TUTORIAL TOPIC: SCRIPTING
TUTORIAL TITLE: UNDERSTANDING SPRMs
 

 

Every DVD player contains 16 General Parameter Register Memories (GPRMs, or “germs”). Similar to RAM in your computer, these are generic memory locations the player can use like a scratchpad to read and write information while it is operating. Scripting gives you to access 8 of these GPRMs, numbered GPRM 0 – GPRM 7. The remaining 8 are unavailable for scripting purposes. GPRMs have no specific preset function – any GPRM can be used for a particular function in one disc, and then may be used for a completely different function in another project. You will find that you will use GPRMs a LOT in Scripting. They are extremely powerful, and very versatile.

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Understanding General Parameters - GPRMs 

Each DVD player contains 16 General Parameter Register Memories (GPRMs, or “germs”). Similar to RAM in your computer, these are generic memory locations the player can use like a scratchpad to read and write information while it is operating. DVD Studio Pro scripting allows you to access 8 of these GPRMs, numbered GPRM 0 – GPRM 7. The remaining 8 are unavailable for scripting purposes. All GPRMs default to a value of 0 initially.

GPRMs have no specific preset function – any GPRM can be used for a particular function in one disc, and then may be used for a completely different function in another disc. You will find that you will use them a LOT in Scripting. They are extremely powerful, and very versatile.  

GPRM “issues” to be aware of

GPRMs can store numerical values between 0 and 65535 (the largest
value possible in 16 bits), but you will want to be careful if you are planning on performing math operations on GPRM values.  

Overflow and Underflow  

The DVD command set has no provision for overflow or underflow conditions caused by math operations on GPRMs, so you will find that operations that result in values less than zero (0) or greater than 65535 will “wrap around” that end of the numerical scale, and your resulting operations may not be what you expect!  As an example, subtracting 4 from 3 would normally result in –1, but in the GPRM, without underflow protection, the –1 will instead become 65535! To prevent this behavior from being a problem, you can program your script to check for this condition before it happens.  

GPRM volatility 

Again like the RAM in your Computer, GPRM contents are “volatile”. If the player power goes off, or the disc is stopped, the contents of the GPRMs will generally revert to zero. This is disconcerting if the user stops your disc in the middle of a highly interactive section, utilizing lots of GPRMs to track what they have seen. Those GPRM values will disappear, making continued operation of the disc unpredictable, or impossible. This should be kept in mind when you are creating a DVD that relies heavily on scripting to keep track of GPRM values. Locking the DVD remote to prevent selecting “STOP” isn’t the way. But perhaps an advisory screen letting the user know that stopping the DVD will cause problems continuing the program might be the way.  

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Bruce - "the Digital Guy"