Reason for playing games with vector number on x86-64 interrupt handling

[Ilya Lesokhin]

Hi,

I have a question related to interrupt handling on x86-64.

entry_64.s cains the following code:
     pushq_cfi $(~vector+0x80)       /* Note: always in signed byte range */
and later
     addq $-0x80,(%rsp)              /* Adjust vector to [-256,-1] range *
and then finally in irq.c
      unsigned vector = ~regs->orig_ax;

if I'm not mistaken regs->orig_ax contains ((~vector+0x80) -0x80) =
~vector and applying ~ again gives vector.

I was wondering, what is the reason for all those games with vector?
why not just push vector in the begging and save later instructions?

Please CC me in your answers.
Thanks,
Ilya
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