Correctly implemented write protection

equi.c

@@ -125,6 +125,9 @@ static struct EquiRAM ram;
 /* Also create an alternative view of the same RAM area for direct offset-based access */
 static uchar* flatram = (uchar *)&ram;
 
+/* write protection threshold RAM address */
+static ushort write_protection_threshold = 0;
+
 /* Error reporting codes */
 enum EquiErrors {
   SUCCESS=0,
@@ -329,8 +332,11 @@ ushort persistOp(FILE *pfd, ushort maddr, ushort dataLen, ushort blk, uchar isWr
     fseek(pfd, ((unsigned long) blk << 10), SEEK_SET); /* blocks are 1K-aligned */
     if(isWrite) /* writing to the persistent area from the memory */
       proc = (ushort) fwrite(&flatram[maddr], 1, dataLen, pfd);
-    else /* reading from the persistent area into the memory */
+    else { /* reading from the persistent area into the memory */
+      if(maddr < write_protection_threshold)
+        trapout(RESTRICTED_WRITE_ERROR);
       proc = (ushort) fread(&flatram[maddr], 1, dataLen, pfd);
+    }
     if(proc != dataLen)
       status = PERSIST_IO_ERROR;
   }
@@ -459,6 +465,8 @@ void equi_main_loop() {
       case INS_STORE: /* main stack -> mem (word) */
         pbuf = popMain();
         pbuf2 = popMain();
+        if(pbuf < write_protection_threshold)
+          trapout(RESTRICTED_WRITE_ERROR);
         flatram[pbuf] = pbuf2 >> 8U;
         flatram[pbuf + 1U] = pbuf2 & 255U;
         break;
@@ -614,6 +622,7 @@ int main(int argc, char* argv[]) {
   ram.cmd_start = (uchar *)&ram.cmdbuf - (uchar *)&ram.stack_size;
   ram.cmd_size = CMD_BUF_SIZE;
   ram.II = ram.CM = ram.IM = ram.MM = 0; /* reset all flags */
+  write_protection_threshold = ram.gpd_start; /* setup write protection */
   /* process command line params */
   if(argc > 1 && argv[1][0] == 'm') /* enter minification mode, don't run the programs */
     ram.MM = 1;
@@ -638,7 +647,6 @@ int main(int argc, char* argv[]) {
   if(host_islittle) {
     ram.stack_size = tobig(ram.stack_size);
     ram.clt_start = tobig(ram.clt_start);
-    ram.gpd_start = tobig(ram.gpd_start);
     ram.cmd_start = tobig(ram.cmd_start);
     ram.cmd_size = tobig(ram.cmd_size);
   }