Add segment type "overlay".

src/ld65/bin.c

@@ -193,9 +193,14 @@ static void BinWriteMem (BinDesc* D, MemoryArea* M)
                     NewAddr += M->Start;
                 }
                 if (DoWrite || (M->Flags & MF_FILL) != 0) {
+                    /* Seek back for "overlay" segments */
+                    if (NewAddr < Addr) {
+                        fseek(D->F, NewAddr - M->Start, SEEK_SET);
+                    } else {
                         WriteMult (D->F, M->FillVal, NewAddr-Addr);
                         PrintNumVal ("SF_OFFSET", NewAddr - Addr);
                     }
+                }
                 Addr = NewAddr;
             }
 

src/ld65/config.c

@@ -653,6 +653,7 @@ static void ParseSegments (void)
         {   "RW",               CFGTOK_RW               },
         {   "BSS",              CFGTOK_BSS              },
         {   "ZP",               CFGTOK_ZP               },
+        {   "OVERLAY",          CFGTOK_OVERLAY          },
     };
 
     unsigned Count;
@@ -757,6 +758,7 @@ static void ParseSegments (void)
                         case CFGTOK_RW:    /* Default */                    break;
                         case CFGTOK_BSS:   S->Flags |= SF_BSS;              break;
                         case CFGTOK_ZP:    S->Flags |= (SF_BSS | SF_ZP);    break;
+                        case CFGTOK_OVERLAY: S->Flags |= SF_OVERLAY;        break;
                         default:           Internal ("Unexpected token: %d", CfgTok);
                     }
                     CfgNextTok ();
@@ -1795,6 +1797,7 @@ unsigned CfgProcess (void)
     for (I = 0; I < CollCount (&MemoryAreas); ++I) {
         unsigned J;
         unsigned long Addr;
+        unsigned Overlays = 0;
 
         /* Get the next memory area */
         MemoryArea* M = CollAtUnchecked (&MemoryAreas, I);
@@ -1848,6 +1851,29 @@ unsigned CfgProcess (void)
             /* Remember the start address before handling this segment */
             unsigned long StartAddr = Addr;
 
+            /* Take note of overlayed segments and make sure there are no other
+            ** segment types following them in current memory region.
+            */
+            if (S->Flags & SF_OVERLAY) {
+            {
+                if (S->Flags & (SF_OFFSET | SF_START)) {
+                    ++Overlays;
+                } else {
+                    CfgError (GetSourcePos (M->LI),
+                              "Segment `%s' of type `overlay' requires either"
+                              " `Start' or `Offset' argument to be specified.",
+                              GetString (S->Name));
+                }
+            }
+            } else {
+                if (Overlays > 0) {
+                    CfgError (GetSourcePos (M->LI),
+                              "Segment `%s' is preceded by at least one segment"
+                              " of type `overlay'",
+                              GetString (S->Name));
+                }
+            }
+
             /* Some actions depend on whether this is the load or run memory
             ** area.
             */
@@ -1896,6 +1922,16 @@ unsigned CfgProcess (void)
                         /* An offset was given, no address, make an address */
                         NewAddr += M->Start;
                     }
+
+                    if (S->Flags & SF_OVERLAY) {
+                        if (NewAddr < M->Start) {
+                            CfgError (GetSourcePos (S->LI),
+                                      "Segment `%s' begins before memory area `%s'.",
+                                      GetString (S->Name), GetString (M->Name));
+                        } else {
+                            Addr = NewAddr;
+                        }
+                    } else {
                         if (NewAddr < Addr) {
                             /* Offset already too large */
                             ++Overflows;
@@ -1914,6 +1950,7 @@ unsigned CfgProcess (void)
                             Addr = NewAddr;
                         }
                     }
+                }
 
                 /* Set the start address of this segment, set the readonly flag
                 ** in the segment, and remember if the segment is in a

src/ld65/config.h

@@ -96,6 +96,7 @@ struct SegDesc {
 #define SF_RUN_DEF      0x0200          /* RUN symbols already defined */
 #define SF_LOAD_DEF     0x0400          /* LOAD symbols already defined */
 #define SF_FILLVAL      0x0800          /* Segment has separate fill value */
+#define SF_OVERLAY      0x1000          /* Segment can be overlayed on another one */
 
 
 

src/ld65/scanner.h

@@ -105,6 +105,7 @@ typedef enum {
     CFGTOK_RW,
     CFGTOK_BSS,
     CFGTOK_ZP,
+    CFGTOK_OVERLAY,
 
     CFGTOK_O65,
     CFGTOK_BIN,