CBM BASIC loader should use the actual address of main (fixes #111)

docs/api/commodore-programming-guide.md

@@ -2,6 +2,11 @@
 
 ### A note about Commodore 64
 
+#### Unusual main function location
+
+If you're creating a prg file and your `main` function may be located at address $2710 hex (10000 decimal) or higher,
+you might need to define the `DISPLACED_MAIN=1` preprocessor feature. 
+
 #### Multifile programs
 
 A multifile program is a program stored on a disk that consists of the main program file that is executed first

docs/lang/preprocessor.md

@@ -103,6 +103,8 @@ Some libraries may require that some of these be defined.
 
 * `KEYBOARD` – 1 if the target has a keyboard, 0 otherwise
 
+* `DISPLACED_MAIN` – set this to 1 if the `main` function is in a very unusual location for the target
+
 * `USE_MOUSE_MBM` –  set this to 1 if you want to enable middle button support for the mouse. 
 
 * `JOYSTICKS` – the maximum number of joysticks using standard hardware configurations, may be 0

include/cbm/basic_loader.mfk

@@ -0,0 +1,31 @@
+#template $ADDR$
+
+#if not(CBM)
+#warn cbm/basic_loader module should be only used on Commodore targets
+#endif
+
+const array _basic_loader @ $ADDR$ = [
+#if DISPLACED_MAIN
+    @word_le [
+        $ADDR$ + 0xB
+    ],
+#else
+    @word_le [
+        $ADDR$ + if(main.addr >= 10000, 1/0, 0xA) // use -D DISPLACED_MAIN=1 if you get an error here
+    ],
+#endif
+    10,
+    0,
+    $9e,
+#if DISPLACED_MAIN
+    $30 + (main.addr/10000)%%10,
+#endif
+    $30 + (main.addr/1000)%%10,
+    $30 + (main.addr/100)%%10,
+    $30 + (main.addr/10)%%10,
+    $30 + (main.addr/1)%%10,
+    0,
+    0,
+    0
+    ]
+

include/loader_0401.mfk

@@ -1,19 +1 @@
-#if not(CBM)
-#warn loader_0401 module should be only used on Commodore targets
-#endif
-
-const array _basic_loader @$401 = [
-    $0b,
-    4,
-    10,
-    0,
-    $9e,
-    $31,
-    $30,
-    $33,
-    $37,
-    0,
-    0,
-    0
-    ]
-
+import cbm/basic_loader<$401>

include/loader_0801.mfk

@@ -1,19 +1 @@
-#if not(CBM)
-#warn loader_0801 module should be only used on Commodore targets
-#endif
-
-const array _basic_loader @$801 = [
-    $0b,
-    $08,
-    10,
-    0,
-    $9e,
-    $32,
-    $30,
-    $36,
-    $31,
-    0,
-    0,
-    0
-    ]
-
+import cbm/basic_loader<$801>

include/loader_0801_16bit.mfk

@@ -22,4 +22,5 @@ asm void __init_16bit() @$80D {
     clc
     xce
     sep #$30
-}
+    jmp main
+}

include/loader_1001.mfk

@@ -1,19 +1 @@
-#if not(CBM)
-#warn loader_1001 module should be only used on Commodore targets
-#endif
-
-const array _basic_loader @$1001 = [
-    $0b,
-    $10,
-    10,
-    0,
-    $9e,
-    $34,
-    $31,
-    $30,
-    $39,
-    0,
-    0,
-    0
-    ]
-
+import cbm/basic_loader<$1001>

include/loader_1201.mfk

@@ -1,19 +1 @@
-#if not(CBM)
-#warn loader_1201 module should be only used on Commodore targets
-#endif
-
-const array _basic_loader @$1201 = [
-    $0b,
-    $12,
-    10,
-    0,
-    $9e,
-    $34,
-    $36,
-    $32,
-    $31,
-    0,
-    0,
-    0
-    ]
-
+import cbm/basic_loader<$1201>

include/loader_1c01.mfk

@@ -1,19 +1 @@
-#if not(CBM)
-#warn loader_1c01 module should be only used on Commodore targets
-#endif
-
-const array _basic_loader @$1C01 = [
-    $0b,
-    $1C,
-    10,
-    0,
-    $9e,
-    $37,
-    $31,
-    $38,
-    $31,
-    0,
-    0,
-    0
-    ]
-
+import cbm/basic_loader<$1c01>