Most Tandy video registers are write-only, yet Troubadours likes to read/modify/write. Add log file message whenever those are read, and return 0x00 for specific registers to help the game and the hand-rolled modesetting (someone doesn't like using INT 10 to do it?) set up 320x200 16-color mode, even though the game crashes anyway [https://github.com/joncampbell123/dosbox-x/issues/5867]

CHANGELOG

@@ -1,5 +1,21 @@
 NEXT VERSION
-  - 
+  - Tandy machine type: Many registers are WRITE ONLY according to SX
+    documentation, yet "Troubadours" likes to read/modify/write those
+    registers in what appears to be hand-rolled manual modesetting code
+    to set up 320x200 16-color mode, so, add code to respond to reads
+    from those ports with zeros and print a debug message to the log
+    about how DOS games are not supposed to read those registers. Make
+    the hand-rolled modesetting work by returning 0x00, rather than 0xFF.
+    The code will not set up the mode correctly if 0xFF is returned, but
+    will if 0x00 is returned. A side effect of this is that, since one
+    of the registers controls the RAM address used as video memory, this
+    moves it down to 512KB from base memory as well, which of course
+    easily corrupts the very end of the MCB chain unless you enable more
+    than 640KB of RAM and ask DOSBox-X to emulate the full Tandy 768KB
+    of conventional memory. Though the game still crashes very easily,
+    and in the exact same manner in both DOSBox-X and DOSBox SVN, this
+    fix does solve the problem where the game was only able to draw
+    2 out of 4 scanlines on the screen. (joncampbell123).
 
 2025.10.07
   - INT 21h AH=4Ah resize memory: Compact free blocks only at or after the

src/hardware/vga_other.cpp

@@ -944,6 +944,27 @@ static void write_tandy_reg(uint8_t val) {
 	}
 }
 
+/* You're not SUPPOSED to read Tandy registers 3D8, 3DE, 3DF, they're documented WRITE ONLY! */
+static Bitu read_tandy(Bitu port,Bitu /*iolen*/) {
+	LOG(LOG_VGAMISC,LOG_DEBUG)("DOS applications are NOT SUPPOSED to read Tandy register %x",(unsigned int)port);
+
+	switch (port) {
+		/* "Troubadours" does not switch into Tandy 320x200 16-color by calling INT 10h.
+		 * Instead, it has it's own hand-written code to modify registers directly.
+		 * For whatever reason, it modifies 3DD and 3DF by reading, modifying, and writing
+		 * the registers EVEN THOUGH Tandy documents the registers as write only.
+		 * Returning the last written value doesn't work, because the modified values
+		 * come out incorrect. 3DFh after modification comes out to 0xFF which maps the
+		 * video RAM to B800h as repeating 16KB banks for example. The game works correctly
+		 * if this code provides 0x00, not 0xFF, when it reads these specific registers. */
+		case 0x3DD:
+		case 0x3DF:
+			return 0;
+	}
+
+	return ~0u;
+}
+
 static void write_tandy(Bitu port,Bitu val,Bitu /*iolen*/) {
 	switch (port) {
 	case 0x3d8:
@@ -981,7 +1002,6 @@ static void write_tandy(Bitu port,Bitu val,Bitu /*iolen*/) {
 		// appears to be ORed with CPU A14 (to preserve some sort of
 		// backwards compatibility?), resulting in odd pages being mapped
 		// as 2x16kB. Implemented in vga_memory.cpp Tandy handler.
-
 		vga.tandy.line_mask = (uint8_t)(val >> 6);
 		vga.tandy.draw_bank = val & ((vga.tandy.line_mask&2) ? 0x6 : 0x7);
 		vga.tandy.mem_bank = (val >> 3) & 7;
@@ -1297,6 +1317,18 @@ void VGA_SetupOther(void) {
 	}
 	if (machine==MCH_TANDY) {
 		write_tandy( 0x3df, 0x0, 0 );
+
+		// according to Tandy SX documentation a lot of registers are write-only
+		IO_RegisterReadHandler(0x3d4,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3d5,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3d8,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3d9,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3db,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3dc,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3dd,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3de,read_tandy,IO_MB);
+		IO_RegisterReadHandler(0x3df,read_tandy,IO_MB);
+
 		IO_RegisterWriteHandler(0x3d8,write_tandy,IO_MB);
 		IO_RegisterWriteHandler(0x3d9,write_tandy,IO_MB);
 		IO_RegisterWriteHandler(0x3da,write_tandy,IO_MB);