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Introducing the Instruction Set Part 3 - Intellivision Wiki

Introducing the Instruction Set Part 3


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(Indirect Branching: "Jump Vectors")
(Jump Vectors)
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Jump vectors are locations in memory that hold addresses of functions to jump to.  For example, when an interrupt occurs, the EXEC does a tiny bit of housekeeping (it saves the registers on the stack), and then it looks in locations $100-$101 for the address of an interrupt service routine to jump to.  This allows the game to change what function handles interrupts to suit its needs, despite the fact that the interrupt is handled initially by the EXEC ROM.
Jump vectors are locations in memory that hold addresses of functions to jump to.  For example, when an interrupt occurs, the EXEC does a tiny bit of housekeeping (it saves the registers on the stack), and then it looks in locations $100-$101 for the address of an interrupt service routine to jump to.  This allows the game to change what function handles interrupts to suit its needs, despite the fact that the interrupt is handled initially by the EXEC ROM.
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The following code illustrates how to set up the interrupt service vector.  Since that vector is in 8-bit memory and program addresses are 16 bits, the vector occupies two locations.
The following code illustrates how to set up the interrupt service vector.  Since that vector is in 8-bit memory and program addresses are 16 bits, the vector occupies two locations.

Revision as of 08:33, 5 November 2007

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