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Hardware Interrupts

Thread Scheduling 4-19

4.2.4 Context and Interrupt Management Within Interrupts

When a hardware interrupt preempts the function that is currently executing,

the HWI function must save and restore any registers it uses or modifies.

DSP/BIOS provides the HWI_enter assembly macro to save registers and the

HWI_exit assembly macro to restore registers. Using these macros gives the

function that was preempted the same context when it resumes running. In

addition to the register context saving/restoring functionality, the

HWI_enter/HWI_exit macros perform the following system level operations:

❏ ensure the SWI and TSK schedulers are called at the appropriate times

❏ disable/restore individual interrupts while the ISR executes

The HWI_enter assembly macro must be called prior to any DSP/BIOS API

calls that could post or affect a software interrupt or semaphore. The

HWI_exit assembly macro must be called at the very end of the function’s

code.

In order to support interrupt routines written completely in C, DSP/BIOS

provides an HWI dispatcher that performs these enter and exit macros for an

interrupt routine. An HWI can handle context saving and interrupt disabling

using this HWI dispatcher or by explicitly calling HWI_enter and HWI_exit.

The HWI configuration properties allow you to choose whether the HWI

dispatcher is used for individual HWI objects. The HWI dispatcher is the

preferred method for handling interrupts.

The HWI dispatcher, in effect, calls the configured HWI function from within

an HWI_enter/HWI_exit macro pair. This allows the HWI function to be written

completely in C. It would, in fact, cause a system crash were the dispatcher

to call a function that contains the HWI_enter/HWI_exit macro pair. Using the

dispatcher therefore allows for only one instance of the HWI_enter and

HWI_exit code.

Note:

The interrupt keyword or INTERRUPT pragma must not be used when

HWI objects are used in conjunction with C functions.The

HWI_enter/HWI_exit macros and the HWI dispatcher contain this

functionality, and the use of the C modifier can cause program failure.

Texas Instruments TMS320 User Manual

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