SEQUENCE OF COMMANDS FOR AN EXPOSURE

expose_callback in expctl_stubs.c is called by the window manager when the
user clicks on the expose button.

Do_expose in expctl.c is then called by expose_callback.

Do_readout_ccd in expctl.c is called from within do_expose.

Within do_readout_ccd, three commands are executed/sent,
	1)  The total time of the image is written to the boards via the
	    function dsp_command.  Dsp_command then expects a DON before
	    returning to do_readout_ccd.
	2)  A reset is sent (send_reset) and nothing is expected.
	3)  The sex (start exposure) command is sent via dsp_command.
Dsp_command doesn't send the sex command.  Instead it calls the function
image_transfer in libdsp.c, and this is where everything goes on.
	1) Send the sex command via send_sex
	2) expect a DON
	3) if the remaining time is less than or equal to 4 seconds, then
	   send a word width of 1 and set/send the timeout for the image
	   transfer.
	   If there are more than 4 seconds left, then send a timeout of 
	   75 usecs and get the remaining time from the utility board.
	   When the time remaining is less than 4 seconds, send a word width
	   of 1 and set/send the timeout for the image transfer.
	4) Fork a child process to perform the read and if the fork fails,
	   perform the read itself.
The Read.
The action of transferring the image is done via the UNIX system call 'read'.
The name of the device driver, the location in user memory that will hold the
image, and the size of the image are the parameters passed to read.  At this
point, the device driver is now responsible for transferring the image.  The
steps to do so are, (for the revision 1.6.x device driver)
	1) Allocate and lock three 64KB buffers
	2) Load the address and size of the first buffer onto the Address
	   Register and the Byte Count Register of the DMA.
	3) Enable the DMA to perform a data transfer by setting the appropriate
	   bits in the DMA's condition status register.
	4) Load the address and the size of the next buffer onto the Next
	   Address register and the Next Byte Count Register.
	5) When the first buffer has filled, the DMA will continue with the
	   buffer in the Next Registers by moving the address and size in the
	   Next Registers into the Address and Byte Count Registers, thus 
	   clearing the Next Registers.  The device driver will load the
	   address and the size of the next buffer onto the Next Address
	   Register and the Next Byte Count Register.
	6) Transfer the data in the first buffer into user memory.
	7) Continue with steps 5 and 6 until the entire image has transferred.
The Fork.
The forking of the child process is a VERY time consuming and cpu expensive
process, as is the time spent while child process is preparing for the image.
In hindsight, step #4 has the potential to lose data.  Therefore, it might be
prudent to fork the child process before the send_sex command, and have the
child process waiting on a semaphore.  When the time comes to transfer the
image, the semaphore could be changed and the child process could begin the
data transfer immediately.

Another Important note: Currently, the Rev 1.6.x device driver does not support
timeouts.  Whenever a timeout is sent, the device driver just ignores it.