Issue #51: extend MZ header to 64 bytes

This lets one append an NE, LE, PE, etc executable to the DOS executable and set the dword at offset 0x3C of the header to point to it, thus making a fat executable containing a DOS part/stub and a part for some other OS/purpoose. Or one can simply store additional data in the header if desired. Note, the DPMI stub does not depend on nor utilizes this mechanism to locate the appended 32-bit protected mode part.
alexfru · Apr 28, 2024 · b120a9c · b120a9c
1 parent 07c78cf
commit b120a9c
Show file tree

Hide file tree

Showing 15 changed files with 39 additions and 23 deletions.
diff --git a/v0100/bind/n2f.exe b/v0100/bind/n2f.exe
diff --git a/v0100/bind/smlrc.exe b/v0100/bind/smlrc.exe
diff --git a/v0100/bind/smlrcc.exe b/v0100/bind/smlrcc.exe
diff --git a/v0100/bind/smlrl.exe b/v0100/bind/smlrl.exe
diff --git a/v0100/bind/smlrpp.exe b/v0100/bind/smlrpp.exe
diff --git a/v0100/bindp/n2f.exe b/v0100/bindp/n2f.exe
diff --git a/v0100/bindp/smlrc.exe b/v0100/bindp/smlrc.exe
diff --git a/v0100/bindp/smlrcc.exe b/v0100/bindp/smlrcc.exe
diff --git a/v0100/bindp/smlrl.exe b/v0100/bindp/smlrl.exe
diff --git a/v0100/bindp/smlrpp.exe b/v0100/bindp/smlrpp.exe
diff --git a/v0100/binl/smlrl b/v0100/binl/smlrl
diff --git a/v0100/binm/smlrl b/v0100/binm/smlrl
diff --git a/v0100/binw/smlrl.dat b/v0100/binw/smlrl.dat
diff --git a/v0100/lib/dpstub.exe b/v0100/lib/dpstub.exe
diff --git a/v0100/smlrl.c b/v0100/smlrl.c
@@ -411,11 +411,12 @@ typedef struct
   uint16 InitCs;
   uint16 ReloOff;
   uint16 OverlayNo;
-  uint16 FirstRelo[2];
+  uint16 Reserved[16];
+  uint32 NewFmtHdrOff;
 } tDosExeHeader;
 
 #ifndef __SMALLER_C__
-C_ASSERT(sizeof(tDosExeHeader) == 32);
+C_ASSERT(sizeof(tDosExeHeader) == 64);
 #endif
 
 typedef struct
@@ -1811,21 +1812,22 @@ void RelocateAndWriteSection(FILE* outStream, FILE* inStream, size_t size, tElfM
 
 tDosExeHeader DosExeHeader =
 {
-  { "MZ" }, // Signature
-  0,        // PartPage
-  0,        // PageCnt
-  0,        // ReloCnt
-  2,        // HdrSize
-  0,        // MinAlloc
-  0,        // MaxAlloc
-  0,        // InitSs
-  0xFFFC,   // InitSp
-  0,        // ChkSum
-  0,        // InitIp
-  0,        // InitCs
-  28,       // ReloOff
-  0,        // OverlayNo
-  { 0, 0 }  // FirstRelo
+  { "MZ" },                     // Signature
+  0,                            // PartPage
+  0,                            // PageCnt
+  0,                            // ReloCnt
+  sizeof(tDosExeHeader) / 16,   // HdrSize
+  0,                            // MinAlloc
+  0,                            // MaxAlloc
+  0,                            // InitSs
+  0xFFFC,                       // InitSp
+  0,                            // ChkSum
+  0,                            // InitIp
+  0,                            // InitCs
+  28,                           // ReloOff (offsetof(tDosExeHeader, Reserved))
+  0,                            // OverlayNo
+  { 0 },                        // Reserved
+  0                             // NewFmtHdrOff
 };
 
 uint8 DosMzExeStub[128] =
@@ -2651,6 +2653,13 @@ void RwDosExe(void)
   switch (OutputFormat)
   {
   case FormatDosExeSmall:
+    // The very first byte after the MZ header is the first instruction in the
+    // code segment. Its CS:IP (CS is relative to the actual load location)
+    // is going to be -sizeof(tDosExeHeader)/16 : sizeof(tDosExeHeader).
+    // (If you compute this CS*16+IP modulo 2**20, you'll get relative physical
+    // address 0)
+    // Such a setup helps a bit with debugging: IP of every instruction in the
+    // code segment is equal to instruction's offset within the .EXE.
     Origin = sizeof(tDosExeHeader);
     hdrsz = sizeof(tDosExeHeader);
     if (StackSize == 0xFFFFFFFF)
@@ -2664,6 +2673,7 @@ void RwDosExe(void)
     break;
   case FormatDosExeHuge:
   case FormatDosExeUnreal:
+    // See the above comment on CS:IP setup.
     Origin = sizeof(tDosExeHeader);
     hdrsz = sizeof(tDosExeHeader);
     if (StackSize == 0xFFFFFFFF)
@@ -2713,9 +2723,12 @@ void RwDosExe(void)
       DosExeHeader.PartPage = fsz % 512;
       DosExeHeader.PageCnt = (fsz + 511) / 512;
       DosExeHeader.InitIp = FindSymbolAddress(EntryPoint);
-      DosExeHeader.InitCs = 0xFFFE;
-      DosExeHeader.InitSs = (pSectDescrs[SectCnt].Stop + 15) / 16 - 2; // data/stack segment starts right after code segment's padding
-      DosExeHeader.MaxAlloc = DosExeHeader.MinAlloc = 4096 - dsz / 16; // maximum stack size = data segment size - data size
+      DosExeHeader.InitCs = 0xFFFF - sizeof(tDosExeHeader) / 16 + 1;
+      // data/stack segment starts right after code segment's padding:
+      DosExeHeader.InitSs = (pSectDescrs[SectCnt].Stop + 15) / 16 -
+                            sizeof(tDosExeHeader) / 16;
+      // maximum stack size = data segment size - data size:
+      DosExeHeader.MaxAlloc = DosExeHeader.MinAlloc = 4096 - dsz / 16;
       Fwrite(&DosExeHeader, sizeof DosExeHeader, fout);
     }
     break;
@@ -2743,10 +2756,13 @@ void RwDosExe(void)
       DosExeHeader.PartPage = fsz % 512;
       DosExeHeader.PageCnt = (fsz + 511) / 512;
       DosExeHeader.InitIp = ip & 0xF;
-      DosExeHeader.InitCs = (ip >> 4) - 2;
-      DosExeHeader.InitSs = (sz + 15) / 16 - 2; // stack segment starts right after data segment's padding
+      DosExeHeader.InitCs = (ip >> 4) - sizeof(tDosExeHeader) / 16;
+      // stack segment starts right after data segment's padding:
+      DosExeHeader.InitSs = (sz + 15) / 16 - sizeof(tDosExeHeader) / 16;
       DosExeHeader.InitSp = StackSize;
-      DosExeHeader.MaxAlloc = DosExeHeader.MinAlloc = (sz - fsz + 15) / 16 + (StackSize + 15) / 16 + 1; // maximum stack size = 64KB
+      // maximum stack size = 64KB:
+      DosExeHeader.MaxAlloc = DosExeHeader.MinAlloc =
+        (sz - fsz + 15) / 16 + (StackSize + 15) / 16 + 1;
       Fwrite(&DosExeHeader, sizeof DosExeHeader, fout);
     }
     break;