S3: Mask Linear Address Window by memory size as documented by S3

CHANGELOG

@@ -1,4 +1,11 @@
 NEXT
+  - S3 emulation: Follow S3 Trio64 documentation and mask the Linear
+    Window Position bits according to the size of the memory. You
+    can only place it on a multiple of the memory size. This silences
+    warnings caused by Line Wars II which appears to modify only the
+    upper byte but not the lower when modifying this register, which
+    then leaves 0xA0 in the lower half. That lower half would be
+    ignored for 1MB or larger video memory sizes. (joncampbell123).
   - S3 XGA emulation: Add XGA acceleration for 4bpp packed 16-color
     SVGA modes. A CAD package has S3 support drivers that use the
     16-color 4bpp packed modes and expects XGA acceleration to work.

src/hardware/vga_memory.cpp

@@ -2945,30 +2945,40 @@ void VGA_StartUpdateLFB(void) {
 		// FIXME: What about the 8MB window?
 	}
 
-    /* The LFB register has an enable bit */
-    if (!(vga.s3.reg_58 & 0x10)) {
-        vga.lfb.page = (unsigned int)vga.s3.la_window << 4u;
-        vga.lfb.addr = (unsigned int)vga.s3.la_window << 16u;
-        vga.lfb.handler = NULL;
-        MEM_SetLFB(0,0,NULL,NULL);
-    }
-    /* if the DOS application or Windows 3.1 driver attempts to put the linear framebuffer
+	/* NTS: 64KB winsz = 0x10000 => winmsk = 0xFFFF0000 => 0xFFFF
+	 *      1MB winsz = 0x100000 => winmsk = 0xFFF00000 => 0xFFF0
+	 *      2MB winsz = 0x200000 => winmsk = 0xFFE00000 => 0xFFE0
+	 *      and so on.
+	 *
+	 * From the S3 Trio32/Trio64 documentation regarding the Linear Address Window Position Registers:
+	 * "The Linear Address Window resides on a 64KB, 1MB, 2MB, or 4MB (Trio64 only) memory boundary (size-aligned) ...
+	 *  Some LSBs of this register are ignored because of the size-aligned boundary scheme" */
+	const unsigned int la_winmsk = ~((winsz - 1u) >> 16u); /* Register holds the upper 16 bits of the linear address */
+
+	/* The LFB register has an enable bit */
+	if (!(vga.s3.reg_58 & 0x10)) {
+		vga.lfb.page = (unsigned int)(vga.s3.la_window & la_winmsk) << 4u;
+		vga.lfb.addr = (unsigned int)(vga.s3.la_window & la_winmsk) << 16u;
+		vga.lfb.handler = NULL;
+		MEM_SetLFB(0,0,NULL,NULL);
+	}
+	/* if the DOS application or Windows 3.1 driver attempts to put the linear framebuffer
 	 * below the top of memory, then we're probably entering a DOS VM and it's probably
 	 * a 64KB window. If it's not a 64KB window then print a warning. */
-    else if ((unsigned long)(vga.s3.la_window << 4UL) < (unsigned long)MEM_TotalPages()) {
+	else if ((unsigned long)(vga.s3.la_window << 4UL) < (unsigned long)MEM_TotalPages()) {
 		if (winsz != 0x10000) // 64KB window normal for entering a DOS VM in Windows 3.1 or legacy bank switching in DOS
 			LOG(LOG_MISC,LOG_WARN)("S3 warning: Window size != 64KB and address conflict with system RAM!");
 
-		vga.lfb.page = (unsigned int)vga.s3.la_window << 4u;
-		vga.lfb.addr = (unsigned int)vga.s3.la_window << 16u;
+		vga.lfb.page = (unsigned int)(vga.s3.la_window & la_winmsk) << 4u;
+		vga.lfb.addr = (unsigned int)(vga.s3.la_window & la_winmsk) << 16u;
 		vga.lfb.handler = NULL;
 		MEM_SetLFB(0,0,NULL,NULL);
 	}
 	else {
-		vga.lfb.page = (unsigned int)vga.s3.la_window << 4u;
-		vga.lfb.addr = (unsigned int)vga.s3.la_window << 16u;
+		vga.lfb.page = (unsigned int)(vga.s3.la_window & la_winmsk) << 4u;
+		vga.lfb.addr = (unsigned int)(vga.s3.la_window & la_winmsk) << 16u;
 		vga.lfb.handler = &vgaph.lfb;
-		MEM_SetLFB((unsigned int)vga.s3.la_window << 4u,(unsigned int)vga.mem.memsize/4096u, vga.lfb.handler, &vgaph.mmio);
+		MEM_SetLFB((unsigned int)(vga.s3.la_window & la_winmsk) << 4u,(unsigned int)vga.mem.memsize/4096u, vga.lfb.handler, &vgaph.mmio);
 	}
 }