[FREELDR] Adapt bootsectors and multiboot code for future base address change (reactos#7785)

See also PRs reactos#8439 and reactos#7530.
CORE-19882

Author: iLauncherDev

boot/freeldr/bootsect/btrfs.S

@@ -386,7 +386,7 @@ FreeLdrFound:
     //mov  dh, 0    // Load boot partition into DH (not needed, FreeLbr detects it itself)
 
     /* Transfer execution to the bootloader */
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 
 // Insert chunk into chunk map (located at DS:[CHUNK_MAP_OFFSET])

boot/freeldr/bootsect/fat.S

@@ -48,10 +48,10 @@ DataAreaStartLow        =   4
 BiosCHSDriveSizeHigh    =   6
 BiosCHSDriveSizeLow     =   8
 BiosCHSDriveSize        =   8
-ReadSectorsOffset       =   10
-ReadClusterOffset       =   12
-PutCharsOffset          =   14
-BootSectorStackTop      =   HEX(7c00) - 16
+ReadSectorsOffset       =   12
+ReadClusterOffset       =   16
+PutCharsOffset          =   20
+BootSectorStackTop      =   HEX(7c00) - 22
 
 
 // org 7c00h
@@ -218,17 +218,22 @@ FoundFreeLoader:
 
     // Save the addresses of needed functions so
     // the helper code will know where to call them.
-    mov word ptr [bp-ReadSectorsOffset], offset ReadSectors     // Save the address of ReadSectors
-    mov word ptr [bp-ReadClusterOffset], offset ReadCluster     // Save the address of ReadCluster
-    mov word ptr [bp-PutCharsOffset],    offset PutChars        // Save the address of PutChars
+    xor bx, bx
+    mov word ptr [bp-(ReadSectorsOffset-2)], bx                 // Save the segment of ReadSectors
+    mov word ptr [bp-(ReadClusterOffset-2)], bx                 // Save the segment of ReadCluster
+    mov word ptr [bp-(PutCharsOffset-2)],    bx                 // Save the segment of PutChars
+    mov word ptr [bp-ReadSectorsOffset],     offset ReadSectors // Save the address of ReadSectors
+    mov word ptr [bp-ReadClusterOffset],     offset ReadCluster // Save the address of ReadCluster
+    mov word ptr [bp-PutCharsOffset],        offset PutChars    // Save the address of PutChars
+    mov byte ptr [PatchedRet],               HEX(CB)            // Patch 'ret' with 'retf'
 
     // Now AX has start cluster of FreeLoader and we
     // have loaded the helper code in the first 512 bytes
     // of FreeLoader to 0000:F800. Now transfer control
     // to the helper code. Skip the first three bytes
     // because they contain a jump instruction to skip
     // over the helper code in the FreeLoader image.
-    ljmp16 0, FREELDR_BASE + 3
+    ljmp16 FREELDR_BASE / 16, 3
 
 
 
@@ -249,13 +254,11 @@ Reboot:
 PutChars:
     lodsb
     or al,al
-    jz short Done
+    jz PatchedRet
     mov ah, HEX(0e)
     mov bx, 7
     int HEX(10)
     jmp short PutChars
-Done:
-    ret
 
 // Displays a bad boot message
 // And reboots
@@ -265,7 +268,6 @@ BadBoot:
 
     jmp short Reboot
 
-
 // Reads cluster number in AX into [ES:0000]
 ReadCluster:
     // StartSector = ((Cluster - 2) * SectorsPerCluster) + ReservedSectors + HiddenSectors;
@@ -354,7 +356,7 @@ ReadSectorsLBALoop:
 
     loop ReadSectorsLBALoop                 // Read next sector
 
-    ret
+    jmp PatchedRet
 
 
 // Reads logical sectors into [ES:BX]
@@ -403,11 +405,12 @@ NoCarryCHS:
                             // Increment read buffer for next sector
     loop ReadSectorsCHSLoop // Read next sector
 
+PatchedRet:
     ret
 
-
 msgDiskError:
-    .ascii "Disk error", CR, LF, NUL
+//  .ascii "Disk error", CR, LF, NUL
+    .ascii "ERR", CR, LF, NUL
 msgFreeLdr:
     .ascii "Ldr not found", CR, LF, NUL
 // Sorry, need the space...

boot/freeldr/bootsect/fat32.S

@@ -425,7 +425,7 @@ LoadFileDone:
     mov  dh, byte ptr ds:[BootPartition]    // Load boot partition into DH
 
     /* Transfer execution to the bootloader */
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 // Returns the FAT entry for a given cluster number
 // On entry EAX has cluster number

boot/freeldr/bootsect/faty.S

@@ -400,7 +400,7 @@ main:
 
     /* Now the complete freeldr imag is loaded.
        Jump to the realmode entry point. */
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 
 

boot/freeldr/bootsect/isoboot.S

@@ -303,7 +303,7 @@ found_drive:
 
 .jump_to_freeldr:
     // Transfer execution to the bootloader.
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 
 /* FUNCTIONS *****************************************************************/

boot/freeldr/bootsect/ntfs.S

@@ -417,7 +417,7 @@ StartSearch:
 
     mov    dl, byte ptr [BootDrive]
     mov    dh, byte ptr [BootPartition]
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 // Error message if Freeldr is compressed, encrypted or sparse
 CompressedFreeldr:

boot/freeldr/bootsect/pc98/fat12fdd.S

@@ -25,10 +25,10 @@ DataAreaStartLow     = 4
 BiosCHSDriveSizeHigh = 6
 BiosCHSDriveSizeLow  = 8
 BiosCHSDriveSize     = 8
-ReadSectorsOffset    = 10
-ReadClusterOffset    = 12
-PutCharsOffset       = 14
-BootSectorStackTop   = HEX(7C00) - 16
+ReadSectorsOffset    = 12
+ReadClusterOffset    = 16
+PutCharsOffset       = 20
+BootSectorStackTop   = HEX(7C00) - 22
 
 if 0
 .macro DEBUG_STOP
@@ -242,9 +242,14 @@ FoundFreeLoader:
      * Save the addresses of needed functions so
      * the helper code will know where to call them
      */
-    mov word ptr [bp - ReadSectorsOffset], offset ReadSectors   // Save the address of ReadSectors
-    mov word ptr [bp - ReadClusterOffset], offset ReadCluster   // Save the address of ReadCluster
-    mov word ptr [bp - PutCharsOffset], offset PrintString      // Save the address of PrintString
+    xor bx, bx
+    mov word ptr [bp-(ReadSectorsOffset-2)], bx                 // Save the segment of ReadSectors
+    mov word ptr [bp-(ReadClusterOffset-2)], bx                 // Save the segment of ReadCluster
+    mov word ptr [bp-(PutCharsOffset-2)],    bx                 // Save the segment of PrintString
+    mov word ptr [bp-ReadSectorsOffset],     offset ReadSectors // Save the address of ReadSectors
+    mov word ptr [bp-ReadClusterOffset],     offset ReadCluster // Save the address of ReadCluster
+    mov word ptr [bp-PutCharsOffset],        offset PrintString // Save the address of PrintString
+    mov byte ptr [PatchedRet],               HEX(CB)            // Patch 'ret' with 'retf'
 
     /*
      * Now AX has start cluster of FreeLoader and we
@@ -254,7 +259,7 @@ FoundFreeLoader:
      * because they contain a jump instruction to skip
      * over the helper code in the FreeLoader image
      */
-    ljmp16 0, FREELDR_BASE + 3
+    ljmp16 FREELDR_BASE / 16, 3
 
 /*
  * Reads cluster number in AX into [ES:BX]
@@ -345,7 +350,7 @@ ReadSectors:
         pop bx
     loop .ReadSectorsLoop                       // Increment read buffer for next sector, read next sector
 
-    ret
+    jmp PatchedRet
 
 /*
  * Prints a character
@@ -380,7 +385,7 @@ PrintString:
     lodsb                                       // Get a single char from a ptr
 
     or al, al
-    jz short .PrintEnd                          // Found NULL
+    jz short PatchedRet                         // Found NULL
 
     cmp al, HEX(0D)
     jz short .PrintStringHandleCR               // Found CR
@@ -398,7 +403,7 @@ PrintString:
         inc si                                  // Skip the next LF character
      jmp short PrintString
 
-.PrintEnd:
+PatchedRet:
     ret
 
 if 0
@@ -476,7 +481,8 @@ msgDiskError:
 msgNotFoundError:
     .ascii "NFE", CR, LF, NUL
 msgAnyKey:
-    .ascii "Press any key", NUL
+//  .ascii "Press any key", NUL
+    .ascii "Press key", NULL
 filename:
     .ascii "FREELDR SYS"
 

boot/freeldr/freeldr/arch/i386/multiboot.S

@@ -3,7 +3,7 @@
  * LICENSE:     GPL-2.0-or-later (https://spdx.org/licenses/GPL-2.0-or-later)
  * PURPOSE:     Support for the Multiboot v1 specification.
  * COPYRIGHT:   Copyright 1998-2002 Brian Palmer <[email protected]>
- *              Copyright 2024 Daniel Victor <[email protected]>
+ *              Copyright 2024-2025 Daniel Victor <[email protected]>
  */
 
 #include <asm.inc>
@@ -190,7 +190,7 @@ mb4:
     mov cr0, rax
 
     /* Jump to real entry point */
-    ljmp16 0, FREELDR_BASE
+    ljmp16 FREELDR_BASE / 16, 0
 
 
     /* Force 8-byte alignment */

boot/freeldr/freeldr/arch/realmode/fathelp.inc

@@ -14,9 +14,13 @@
 #define BiosCHSDriveSizeHigh    6
 #define BiosCHSDriveSizeLow     8
 #define BiosCHSDriveSize        8
-#define ReadSectorsOffset       10
-#define ReadClusterOffset       12
-#define PutCharsOffset          14
+#define ReadSectorsOffset       12
+#define ReadClusterOffset       16
+#define PutCharsOffset          20
+
+#define ReadSectors dword ptr ss:[bp-ReadSectorsOffset]
+#define ReadCluster dword ptr ss:[bp-ReadClusterOffset]
+#define PutChars    dword ptr ss:[bp-PutCharsOffset]
 
 #define OEMName                 3
 #define BytesPerSector          11
@@ -65,8 +69,8 @@ FatHelperEntryPoint:
     push ax
 
     /* Display "Loading FreeLoader..." message */
-    mov si, offset msgLoading
-    call word ptr [bp-PutCharsOffset]
+    mov si, offset msgLoading - FREELDR_BASE
+    call CS_PutChars
 
     call ReadFatIntoMemory
 
@@ -92,7 +96,7 @@ LoadFile3:
     push ax
     xor  bx,bx              // Load ROSLDR starting at 0000:8000h
     push es
-    call word ptr [bp-ReadClusterOffset]
+    call ReadCluster
     pop  es
 
     xor  bx,bx
@@ -133,7 +137,7 @@ ReadFatIntoMemory:
     mov bx, HEX(7000)
     mov es,bx
     xor bx,bx
-    call word ptr [bp-ReadSectorsOffset]
+    call ReadSectors
     ret
 
 
@@ -216,6 +220,22 @@ IsFat12_2:
 IsFat12_Done:
     ret
 
+CS_PutChars:
+    /* Save necessary registers */
+    push ax
+    push ds
+
+    /* Prepare ds before PutChars call */
+    mov ax, cs
+    mov ds, ax
+
+    /* Display the message */
+    call PutChars
+
+    /* Restore necessary registers and return */
+    pop ds
+    pop ax
+    ret
 
 msgLoading:
     .ascii "Loading FreeLoader...", CR, LF, NUL