ALIA Background Information

What is ALIA ?
ALIA is a seven process scanning program which is used to operate the CASB GAMMA microdensitometers. Each of the seven processes runs a program to operate a particular subsystem of the hardwareand they communicate via shared memory (global sections) and event flags. The user interacts only with the controller program.

This software architecture allows each individual program to be relatively simple, and thus maintainable. Additionally, a change to the hardware of one subsystem can be easily accomodated by modifying or replacing one program with minimal impact on the rest of the software.


What is the SAC?
SAC stands for Stand Alone Console. This program operates the GAMMA hardware when a scan is not in progress, for example, updating position displays and performing specific functions requested by the user.

Although the SAC can run independently of ALIA -- hence its name -- ALIA cannot run independently of the SAC . The controller program of ALIA interacts with the SAC to perform all hardware operations during scan setup.

What are the seven processes that comprise ALIA ?
The Controller
This is the program that the user interacts with. It performs all setup functions (in conjuction with the SAC , initializes all variables in the global sections , spawns the other processes, and feeds them the relevant data for each pass until the scan is complete.
The Data Collector
Manages the data collection subsystem: the PPG, the scaler which counts the pixels, and the FIFO which collects the pixels. Performs intermediate reads during a pass so as not to overflow the FIFO, and stores all pixel data in the appropriate buffer in the data section.

The data collector also acts as gatekeeper for the other processes at the end of each pass. It waits to hear that all other processes are ready to proceed with the next pass before opening the gate and instructing the other processes to go ahead.

The Servo
Manages the x and y motion of the table, and defines the x position at which we begin taking data for each pass.
The Laser Monitor
Manages the laser monitoring subsystem, namely the transient recorder CAMAC module. Collects laser intensity data before the beam passes through the plate, and saves it into the appropriate buffer in the data section.
The AO controller
Manages the acousto-optical crystal subsystem. Sends commands to the DSP (Digital Signal Processor) to step through the frequency table in the appropriate direction.
The Scanning Console
Manages the status display in room 133. Will someday be connected to the scan pause request button, and collect diagnostic data on the GAMMA hardware.
The Laser Data Processor
Manipulates the data and writes it to disk. Manipulation includes unscrambling the pixels, correcting for laser flicker, applying shading to correct for vignetting, and transforming to a 15 bit density scale.

What logical names are needed for ALIA to function properly, and how are they defined?
All needed logical names are defined in the group logical name table. This occurs automatically at boot time. The permanent global sections are created at the same time.

Of course, it is always possible that something has gone wrong and the things that should have happened at boot time, didn't. If this seems to have happened, you can manually perform the task by executing the com file disk$schmidt:[plateproc.control]pixel1_bootme.com.

What disks are crossmounted to and from the cluster, and why?
We have arranged with the system managers to mount several important production disks so that they are accessible from the Pixels - namely, disk$gsc2 which has databases on it, disk$catalogs which holds the catalogs, and disk$prod which has additional tools and databases.

As a reminder that these disks are not really local, they have been mounted with the prefix h, eg, disk$hgsc2. The h stands for Haven. If Haven is down, the disks will be inaccessible to the Pixels, even if they are accessible to the rest of the cluster.

Similarly, the pixel disks have been crossmounted to Haven. This was done primarily to support computer ops backups.

If the crossmounted disks are not available, ALIA itself will not be impacted; however, further pipeline processing and database updates will most likely be affected.


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Vicki Laidler -- laidler@stsci.edu
Last updated 8/29/95