From: james.harris.1@gmail.com   
      
   I've just started looking at booting from FAT16 and it's proving to be a   
   bit of a challenge. That's because of what I want it to do and because   
   space is soooo limited! I've set out my idea below. How does it look to   
   you?   
      
   For nomenclature, the sequence to boot from a hard disk can be seen as   
      
      MBR --> VBR --> OSloader   
      
   i.e. Master Boot Record (first sector of a disk) loads the Volume Boot   
   Record (first sector of a partition). It, in turn, loads the (larger) OS   
   loader which is big enough to include code to do a lot more.   
      
   As a reminder, the layout of a FAT16 volume is   
      
         
      
   AIUI there are two 'traditional' approaches. One is to make the reserved   
   area large enough for plenty of code. The other (taken by LILO, IIRC) is   
   to have the os loader in unmovable sectors of the filespace and to   
   'hardcode' their location into the VBR.   
      
   It is /possible/ for there to be plenty of space in the reserved section   
   of a FAT volume but that cannot be counted on. AIUI it depends on how it   
   has been formatted. In an already formatted volume the reserved area   
   might be only one sector, the VBR itself and nothing else. So I don't   
   believe I can rely on there being any more than 512 bytes in that space.   
      
   I'd also like to avoid the LILO approach. It's a pain when a new OS   
   loader is installed, requiring something to update the hardcoded block   
   addresses in the VBR.   
      
   So, what can be done?   
      
   Going back to the boot steps the MBR will be 'standard', i.e. although   
   the BIOS will have loaded it to 0x7c00 it will move itself out of the   
   way (often to 0x600) and load the VBR to 0x7c00. It could be any MBR. It   
   would not need to be specific to a particular OS.   
      
   Effectively, the first point I will get control is when my VBR is loaded   
   to 0x7c00 and started.   
      
   I will then need to load the OS loader but than can be difficult in just   
   512 bytes as I would like it to be a normal file. No special sectors. No   
   unmovable sectors. My plan, then, is to split the work as follows.   
      
   The VBR will look through the root directory until it finds the OS   
   loader by name. When it finds it it will load just its first sector and   
   will transfer control to that sector.   
      
   The first sector of the OS loader won't have to include code to read the   
   root directory again meaning it will /hopefully/! have enough program   
   space to follow the FAT chain to load more of itself.   
      
   So loading would be in at least these parts:   
      
   1. VBR finds OS loader file and loads 1st sector of it.   
      
   2. OS loader's first sector follows the FAT chain to load more of (maybe   
   all of) OS loader.   
      
   3. The OS loader is then large enough to do whatever's necessary.   
      
      
   That's the plan. What do you think of it? Any better ideas? Suggestions   
   welcome.   
      
      
      
   --   
   James Harris   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)