From: peter.ferrie@gmail.com   
      
   You'd be running already from 0000:7C00 if you're on a PC.  Your read will   
   overwrite your existing code.   
   Since all of the registers will have been set already for the first load, you   
   could just   
   mov ax, 0x201   
   inc cx   
   pushf   
   push es   
   push bx   
   jmp far es:[0x004c]   
   and forget about retry.   
   Otherwise, yes it's possible to have a polyglot bootsector, even on a 5.25".   
   As for the ProDOS loader, block 1 is "reserved" (=unused).  You could put the   
   original sector there.  Then you wouldn't need to fit the actual loader in the   
   boot sector.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)