[apple] remove indirect jmp in sp_dispatch to fix page boundary bug

Author: markjfisher

Changelog.md

@@ -9,6 +9,7 @@
 - [atari] fix appkey arrays/pointers
 - [make] Use new makefile structure
 - [coco] Use same makefiles as other targets
+- [apple2] change sp_init/sp_dispatch to use self modifying code to fix page boundary issue and replace _sp_dispatch_fn for _sp_dispatch_address
 
 ## [4.2.0] - 2024-06-19
 

apple2/src/bus/sp_data.s

@@ -1,7 +1,6 @@
         .export     _sp_count
         .export     _sp_cmdlist
         .export     _sp_dest
-        .export     _sp_dispatch_fn
         .export     _sp_error
         .export     _sp_is_init
         .export     _sp_payload
@@ -17,8 +16,6 @@ _sp_is_init:        .byte 0
 _sp_dest:           .res 1
 _sp_error:          .res 1
 _sp_count:          .res 2
-.align              2
-_sp_dispatch_fn:    .res 2
 _sp_cmdlist:        .res 10
 
 _sp_payload:        .res SP_PAYLOAD_SIZE

apple2/src/bus/sp_dispatch.s

@@ -1,25 +1,28 @@
         .export     _sp_dispatch
+        .export     _sp_dispatch_address
 
         .import     _sp_count
         .import     _sp_cmdlist
-        .import     _sp_dispatch_fn
         .import     _sp_error
 
 ; KEEP THIS FILE AS ASM AS IT DOES TRICKS WITH DATA AND INDIRECT CALLS TO DISPATCH FUNCTION
 
 ; int8_t sp_dispatch(uint8_t cmd)
 ;
 ; returns any error code from the smart port _sp_dispatch function
-.proc _sp_dispatch
+_sp_dispatch:
         sta     dispatch_data
         lda     #<_sp_cmdlist
         sta     dispatch_data+1
         lda     #>_sp_cmdlist
         sta     dispatch_data+2
         
         ; the SP dispatch alters the return address by 3 bytes to skip the data below.
-        ; it returs with any error codes
-        jsr     do_jmp
+        ; it returs with any error codes.
+        .byte   $20             ; JSR - making this a byte so we can get exact location of address being called
+_sp_dispatch_address:
+        ; overwritten in sp_init to correct address
+        .word   $0000
 
 dispatch_data:
         .byte   $00             ; command
@@ -36,7 +39,3 @@ dispatch_data:
         lda     _sp_error
         rts
 
-do_jmp:
-        jmp     (_sp_dispatch_fn)
-
-.endproc

apple2/src/bus/sp_init.c

@@ -33,8 +33,8 @@ uint8_t sp_init() {
 			// If a match is found, calculate the dispatch function address
 			offset = read_memory(base + 0xFF);
 			dispatch_address = base + offset + 3;
-			sp_dispatch_fn[0] = dispatch_address & 0xFF;
-			sp_dispatch_fn[1] = dispatch_address >> 8;
+			sp_dispatch_address[0] = dispatch_address & 0xFF;
+			sp_dispatch_address[1] = dispatch_address >> 8;
 
 			// now find and return the network id. it's stored in sp_network after calling sp_get_network_id.
 			// we need to set sp_is_init to 1 to stop sp_get_network_id from calling init again and recursing.
@@ -48,8 +48,6 @@ uint8_t sp_init() {
 		}
 	}
 
-	// If no match is found, ensure dispatch function is cleared, sp_is_init is already 0, then return 0 for network not found.
-	sp_dispatch_fn[0] = 0;
-	sp_dispatch_fn[1] = 0;
+	// no match is found, return 0 for network not found.
 	return 0;
 }

apple2/src/include/fujinet-bus-apple2.h

@@ -66,8 +66,8 @@ extern uint8_t sp_payload[];
 // cmd data that is communicated to the SP device
 extern uint8_t sp_cmdlist[10];
 
-// the dispatch function used for doing SP calls for a particular card
-extern uint8_t sp_dispatch_fn[2];
+// the location of the dispatch function to be written by sp_init
+extern uint8_t sp_dispatch_address[2];
 
 // invoke smartport command
 int8_t sp_dispatch(uint8_t cmd);