Implement original disk behavior, remove copy protection workaround

Add a pre-access hook for disk access, and emulate disk controller
behavior as necessary to satisfy the game's copy protection. This makes
it possible to remove the periodic poking into gamestate to defuse it.

Thanks to Wayne Holder for explaining how the system works, it would
have been much harder to figure out without that.

This removes the last TODO.

README.md

@@ -190,14 +190,6 @@ have been implemented, Though I cannot guarantee that there aren't any bugs
 left. If so, please report them through the issue tracker, along with a saved
 state if possible.
 
-### To be done
-
-Non-critical:
-
-- The copy protection (integrity check) is mitigated in an ugly and brittle
-  way. It would be better to emulate the underlying mechanism (which is known)
-  in the simulated disk controller.
-
 P-system
 -----------
 

src/psys/psys_rsp.c

@@ -148,7 +148,7 @@ static psys_word unitrw(struct psys_state *state, struct psys_rsp_state *rsp, bo
     } break;
     case PSYS_UNIT_DISK0: {
         unsigned x, srcblk;
-        if (ctrl & 2) { /* physical addressing starts a second earlier */
+        if (ctrl & 2) { /* physical addressing starts a track earlier */
             len    = 512;
             srcblk = block;
         } else {
@@ -170,6 +170,9 @@ static psys_word unitrw(struct psys_state *state, struct psys_rsp_state *rsp, bo
             if (PDBG(state, RSP)) {
                 psys_debug("%s 0x%04x 0x%04x 0x%04x\n", wr ? "write" : "read", buf_addr + x, srcblk * PSYS_BLOCK_SIZE, remainder);
             }
+            if (rsp->pre_access_hook) {
+                rsp->pre_access_hook(rsp->pre_access_hook_data, unit, srcblk, wr);
+            }
             if (wr) {
                 memcpy(rsp->disk0 + srcblk * PSYS_BLOCK_SIZE, buf + x, remainder);
             } else {
@@ -766,6 +769,13 @@ void psys_rsp_set_disk(struct psys_binding *b, int n, void *data, size_t size, s
     rsp->disk0_wrap  = wrap;
 }
 
+void psys_rsp_set_pre_access_hook(struct psys_binding *b, psys_rsp_pre_access_hook *pre_access_hook, void *data)
+{
+    struct psys_rsp_state *rsp = (struct psys_rsp_state *)b->userdata;
+    rsp->pre_access_hook       = pre_access_hook;
+    rsp->pre_access_hook_data  = data;
+}
+
 void psys_rsp_event(struct psys_binding *b, psys_word event, bool taskswitch)
 {
     struct psys_rsp_state *rsp = (struct psys_rsp_state *)b->userdata;

src/psys/psys_rsp.h

@@ -83,6 +83,8 @@ enum {
     GDIR_ENTRY_SIZE  = 0x1a,
 };
 
+typedef void(psys_rsp_pre_access_hook)(void *data, int disk, unsigned blk, bool wr);
+
 #define PSYS_MAX_EVENTS 64
 
 struct psys_rsp_state {
@@ -97,13 +99,18 @@ struct psys_rsp_state {
 
     /* simulated inputs */
     unsigned time;
+
+    /* hooks */
+    psys_rsp_pre_access_hook *pre_access_hook;
+    void *pre_access_hook_data;
 };
 
 /* Fixed I/O block size */
 #define PSYS_BLOCK_SIZE 512
 
 extern struct psys_binding *psys_new_rsp(struct psys_state *state);
 extern void psys_rsp_set_disk(struct psys_binding *b, int n, void *data, size_t size, size_t track, bool wrap);
+extern void psys_rsp_set_pre_access_hook(struct psys_binding *b, psys_rsp_pre_access_hook *pre_access_hook, void *data);
 extern void psys_destroy_rsp(struct psys_binding *b);
 
 /* Trigger event */