stackoverflow January 20, 2026 tech Rep: 35

Why does this code save registers before interrupts which don't destroy these registers?

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January 20, 2026 Score: 3 Rep: 94,933 Quality: Low Completeness: 40%

Most likely the developers simply did not think about whether the BIOS call would preserve SI or not and just saved it out of habit. It could also be that some early BIOS version would indeed clobber SI. For the same reason they probably just assume DF is clear, as it's very unusual for it to be set.

BH needs to be clear, as INT 10h/AH=0Eh takes the video page number from BH in text mode.

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