OSDev.org

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May be this can help you

; code located at 0000:7C00, adjust segment registers
cli
mov ax, 0x07C0
mov ds, ax
mov es, ax
mov fs, ax
mov gs, ax
sti

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one thing that could help you is adding a checksum at the end of your kernel: edit the last 2 bytes so that the sum of your kernel words is 0. When loading the kernel (or your 2nd stage. that's roughly the same ), check this sum is still 0 before jumping: it should help you telling whether you loaded the whole file correctly or not.