Support changing the default base address by using the DOS environment variable RESETOPL

If a command line parameter is not specified, use the RESETOPL DOS environment variable as the source for the base address of the OPL2/3 chip. Useful for users of Ad Lib-compatible sound cards (including the Resound cards from TexElec), as they need only put "SET RESETOPL=0x388" in their autoexec.bat file and now they save having to type the base address each time they need to reset their OPL chip.

Author: jdmcs

README.md

@@ -16,6 +16,20 @@ C:\> resetopl 0x240
 Resetting OPL3 at 0x240
 ```
 
+You can change the base by setting the DOS environment variable `resetopl`:
+```
+C:\> set resetopl=0x388
+C:\> resetopl
+Resetting OPL3 at 0x388
+```
+
+The argument still overrides the base specified in the environment variable:
+```
+C:\> set resetopl=0x388
+C:\> resetopl 0x240
+Resetting OPL3 at 0x240
+```
+
 The utility detects if the OPL-chip is present and will abort if the chip
 is not detected:
 ```

resetopl.c

@@ -52,11 +52,23 @@ void set_registers(unsigned int base, unsigned char low, unsigned char high, uns
 int main(int argc, char **argv) {
 	unsigned int base;
 	unsigned char val1, val2;
+	char *dosenvvar = NULL;
 	int opl3 = 0;
 
 	if (argc == 1) {
-		base = 0x220;
-		printf("No argument given, assuming base at 0x%x\n", base);
+		dosenvvar = getenv("RESETOPL");
+		if (dosenvvar == NULL) {
+			base = 0x220;
+			printf("No argument given, assuming base at 0x%x\n", base);
+		}
+		else {
+			char *endp;
+			base = strtoul(dosenvvar, &endp, 0);
+			if (*endp != '\0') {
+				fprintf(stderr, "Invalid base address %s in environment variable!\n", dosenvvar);
+				exit(1);
+			}
+		}
 	} else if (argc == 2) {
 		char *endp;
 		base = strtoul(argv[1], &endp, 0);