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(*
Title: Standard Basis Library: Word8 Structure
Author: David Matthews
Copyright David Matthews 1999
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA
*)
(* G&R 2004 status: checked, no change *)
structure Word8 :> WORD =
struct
(* This structure is largely derived from Word but we use opaque
signature matching so that it is a different type. *)
open Word
(* Values of type Word8.word can be in the range 0..255 and
are implemented using tagged integers. *)
(* There seem to be two ways to implement this, given that the underlying
representation is a tagged 30/31 bit integer. We could either ensure
that the top 22/23 bits are always zero and mask them after every
operation (or if the code-generator was clever, after several) or
we could allow these bits to be undefined and mask them before any
operation which could expose them. For the moment we choose the
former. *)
(* N.B. The Byte structure contains functions which map between
Word8.word values and Char.char. It assumes that the underlying
representation is the same. *)
val wordSize = 8
val maxWord = 255
val maxWordAsWord: word = RunCall.unsafeCast maxWord
infix 8 << >> ~>>
(* Comparison operations, min, max and compare, fmt, toString,
orb, andb, xorb can be inherited directly from Word.
Similarly div and mod since the results will always be no
larger than the arguments. *)
(* Not the same as Word.notb because it only affects the bottom 8 bits. *)
fun notb x = xorb(maxWordAsWord, x)
(* Internal function to convert from Word.word. *)
fun fromWord (w: Word.word) = andb(w, maxWordAsWord)
(* Converting from LargeWord.word. First convert to Word.word and
then mask. *)
val fromLargeWord = fromWord o Word.fromLargeWord
and fromInt = fromWord o Word.fromInt
and fromLargeInt = fromWord o Word.fromLargeInt
val fromLarge = fromLargeWord
(* Arithmetic shift - sign extends. *)
(* TODO: Replace by built-in? We need a separate function from
arithmetic shift for Word because the sign bit is in a different
place. *)
(* Shift the "sign" bit into the real sign bit position then
shift right down again. *)
local
val toSignBit = Word.fromInt(Int.-(Word.wordSize,wordSize))
in
fun op ~>> (a: word, b: Word.word): word =
fromWord(Word.~>>(Word.<<(a, toSignBit), Word.+(b, toSignBit)))
end
(* TODO: Replace with built-in? We need to mask the result so
that it remains in the range 0..255 *)
fun op << (a: word, b: Word.word): word = andb(Word.<<(a,b), maxWordAsWord)
(* Conversion to unsigned integer. This is simpler than for Word
because all Word8 values correspond to short integers. *)
val toInt: word->int = RunCall.unsafeCast
(* Conversion to signed integer. *)
(* TODO: This could be implemented using shifts. i.e logical shift
left by (Word.wordSize-Word8.wordSize) then arithmetic shift
right by the same amount. *)
fun toIntX (x: word) : int =
let
val intx = toInt x
open Int (* We want integer arithmetic here. *)
in
if intx >= 128
then intx-maxWord-1
else intx
end
val toLargeInt = LargeInt.fromInt o toInt
and toLargeIntX = LargeInt.fromInt o toIntX
(* Convert to a large word by sign extending. *)
fun toLargeWordX (w: word): LargeWord.word =
LargeWord.fromInt(toIntX w);
val toLargeX = toLargeWordX
(* Use Word.scan but check that the result is in the range. *)
val wordScan = scan;
fun scan radix getc src =
case wordScan radix getc src of
NONE => NONE
| SOME(res, src') =>
if res > maxWordAsWord
then raise General.Overflow
else SOME(res, src')
val fromString = StringCvt.scanString (scan StringCvt.HEX)
(* TODO: Replace by built-ins? *)
fun op + (a, b) = fromWord(Word.+(a, b))
and op - (a, b) = fromWord(Word.-(a, b))
and op * (a, b) = fromWord(Word.*(a, b))
fun ~ x = 0w0 - x
end;
(* Because we are using opaque signature matching we have to install
type-dependent functions OUTSIDE the structure. *)
local
(* The string may be either 0wnnn or 0wxXXX *)
fun convWord s : Word8.word =
let
val radix =
(* The word value must consist of at least 0w and a digit. *)
if String.sub(s, 2) = #"x" then StringCvt.HEX else StringCvt.DEC
in
case StringCvt.scanString (Word8.scan radix) s of
NONE => raise RunCall.Conversion "Invalid word8 constant"
| SOME res => res
end
(* Install the pretty printer for Word8.word *)
fun pretty _ _ x = PolyML.PrettyString("0wx" ^ Word8.toString x)
in
val () = RunCall.addOverload convWord "convWord"
val () = PolyML.addPrettyPrinter pretty
end;
(* Add the overloaded operators. *)
val () = RunCall.addOverload Word8.~ "~";
val () = RunCall.addOverload Word8.+ "+";
val () = RunCall.addOverload Word8.- "-";
val () = RunCall.addOverload Word8.* "*";
val () = RunCall.addOverload Word8.div "div";
val () = RunCall.addOverload Word8.mod "mod";
val () = RunCall.addOverload Word8.< "<";
val () = RunCall.addOverload Word8.> ">";
val () = RunCall.addOverload Word8.<= "<=";
val () = RunCall.addOverload Word8.>= ">=";
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