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Asked: 2026-05-20T21:31:14+00:00

I’m trying to optimize some functions and I realized that I know next to

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I’m trying to optimize some functions and I realized that I know next to nothing about how long certain things take.

I can ask all the questions here, but I’d rather just find a good article on the subject if anyone knows one.

I’m using IAR to write a program in C for an ATMEL SAM7S processor. I have a sort function that takes 500uS or so, and I wanted to see if I could speed it up. I could also just post it here but I was hoping to learn for myself.

Like, is it any faster to subtract two 16 bit integers than it is to subtract two 32 bit integers? And how long does an operation like that take? Just one cycle or more? How long does multiplication take compared to subtraction?

Anyone know a place to look? I tried googling for some stuff but I couldn’t come up with any useful search terms.

If anyone has an ideas on my specific function, I can post details. I’m basically trying to match two analog values to the closest index in a table of calibrated values. Right now I iterate through the whole table and use least squares to determine the closest match. Its pretty straightforward and I’m not sure there is a faster way without applying some extra logic to my table. But if I at least knew how long certain things took, I could probably optimize it myself.

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1 Answer

  1. Editorial Team

    is it any faster to subtract two 16 bit integers than it is to subtract two 32 bit integers?

    Not on an ARM architecture which has native 32-bit registers, no.

    Anyone know a place to look?

    The canonical place for instruction cycle timings would be the Tech Ref Manual for the particular architecture your chip implements, eg. ARM7TDMI; timings for simple alu ops here and yes, it is one cycle. This is not friendly doc to be reading if you’re not already well familiar with the instruction set, though…

    Right now I iterate through the whole table

    You’ll be much better off looking at algorithmic optimisations here (eg indexing the table, sorting by one co-ordinate to narrow it down, etc) than worrying about instruction-level micro-optimisations.

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