I found this whilst surfing the web, I found it useful and as it was not on Aminet,
here it is. It contains assembler function, callable from a C progrm to access the 
Amiga parallel port.

You can use the supplied software with DICE, PhxAss and ManxC. I could not get it to work 
with Storm C but I have only recently got it on my Developer CD.

Original Readme below.

      pardemo.h    - Demo source
      pardemo.c    - Demo source
      PARdemo.doc  - Demo documentation
      Port.txt     - 8520 CIA and port overview, precautions, and guidelines
      ReadMe

   The following C functions are provided:

      getport  -- Get   parallel port access
      freeport -- Free  parallel port access
      portdir  -- Set   parallel port data direction      (0/1 = Input/Output)
      portddr  -- Set   parallel port data direction bits ($00-$FF)
      rdport   -- Read  parallel port data                ($00-$FF)
      wrport   -- Write parallel port data                ($00-$FF)
      busydir  -- Set   BUSY control line data direction  (0/1 = Input/Output)
      rdbusy   -- Read  BUSY control line state           (0/1 = Low/High)
      setbusy  -- Set   BUSY control line state           (0/1 = Low/High)
      poutdir  -- Set   POUT control line data direction  (0/1 = Input/Output)
      rdpout   -- Read  POUT control line state           (0/1 = Low/High)
      setpout  -- Set   POUT control line state           (0/1 = Low/High)
      seldir   -- Set   SEL  control line data direction  (0/1 = Input/Output)
      rdsel    -- Read  SEL  control line state           (0/1 = Low/High)
      setsel   -- Set   SEL  control line state           (0/1 = Low/High)

   Tom Handley / 18 June 93
   BIX     : thandley
   Internet: thandley@bix.com or thandley@nesbbx.rain.com

 PERMSSN    UID  GID    PACKED    SIZE  RATIO METHOD CRC     STAMP          NAME
---------- ----------- ------- ------- ------ ---------- ------------ -------------
[generic]                 5935   11944  49.7% -lh5- c05b Jun 20  1993 PARIO/PARdemo
[generic]                 1969    9086  21.7% -lh5- ae9f Jun 20  1993 PARIO/pardemo.c
[generic]                 1687    6308  26.7% -lh5- d008 Jun 20  1993 PARIO/PARdemo.doc
[generic]                 1322    4212  31.4% -lh5- 8a2c Jun 20  1993 PARIO/pardemo.h
[generic]                 2874   13160  21.8% -lh5- 4288 Jun 20  1993 PARIO/pario.asm
[generic]                 2541   11832  21.5% -lh5- c263 Jun 20  1993 PARIO/PARIO.doc
[generic]                  526    1056  49.8% -lh5- a9ba Jun 20  1993 PARIO/pario.o
[generic]                 3148    7285  43.2% -lh5- 4359 Jun 20  1993 PARIO/Port.txt
[generic]                 1059    2955  35.8% -lh5- 3a78 Jun 20  1993 PARIO/ReadMe
---------- ----------- ------- ------- ------ ---------- ------------ -------------
 Total         9 files   21061   67838  31.0%            Dec 16  2000