Topic       : TOS - The Operating System
Author      : 
Version     : tos.hyp (December 19, 2008)
Subject     : Programmieren/Atari
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Compiled on : Atari
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View Ref-File13.6.2  Definition of the driver daisy chain                       TOS

Basis of the GDPS is the vector $41C which is unused by the operating 
system. Normally this vector contains 0L. In GDPS this vector points 
to the header of the first device driver in the chain. This header is 
built up as follows (rAdr=relative Address, S=Size):

     rAdr S  Meaning

      OH  L  Pointer to the next driver header, or 0 in last driver
             of the chain
      4H  L  'GDPS' = 0x47445053 magic string as identifier for GDPS

From this follows the method for linking a driver in and out of the 
chain.

  1. Linking in of a driver

     The driver copies the vector placed at 0x41C into its header. 
     Subsequently it enters the address of its header into 0x41C.

     Assembler example (to be executed in supervisor-mode):

         len     header(pe),a0  * Address of the GDPS header
         clr.l   (a0)           * Initialize NEXT to 0
         cmp.l   0x41C,a0       * = old value
         beq     unten          * Yes, no endless chaining!
         move.l  0x41C,(a0)     * sonst NEXT eintragen
         move.l  a0,0x41C       * and link header into the chain

  2. Unlinking of a driver

     The driver searches the chain for the pointer to its header. It 
     then replaces it with a pointer found in its header.

     Assembler example (to be executed in supervisor-mode):

                    move.l  #0x41C,a0
                    lea     header(pe),a1
        search:     cmp.l   (a0),a1
                    beq     found
                    move.l  (a0),a0
                    bra     search
        found:      move.l  (a1),(a0)

  3. Searching for a driver

     Searches for a driver too should be performed in supervisor-mode. 
     As the operating system does not set the variable 0x41C to 0 at a 
     warm start, one has to check for the magic string for each vector 
     found.

     There follows a sample routine in GFA Basic notation that writes 
     the found and valid vectors into an integer array:

        DIM vector%(31)                   ! Here addresses of the drivers lie
        index%=0                          ! Initialize counter to 0
        adr%=SLPEEK($41C)                 ! Address of the first driver
        WHILE adr%<>0 AND SLPEEK(adr%+4)=$47445053
                                          ! Test: Address valid and magic OK?
           vector%(index%)=adr%           ! If yes, remember address
           inc index%                     ! Increment counter by one
        WEND
        PRINT index%;" Driver found!"