
+                BX_CIRRUS_THIS s.memory[offset << 2] = BX_CIRRUS_THIS s.graphics_ctrl.latch[0];
+              if (sequ_map_mask & 0x02)
+                BX_CIRRUS_THIS s.memory[(offset << 2) + 1] = BX_CIRRUS_THIS s.graphics_ctrl.latch[1];
+              if (sequ_map_mask & 0x04)
+                BX_CIRRUS_THIS s.memory[(offset << 2) + 2] = BX_CIRRUS_THIS s.graphics_ctrl.latch[2];
+              if (sequ_map_mask & 0x08)
+                BX_CIRRUS_THIS s.memory[(offset << 2) + 3] = BX_CIRRUS_THIS s.graphics_ctrl.latch[3];
+            }
+          }
+        } else {
+          *(BX_CIRRUS_THIS s.memory + offset) = value;
+        }
       } else {
         if ((BX_CIRRUS_THIS control.reg[0x0b] & 0x14) != 0x14) {
           mem_write_mode4and5_8bpp(mode, offset, value); 

Result with these changes applied is not perfect - some movements still produce corruptions: either because my implementation have bugs, or because there are more unimplemented things left.

Screenshot for problem 1 (small window with WinMode icon was moved):

Screenshot for problem 2 (same window):

Version: 60491a6.