September 30, 2003
トリビア的ASCIIの秘密
最近仕事が忙しくて、すっかり更新をサボリ気味なので、小ネタで…
ASCII コードで DEL(削除) が127番なのって、何でか知ってる?
「ていうか ASCII なんて暗記してねーよ」てな諸兄、歴史的な Vint Cerf博士の RFC20 を探すのもよいが、Unix 系(とか言うと SCO に訴えられたりするのだろうか)な環境をお使いならば、今すぐ man ascii しよう。
#関係ないけど MacOS X って man page ついてるのだろうか?
DEL 以外の制御文字(ISO-2022-JP符号化方式でおなじみの ESC とか、ビープを鳴らす BEL とか)は、コードの先頭のほうに並んでる。Hex で言うと、きれいに 0x00~0x1f までが制御文字。0x20 が SPC(空白文字)で、0x21 から記号とアルファベットがはじまる、という感じ。
しかし何故か、文字削除の DEL だけ、コードの終端に鎮座ましましている。伝説によると、この歴史は古くパンチカードの時代に遡るらしい…
生まれてはじめて触ったPCが日立の Basic Master で、高校を卒業したら Sun 3 だった自分には想像もつかないほどイニシエの時代だが、その昔プログラミングは紙に穴を開けていたらしい。2進数で、穴がなければゼロ、穴が開いてればイチ。
ちなみにパンチカードシステムは、アメリカの国勢調査のために110年ほど前(なんと2世紀前だ!)に生まれて、当時は超ハイテクとして大成功し、IBMが生まれることになったのだ…
てな話はさておき。
コードを書いてりゃ打ち間違えます。これはキーボードもパンチカードも同じこと。キャラクタ端末なら、DEL キー(または Backspace)を押して一文字削除すればよいわけだが、パンチカードでは、一度開けた穴は元に戻らない。おっと困った。
ではどうしていたかというと、間違えた文字の場所は「全部に穴を開けていた」。全部に穴があると、その文字は読み取らない=削除された、ということらしい。
全部に穴を開けるってことは、全ビットに1がたつってこと。つまり 7bit の ASCII コードなら、01111111(2) つまり 0x7f よって10進数なら 127 になるってことだ。
というわけで、歴史的理由により DEL だけは ASCII の 0x7f に居座ることになったという話。
ちなみに、穴を開けてリリースしちゃったパンチカードに、あとから間違い(=バグ)を見つけたときは、それを修正するために、穴にシールを貼っていた。このシールのことをパッチと呼ぶことから、プログラムの修正をさして「パッチを当てる」というようになった、というのは有名な話。
決して「バッチ」を当ててはいけません。罰があたりそうだ…
ついでに、定期的にまとめて処理するプログラムは「バッチ処理」。一括の意のある batch です。決してバッジ処理ではありません。記章を処理してどうする ;-)
「へぇー」と思った方は、思った回数だけ、コメントへどうぞ。
Posted by ryo at September 30, 2003 05:51 PM | TrackBack (79)高校時代、情報処理の実習に「紙テープ穿孔機」を使っとりました。
打ち間違えたら、一キャラクタ分、穿孔位置を戻して DEL キーを押します。すると全部の穴が空いていました。
ステートメントごと直すには、新たに正しいステートメントを紙テープに打ち込んでから、間違えたステートメントの部分を文字通り「切り貼り」しました。
ある日のこと、県の情報処理センターで研修できることになりました。
そこにはカードパンチャーがあって、これだとステートメントを間違えても、新しいカードに正しいステートメントを打ち込み、間違えたステートメントのカードと「差し替え」ればよかったのです。
「なんて楽チンなんだ!ハイテク万歳!」などと感動したのはウン十年前の出来事であります…。
へぇへぇ~×8
パッチのことの方がへぇ~でした。
Posted by: noname at September 30, 2003 06:12 PM紙テープ!
僕はTVのウルトラマンの中でしかお目にかかったことがありません ;-)
科学特捜隊の本部壁面いっぱいに備えられた、あの巨大な磁気テープと意味不明なランプの光る電子計算機から出てくる紙テープを見て、「東京湾に怪獣が!」とか何とか叫ぶ隊員の姿に、幼少時代はスゲーと思ったものです。
その後、すこし計算機をカジって「あんなん穴見ただけでわかるわけないじゃん、まったくTVってやつは技術的正確性を欠く描写を…云々」とか偉ぶってみた若い頃もありました。
しかし今になって冷静に12桁のホレリスコードを眺めてみると、モールス信号が読める程度には、十分読めそうなんですね。
ホレリスコードと穴の位置の関係;
http://www.cs.uiowa.edu/~jones/cards/codes.html
これでキミも科学特捜隊になれる!かも(?)
Posted by: ryo at September 30, 2003 11:47 PMUX50でコメント書いたら途中で吹っ飛んだ。
うっきーーーー!!!!
ということで、PCから再チャレンジ。
うちの研究室には空かずのタンスみたいなラックが鎮座していました。あるとき、ふと開けて見ると、、、
ラック一杯に紙テープが!!
教授の古のプログラムがはいっていたんですねぇ
各々の紙テープの端には 3KBとかかいてあったので、ラック一杯全部あわせても1MBいかないでしょうくらいな容量(^^)
邪魔だから燃やしちまえ!とかいってました(笑)
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