Please read the commented section of the code for more information.

/*
mIRC Hex-Editor Snippet
Type /hex to start

Author Notes:
- Short hex editor snippet, which uses a basic mIRC window, the script
is actually pretty short, but with limited functionality
- Leave a comment behind, also contact at mknobbout@gmail.com

KNOWN BUGS:
- If the scroll bar is used the script will think you clicked on a different byte, so dont use the scroll button. Cannot find a decent way to disable this.

Side Notes:
- If you do not know what a hex editor is or does, this snippet is not for you!
- The snippet writes directly to the file
*/

alias hex {
  set %hex 0 34 22 @Hex-Editor $$sfile($mircdir $+ *.*,Hex Editor)
  xhex %hex
}
menu @Hex-Editor {
  dclick:{
    tokenize 32 %hex
    if ($mouse.y isnum 585-600) {
      if (($mouse.x isnum 0-85) && ($calc($1 - $2 * $3) > 0)) set %hex $calc($1 - $2 * $3) $2-
      elseif (($mouse.x isnum 90-140) && ($calc($1 + $2 * $3) <= $file($+(",$$5-,")))) set %hex $calc($1 + $2 * $3) $2-
      xhex %hex
    }
    var &hex, %x = $int($calc($int($calc(($mouse.x - 56) / 8)) / 3 - 1)), %y = $int($calc(($mouse.y - 22) / 16)), %byte = $calc($1 + $3 * %y + %x)
    if ((%x !isnum $+(0-,$calc($3 - 1))) || (%y !isnum $+(0-,$calc($2 - 1)))) return
    bread $+(",$$5-,") %byte 1 &hex
    var %write = $$input(Replace byte $+(',$$base($bvar(&hex,1),10,16,2),') at address $+(',$base(%byte,10,16,8),') with?,e,Replacement)
    if ($base(%write,16,10) isnum 0-255) {
      bset &hex 1 $v1
      bwrite $+(",$5-,") %byte 1 &hex
      xhex $1-
    }
    else return $input($base($v1,10,16,2) Erronomous Hex,o,error)
  }
}
alias xhex {
  ;/xhex <N (start of file)> <N (loop)> <N (length)> <Window Name> <File>
  if (!$window($4)) window -aCdfk0 +l $4 -1 -1 800 600 Fixedsys 9 | clear $4
  echo $4 $chr(2) $+ Double-Click on a byte to edit, $chr(3) $+ 02Addresses are Blue, $chr(3) $+ 01Bytes are Black, $chr(3) $+ 04ASCII is Red
  var &hex, %a = $1, %b = 1, %c = $+(",$$5-,")
  if (!$isfile(%c)) return
  while (%b <= $2) {
    if (%a <= $file(%c)) {
      .bread %c %a $3 &hex
      echo $4 $+($chr(3),02,$base(%a,10,16,8),$chr(3)) $asc2(&hex).hex $+($chr(3),04,$asc2(&hex).chr)
    }
    else echo $4 $chr(160)
    inc %a $3 | inc %b
  }
  :error
  echo $4 $str(-,17)
  echo $4 $+($chr(3),00,$chr(44),01<<Previous,$chr(3)) $+($chr(3),00,$chr(44),01Next>>,$chr(3))
}
alias asc2 {
  var %a = 1, %b, %c = $chr(160)
  while (%a <= $bvar($$1,0)) {
    %b = $iif($prop = hex,%b $base($bvar($1,%a),10,16,2),%b $+ $iif($chr($bvar($1,%a)),$iif($v1 = $chr(32) || $v1 = $chr(9),%c,$v1),%c))
    inc %a
  }
  return %b
}