Features of the FIQ are,. Some have quoted the difficulty of coding in assembler to handle the FIQ. Here is an example,. The main code needs to poll the FIQ r10 to determine when the buffer is finished. Typically RTOS interrupt latency will be instructions. For Linux, it maybe instructions. Real DMA is always preferable, however, for high frequency simple interrupts like a buffer transfer , the FIQ can provide a solution.
As the FIQ is about speed, you shouldn't consider it if you aren't secure in coding in assembler or willing to dedicate the time. Assembler written by an infinitely running programmer will be faster than a compiler. Having GCC assist can help a novice. As the FIQ has a separate mask bit it is almost ubiquitously enabled. Often the service time is not critical for an interrupt, but the time between signalling and servicing. Here, the FIQ also has an advantage.
The FIQ is not scalable. In order to use multiple FIQ sources, the banked registers must be shared among interrupt routines. The FIQ is generally a one trick pony. This is basically the same statement as multiplexing the interrupts. The banked registers give 6 free variables to use which never load from memory. Register are faster than memory. Registers are faster than L2-cache. Registers are faster than L1-cache. Registers are fast.
If you can not write a routine that runs with 6 variables, then the FIQ is not suitable. Active Oldest Votes. FIQ mode has its own dedicated banked registers, r8-r But if your FIQ handler is able to be written such that it only uses r8-r13 , it can take advantage of these banked registers in two ways: One is that it does not incur the overhead of pushing and popping any registers that are used by the interrupt service routine ISR.
This can save a significant number of cycles on both entry and exit to the ISR. Also, the handler can rely on values persisting in registers from one call to the next, so that for example r8 may be used as a pointer to a hardware device and the handler can rely on the same value being in r8 the next time it is called. FIQ location at the end of the exception vector table 0x1C means that if the FIQ handler code is placed directly at the end of the vector table, no branch is required - the code can execute directly from 0x1C.
This saves a few cycles on entry to the ISR. So why do many systems not use FIQ? FIQ handler code typically cannot be written in C - it needs to be written directly in assembly language. If you care sufficiently about ISR performance to want to use FIQ, you probably wouldn't want to leave a few cycles on the table by coding in C in any case, but more importantly the C compiler will not produce code that follows the restriction on using only registers r8-r Code produced by a C compiler compliant with ARM's ATPCS procedure call standard will instead use registers r0-r3 for scratch values and will not produce the correct cpsr restoring return code at the end of the function.
All of the interrupt controller hardware is typically on the IRQ pin. Using FIQ only makes sense if you have a single highest priority interrupt source connected to the nFIQ input and many systems do not have a single permanently highest priority source. There is no value connecting multiple sources to the FIQ and then having software prioritize between them as this removes nearly all the advantages the FIQ has over IRQ.
Improve this answer. Rainald62 9 9 silver badges 16 16 bronze badges. The precise determination of what might be a secure interrupt source and how that should be handled differently from a normal interrupt depends on the threat and implementation models. They will continue using registers r0 to r7 instead of r8 and above.
Also, when calling a function, the compilers will continue to use the standard ABI the function may alter r0 to r3. Pretty quickly, the generated assembly becomes standard and is inefficient. Tail of vector table which is always in cache and mapped by MMU. The last feature also gives a slight advantage over an IRQ which must branch. Latency As the FIQ has a separate mask bit it is almost ubiquitously enabled.
Weakness The FIQ is not scalable. Summary Don't bash my friend the FIQ. However, in this case it functions as a normal interrupt handler, not the traditional SoftDMA role. Charvak That is interrupt controller specific; you are thinking GIC. Some interrupt controllers will automatically clear Ie, uart ready goes not ready when a character is written.
It is a hypothetical example for a generic answer; the OP had an ARM and not an actual working example. Local register variables don't do what you might think they do. They don't reserve the register for a certain variable. In fact, gcc will only guarantee that the variable's value is in the specified registry when inline assembly is executed that has the specified variable an input or output parameter. At any other time, the register may be used for other data. In particular, I believe that gcc does not understand that of src has to be in register r8 when the function returns.
However, global register variables seem to be suitable here, as they do reserve the registers. It is known that GCC tends to use registers r0 to r7 even though r8 and friends could be used. For reference: gcc. Show 1 more comment. From the patent: A method of performing a fast interrupt in a digital data processor having the capability of handling more than one interrupt is provided.
Alex Taylor Alex Taylor 6, 2 2 gold badges 23 23 silver badges 22 22 bronze badges. Not to take away from the answer, but what's in a patent doesn't necessarily say anything about what's implemented, so I wouldn't really consider it an authoritative reference.
Also explain the implementation. The final images produced by compliers contain both bin file and extended loader format ELf file ,what is the difference between the two , especially the utility of ELF file. I'm following along with the Baking Pi course from Cambridge University, in which a simple operating system is built in the ARMv6 instruction set, targeting the From the F32 NEON on two bit floats every other cycle.
I would like to write code depending on whether the target architecture is e. The reason that I can't use I'm using buildroot to create a rootfs running on my ARM platform. I'd like to have a package manager on my platform to easily install packages, In Linux how is the Clock Architecture implemented. I'm newbie on Linux Kernel. Currently, I looked into idle codes and had a quesition. When processor doesn't have any taks in their own runqueue then The ARM9 target I am working with is
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