Page 42 - Designing Autonomous Mobile Robots : Inside the Mindo f an Intellegent Machine

The Basics of Real-time Software (For Mere Mortals)

its execution resumes. This is no more complicated than putting all your work onto
a stack on your desk when a visitor arrives, and when they leave, taking it back off
and resuming what you were doing.

Stack Pointer

Previously
Stored Data Unused Unused Unused Unused Unused

Stack Memory Before Interrupt
Stack Pointer

Address where
Previously
Stored Data Program was Unused Unused Unused Unused
Interrupted
Stack Memory After Interrupt Event

Previously
Unused Unused Unused Unused Unused
Stored Data
Stack Memory After Return from Interrupt

Figure 3.1. A simplified representation of stack operations during interrupts

The interrupt vector is the address to which the program will jump automatically
when an interrupt occurs. This interrupt vector must have been set to point to a valid
interrupt handler routine before the interrupt was enabled or the computer may begin
trying to read and execute memory that is not valid. This has an effect similar to
trying to assemble Christmas toys by discarding the instructions and reading the
Kama Sutra instead; it is amusing but fraught with hazard. The technical term for
this condition is going off into the woodwork. It can cause some very strange behavior.

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