Visible to Intel only — GUID: iga1457455327985
Ixiasoft
Visible to Intel only — GUID: iga1457455327985
Ixiasoft
38.7.4. Positioning the ISR in Vector Table
If have a critical ISR of small size, you can achieve the best performance by positioning the ISR code directly in the vector table. In this way, you eliminate the overhead of branching from the vector table through the HAL funnel to your ISR.This section describes how to modify the VIC Basic example software to create the VIC Table-Resident example. Use this example to ensure that you understand the steps. Then you can make the equivalent changes in your custom code.
Positioning an ISR in a vector table is an advanced and error-prone technique, not directly supported by the HAL. You must exercise great caution to ensure that the ISR code fits in the vector table entry. If your ISR overflows the vector table entry, it corrupts other entries in the vector table, and your entire interrupt handling system.
When locate your ISR in the vector table, it does not need to be registered. Do not call alt_ic_isr_register(), because it overwrites the contents of the vector table.
When the ISR is in the vector table, the HAL does not provide funnel code. Therefore, the ISR code must perform any context-switching actions normally handled by the funnel. Funnel context switching can include some or all of the following actions:
- Saving and restoring registers
- Managing preemption
- Managing the stack pointer
To create the fastest possible ISR, minimize or eliminate the context-switching actions your ISR must perform by conforming to the following guidelines:
- Write the ISR in assembly language
- Assign a shadow register set for the ISR’s use
- Ensure that the ISR cannot be preempted by another ISR using the same register set. By default, preemption within a register set is disabled on the Nios® II processor. You can also ensure this condition by giving the ISR exclusive access to its register set.
The VIC Table-Resident example requires modifying a BSP-generated file, altera_vic1_vector_tbl.S. If you regenerate the BSP after making these modifications, the Nios® II Software Build Tools regenerate altera_vic1_vector_tbl.S, and your changes are overwritten.