User space
-------------------------------------------------------------------------
                                 Kernel space

       |------------|    |------------|    |------------|
upper  |     SD     |    |     SR     |    |     ST     |    |------------|
level  |   disks    |    |cdroms/dvds |    |   tapes    |    |     SG     |
       |block device|    |block device|    |char device |    |pass-through|
       | {sd_mod.o} |    | {sr_mod.o} |    |   {st.o}   |    |char device |
       |------------|    |------------|    |------------|    |   {sg.o}   |
                                                             |------------|


                 |--------------------------------------------|
                 |                  SCSI                      |
mid              |              unifying layer                |
level            |               {scsi_mod.o}                 |
                 |  scsi*.[hc], hosts.[hc], constants.c       |
                 |--------------------------------------------|



          |---------------|                     |---------------|
          |Host (e.g. UW) |-|                   |Pseudo drivers |-|
lower     | Bus Adapter   | |-|                 |for non SCSI   | |-|
level     |  Drivers      | | |                 |buses          | | |
          |(e.g. aic7xxx) | | |                 |(e.g. ide-scsi)| | |
          |---------------| | |                 |---------------| | |
            |---------------| |                   |---------------| |
              |---------------|                     |---------------|


      |-------------------------------------------------|
upper |               Uniform CD-ROM layer              |
level |   __________________________                    |
      |  | DVD, audio, etc          |                   |
      |-------------------------------------------------|


       |------------|    |------------|   |------------|
mid    |   ide-cd   |    |     sr     |   |  Older     |
level  |ATAPI CD-ROM|    |cdroms/dvds |   |generic CD  |
       |driver      |    |block device|   |drivers (sb,|
       | {ide-cd.o} |    | {sr_mod.o} |   |mitsumi,etc)|
       |------------|    |------------|   |------------|

       |------------|    |------------|
low    |    IDE     |    | SCSI mid   |
level  | sub system |    |  level     |
       |driver      |    |            |
       | {ide-cd.o} |    |+ SCSI lower|
       |------------|    |------------|


   +---------------+
   | scsi midlevel |
   +---------------+
     |           |
    ...      +--------+
             | ppscsi |
             +--------+
             /    |    \...
      +------+ +------+
      | bpck | | vpi0 |
      +------+ +------+