Calculating checksums...

I have a snippet of FASM code:

; The last byte (512th) will be the patch_byte for the checksum
; patch_byte is calculated and automagically inserted below
PREV_CHKSUM = 0
repeat $
   load CHKSUM byte from %-1
   CHKSUM = (PREV_CHKSUM + CHKSUM) mod 0x100
   PREV_CHKSUM = CHKSUM
end repeat
store byte (0x100 - CHKSUM) at ($-1)  ; store the patch_byte

How would you suggest I could rewrite this in NASM?
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