Support for older systems (64K ROMs). See issue #22.

Thanks to @mdanh2002 for submitting the patch.

libretro/Retro68Runtime.h

@@ -36,6 +36,19 @@
 #define RETRO68_GET_DISPLACEMENT(DISPLACEMENT) \
 	_RETRO68_GET_DISPLACEMENT(DISPLACEMENT, )
 
+// Calls to the StripAddress() trap are supposed to make sure addresses are 32-bit clean. 
+// But this trap doesn’t exist on old ROMs and old system versions,
+// so programs built with StripAddress will mysteriously crash. 
+// Those systems always run in 24-bit mode, so we can just take the lower 24
+// bits of the 32 bit address.
+// StripAddress24 must not be used on 32-bit systems, or the resulting crashes
+// will be even more mysterious.
+
+#define StripAddress24(x) ((char*) ((unsigned long)(x) & 0x00FFFFFF))
+#define RETRO68_GET_DISPLACEMENT_STRIP24(DISPLACEMENT) \
+	_RETRO68_GET_DISPLACEMENT(DISPLACEMENT, StripAddress24)
+
+// original StripAddress
 #define RETRO68_GET_DISPLACEMENT_STRIP(DISPLACEMENT) \
 	_RETRO68_GET_DISPLACEMENT(DISPLACEMENT, StripAddress)
     

libretro/relocate.c

@@ -30,6 +30,7 @@
 #include <Sound.h>
 #include <Memory.h>
 #include <OSUtils.h>
+#include <Traps.h>
 
 #include "Retro68Runtime.h"
 
@@ -98,16 +99,42 @@ static Retro68RelocState relocState __attribute__ ((nocommon)) = {
 
 void Retro68Relocate()
 {
+	// memory address to retrieve the ROM type (64K or a later ROM)
+	// see for details http://www.mac.linux-m68k.org/devel/macalmanac.php
+	short* ROM85      = (short*) 0x028E;
+	
+	// figure out which trap is supported
+	Boolean is128KROM = ((*ROM85) > 0);
+	Boolean hasSysEnvirons = false;
+	Boolean hasStripAddr = false;
+	if (is128KROM)
+	{
+		UniversalProcPtr trapSysEnv = GetOSTrapAddress(_SysEnvirons);
+		UniversalProcPtr trapStripAddr = GetOSTrapAddress(_StripAddress);
+		UniversalProcPtr trapUnimpl = GetOSTrapAddress(_Unimplemented);
+
+		hasSysEnvirons = (trapSysEnv != trapUnimpl);
+		hasStripAddr = (trapStripAddr != trapUnimpl);
+	}
+					
 	// Figure out the displacement
 	// what is the difference between the addresses in our program code
 	// and an address calculated by PC-relative access?
 	long displacement;
-	RETRO68_GET_DISPLACEMENT_STRIP(displacement);
+	
+	if (hasStripAddr)
+	{
+		RETRO68_GET_DISPLACEMENT_STRIP(displacement); 
+	}
+	else
+	{
+		RETRO68_GET_DISPLACEMENT_STRIP24(displacement);
+	}
 
 	struct Retro68RelocState *rState = (Retro68RelocState*)
 			((char*)&relocState + displacement);
-	// rState now points to the global relocState variable
 			
+	// rState now points to the global relocState variable
 	if(displacement == 0)
 	{
 		if(rState->bssPtr)
@@ -127,7 +154,6 @@ void Retro68Relocate()
 	struct flat_hdr *header = (struct flat_hdr*) (headerOldVirtualAddress + displacement);
 	uint8_t *base = (uint8_t*) (header+1);
 	
-
 	// Recover the handle to the code resource by looking at the
 	// longword before the FLT header. The resource templates in Retro68.r store the offset
 	// from the beginning of the code resource there.
@@ -146,8 +172,6 @@ void Retro68Relocate()
 		}             
 	}
 	
-
-
 	long bss_size = header->bss_end - header->data_end;
 
 	long n = header->reloc_count;
@@ -198,16 +222,22 @@ void Retro68Relocate()
 	
 	// We're basically done.
 	// Now check whether we're on 68040 or later and need to flush the cache.
-	SysEnvRec env;
+	// only do this if SysEnvirons is available.
-
+	// if SysEnvirons is not available, that means we're on an old System or ROM
-	env.processor = 0;
+	// and likely not using a 68040, so we won't do this
-	SysEnvirons(0, &env);
+	if (hasSysEnvirons)
-	if(env.processor >= env68040)
 	{
-		FlushCodeCache();
+		SysEnvRec env;
-	}
-	// accessing globals and calling functions is OK below here.
 
+		env.processor = 0;
+		SysEnvirons(0, &env);
+		if(env.processor >= env68040)
+		{
+			FlushCodeCache();
+		}
+	}
+	
+	// accessing globals and calling functions is OK below here.
 	rState->headerVirtualAddress += displacement;
 }