编程与软件工程garbagecollected.org
四列 ASCII 表
Four Column ASCII
2017 — 2019，它一次次回到首页
黑点是上首页（≥ 50 分）的那次投稿，空心点是没冲上去的投稿，点越大分数越高。
导读
这篇博客把 ASCII 字符表按四列排版展示，让 0–127 的编码与字符对应关系以及其中的位模式规律更容易看出。读者可以借此观察控制字符、数字、大小写字母等区块的排列逻辑。
为什么经久不衰：ASCII 是每天都会碰到的基础编码，这种四列视图把它精心设计的位结构直观呈现出来，所以多年后仍值得一读。
HN 评论精选
DEL 编码为全 1，源于打孔卡无法撤销孔、只能全打孔表示删除。
我很喜欢的一个冷知识是：为什么 DEL 在 127，离其他控制码那么远。因为 0x7F 的 ASCII 七位全是 1。早期打孔卡（和电报？）时代，打错了没法把孔“取消”回 0，但可以把其余位都打孔，表示“忽略这个字符，我已删除它”，即 0b1111111。
warbiscuit查看英文原文
One bit of trivia I really love is why "DEL" is at 127 -- weirdly way away from all the control codes. It's because 0x7F is all ASCII bits set to "1". Back in the early punch card (and telegraph?) days, if there was a typo, you couldn't "unpunch" a hole to make it a 0 again, but you could punch out all the rest of them, indicating "ignore this char, I've deleted it" -- 0b1111111. source: http://www.trafficways.org/ascii/ascii.pdf which is a really neat read if you like that sort of thing :)
ASCII 的位布局允许裁掉一位得到 64 字符的大写子集。
ASCII 是经过精心设计的。我喜欢的一点是：如果不需要小写字母，可以整齐地切出一个 64 字符、6 位的全大写编码（类似 DEC 的做法）。
TazeTSchnitzel查看英文原文
ASCII was very carefully designed. One feature I like is that you can neatly cut out a 64-character 6-bit all-uppercase encoding (like DEC's) if you don't need lowercase. Wikipedia covers some of the design choices here: https://en.wikipedia.org/wiki/ASCII#Internal_organization
ASCII 布局有许多看似可优化但受历史约束的选择，值得追问。
还有更多门道。比如数字映射为自身二进制前面加 011：0 => ...0000，1 => ...0001，2 => ...0010。那为什么数字不从零行开始？为什么 0 是 0110000 而不是 0100000？为什么圆括号不和花括号、方括号同一行？为什么 &（et 的连字）不和 e 同一行？而 $（dollar）却和 d 同一行。
qznc查看英文原文
There is more to it. For example, note that the numerals map to their direct binary notation plus a 011 in front. 0 => ...0000, 1 => ...0001, 2 => ...0010, etc. Now I wonder, why don't they start in the zero row? In other words, why is 0 = 0110000, instead of 0100000? Why are the parens not in the same row as braces and brackets? Why isn't "&" (ligature of "et") not in the same row as "e"? "$" ("dollar") is in the same row as "d".
退格叠打让 ASCII 能用组合方式表示重音等字符，类似早期变长编码。
更多冷知识：ASCII 字符被设计成支持用退格（BS）进行叠打来生成更多字符。例如 á 可以写成 a + BS + '。尖音、重音、抑音和元音变音都可以这样叠加在小写字母上；ñ 是 n + BS + ~，ç 是 c + BS + 逗号。下划线是字符 + BS + 下划线，删除线类似，用破折号或波浪号；粗体是字符 + BS + 同一字符。许多 Compose 键序列就基于此。所以 ASCII 某种意义上可能是最早的变长编码集之一。
cryptonector查看英文原文
More trivia: ASCII characters were designed to support overstrike (using BS) to generate more characters. For example, á (á) can be written as a BS '. Acute, grave, and circumflex accents, and umlauts, can be written this way on lower-case letters. ñ is n BS ~. Cedille (ç) is c BS comma. Underline is character BS underscore; strikethrough is similar, using dash or tilde. Bold is character BS same_character. Many Compose key sequences are based on this. So ASCII, in a way, may well be the first -or one of the first- variable-length codeset. Note that overstriking upper-case letters to add diacritical marks does not work for ASCII (see more below). Also, overstriking was how one typed diacritical marks back in the days of mechanical typewriters. Spanish used to (and may still? I forget) permit upper-case letters to not carry diacritical marks precisely because it was difficult or costly to get typewriters and printers to print such letters. Adding diacritical marks to upper-case letters requires fonts designed for that, and clearly an overstrike sequence cannot work unless the typewriter/printer holds printing the character until it knows whether the next character is BS.
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