From b0c5c68cc628f936fb7856824989c158f70dab39 Mon Sep 17 00:00:00 2001
From: Jonathan Campbell <jonathan@castus.tv>
Date: Wed, 4 Dec 2024 20:49:32 -0800
Subject: [PATCH] INT 10h: If machine=ega, check EGA switches to detect games
 that alter the EGA switches to force 200-line mode

---
 CHANGELOG                |  4 ++++
 src/ints/int10.cpp       |  7 +++++--
 src/ints/int10_modes.cpp | 18 +++++++++++++++++-
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG b/CHANGELOG
index 05436ad07..915f905de 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -4,6 +4,10 @@ Next
     to divide line by 2 in rendering to match double scanned scanlines,
     so that the "graphical" text of "After the War" can render properly
     for machine=vgaonly. (joncampbell123).
+  - INT 10: If machine=ega, check the EGA switches on INT 10h modeset.
+    Some games force the EGA BIOS to use CGA 200-line text modes by
+    modifying those bits and then setting the mode ("After the War").
+    (joncampbell123).
 
 2024.12.04
   - Arrange memory device allocation so that it is possible to allocate
diff --git a/src/ints/int10.cpp b/src/ints/int10.cpp
index 83c50acbe..d6120db6a 100644
--- a/src/ints/int10.cpp
+++ b/src/ints/int10.cpp
@@ -1240,7 +1240,10 @@ static void INT10_Seg40Init(void) {
 	// in Tandy emulation, this byte must be zero.
 	if (IS_EGAVGA_ARCH) {
 		real_writeb(BIOSMEM_SEG,BIOSMEM_VIDEO_CTL,0x60);
-		real_writeb(BIOSMEM_SEG,BIOSMEM_CHAR_HEIGHT,16);
+		if (IS_VGA_ARCH)
+			real_writeb(BIOSMEM_SEG,BIOSMEM_CHAR_HEIGHT,16);
+		else
+			real_writeb(BIOSMEM_SEG,BIOSMEM_CHAR_HEIGHT,ega200 ? 8 : 14);
 		// NTS: This is direclty from the configuration switches. The bits are inverted,
 		//      1=OFF 0=ON.
 		//
@@ -1262,7 +1265,7 @@ static void INT10_Seg40Init(void) {
 		//      [http://hackipedia.org/browse.cgi/Computer/Platform/PC%2c%20IBM%20compatible/Video/EGA/IBM/IBM%20Enhanced%20Graphics%20Adapter%20%281984%2d08%2d02%29%2epdf]
 		//
 		//      Anything else is not valid.
-		real_writeb(BIOSMEM_SEG,BIOSMEM_SWITCHES,0xFF^((!IS_VGA_ARCH && ega200)?0x8:0x6));
+		real_writeb(BIOSMEM_SEG,BIOSMEM_SWITCHES,0xFF^((!IS_VGA_ARCH && ega200)?0x7:0x9));
 		// Set the pointer to video save pointer table
 		real_writed(BIOSMEM_SEG, BIOSMEM_VS_POINTER, int10.rom.video_save_pointers);
 	}
diff --git a/src/ints/int10_modes.cpp b/src/ints/int10_modes.cpp
index 959f7941d..5391ff3d2 100644
--- a/src/ints/int10_modes.cpp
+++ b/src/ints/int10_modes.cpp
@@ -887,7 +887,6 @@ static void FinishSetMode(bool clearmem) {
 		real_writeb(BIOSMEM_SEG,BIOSMEM_NB_ROWS,(uint8_t)(CurMode->theight-1));
 		real_writew(BIOSMEM_SEG,BIOSMEM_CHAR_HEIGHT,(uint16_t)CurMode->cheight);
 		real_writeb(BIOSMEM_SEG,BIOSMEM_VIDEO_CTL,(0x60|(clearmem?0:0x80)));
-		real_writeb(BIOSMEM_SEG,BIOSMEM_SWITCHES,(!IS_VGA_ARCH && ega200)?0x08:0x09);
 		// this is an index into the dcc table:
 #if C_DEBUG
 		if(IS_VGA_ARCH) real_writeb(BIOSMEM_SEG,BIOSMEM_DCC_INDEX,DISP2_Active()?0x0c:0x0b);
@@ -1259,6 +1258,23 @@ static bool ShouldUseVPT(void) {
 
 bool unmask_irq0_on_int10_setmode = true;
 bool INT10_SetVideoMode(uint16_t mode) {
+	if (IS_EGA_ARCH) {
+		/* "After The War" forces EGA into 200-line CGA 8x8 text mode by
+		 * modifying the 16-bits at 0x487-0x488 to alter the EGA switches
+		 * and other info. For this trick to work here, we have to check
+		 * those bits every modeset. */
+		uint8_t egasw = (real_readb(BIOSMEM_SEG,BIOSMEM_SWITCHES)^0xFF)&0xF;
+		bool newega200 = (egasw == 0 || egasw == 1 || egasw == 2 || egasw == 6 || egasw == 7 || egasw == 8);
+
+		if (ega200 != newega200) {
+			LOG(LOG_MISC,LOG_DEBUG)(
+				"EGA: Guest application changed BIOS DATA AREA EGA switches, %s 200-line EGA modes (sw=0x%x)",
+				newega200?"enabled":"disabled",egasw);
+
+			ega200 = newega200;
+		}
+	}
+
 	if (CurMode&&CurMode->mode==7&&!IS_PC98_ARCH) {
 		VideoModeBlock *modelist;