{"id":12563,"date":"2010-10-11T07:00:00","date_gmt":"2010-10-11T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2010\/10\/11\/why-does-each-drive-have-its-own-current-directory\/"},"modified":"2010-10-11T07:00:00","modified_gmt":"2010-10-11T07:00:00","slug":"why-does-each-drive-have-its-own-current-directory","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20101011-00\/?p=12563","title":{"rendered":"Why does each drive have its own current directory?"},"content":{"rendered":"

\nCommenter Dean Earley asks,\n“Why is there a ‘current directory’ AND an current drive<\/a>?\nWhy not merge them?”\n<\/p>\n

\nPithy answer:\nOriginally, each drive had its own current directory,\nbut now they don’t, but it looks like they do.\n<\/p>\n

\nOkay, let’s unwrap that sentence.\nYou actually know enough to answer the question yourself;\nyou just have to put the pieces together.\n<\/p>\n

\nSet the wayback machine to DOS 1.0.\nEach volume was represented by a drive letter.\nThere were no subdirectories.\nThis behavior was carried forward from CP\/M.\n<\/p>\n

\nPrograms from the DOS 1.0 era\ndidn’t understand subdirectories;\nthey referred to files by just drive letter and file name,\nfor example,\nB:PROGRAM.LST<\/code>.\nLet’s fire up the assembler\n(compilers were for rich people)\nand assemble a program whose source code is on the A drive,\nbut sending the output to the B drive.\n<\/p>\n\n\n\n\n\n\n\n
A>asm foo<\/code><\/td>\n     <\/td>\nthe “.asm” extension on “foo” is implied<\/i><\/td>\n<\/tr>\n
Assembler version blah blah blah<\/code><\/td>\n<\/tr>\n
Source File: FOO.ASM<\/code><\/td>\n<\/tr>\n
Listing file [FOO.LST]: NUL<\/u><\/code>\n <\/td>\n<\/td>\nthrow away the listing file<\/a><\/i><\/td>\n<\/tr>\n
Object file [FOO.OBJ]: B:<\/u><\/code><\/td>\n<\/td>\nsend the object file to drive B<\/i><\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

\nSince we gave only a drive letter in response to the\nObject file<\/code> prompt,\nthe assembler defaults to a file name of FOO.OBJ<\/code>,\nresulting in the object file being generated as B:FOO.OBJ<\/code>.\n<\/p>\n

\nOkay, now let’s introduce subdirectories into DOS 2.0.\nSuppose you have want to assemble A:\\SRC\\FOO.ASM<\/code>\nand put the result into\nB:\\OBJ\\FOO.OBJ<\/code>.\nHere’s how you do it:\n<\/p>\n

\nA> B:\nB> CD \\OBJ\nB> A:\nA> CD \\SRC\nA> asm foo\nAssembler version blah blah blah\nSource File: FOO.ASM\nListing file [FOO.LST]: NUL\nObject file [FOO.OBJ]: B:\n<\/pre>\n
\nThe assembler reads from A:FOO.ASM<\/code> and writes\nto B:FOO.OBJ<\/code>,\nbut since the current directory is tracked on a per-drive basis,\nthe results are A:\\SRC\\FOO.ASM<\/code> and\nB:\\OBJ\\FOO.OBJ<\/code> as desired.\nIf the current directory were not tracked on a per-drive basis,\nthen there would be no way to tell the assembler to put its\noutput into a subdirectory.\nAs a result,\nDOS 1.0 programs were effectively limited to operating on\nfiles in the root directory,\nwhich means that nobody would put files in subdirectories\n(because their programs couldn’t access them).\n<\/p>\n
\nFrom a DOS 1.0 standpoint, changing the current directory\non a drive performs the logical equivalent of changing media.\n“Oh look, a completely different set of files!”\n<\/p>\n
\nShort attention span.\n<\/p>\n
\nRemembering the current directory for each drive has been preserved\never since,\nat least for batch files,\nalthough there isn’t actually such a concept as a per-drive current\ndirectory<\/i> in Win32.\nIn Win32, all you have is a<\/i> current directory.\nThe appearance that each drive has its own current directory\nis a fake-out by cmd.exe<\/code>, which uses\n\nstrange environment variables<\/a>\nto create the illusion to batch files that each\ndrive has its own current directory.\n<\/p>\n
\nDean continues,\n“Why not merge them?\nI have to set both the dir and drive if i want a specific working dir.”\n<\/p>\n
\nThe answer to the second question is, “They already are merged.\nIt’s cmd.exe<\/code> that tries to pretend that they aren’t.”\nAnd if you want to set the directory and the drive from the command\nprompt or a batch file, just use the \/D<\/code> option to the\nCHDIR<\/code> command:\n<\/p>\n
\nD:\\> CD \/D C:\\Program Files\\Windows NT\nC:\\Program Files\\Windows NT> _\n<\/pre>\n
\n(Notice that the CHDIR<\/code> command lets you omit quotation\nmarks around paths which contain spaces:\nSince the command takes only one path argument,\nthe lack of quotation marks does not introduce ambiguity.<\/p>\n","protected":false},"excerpt":{"rendered":"
Commenter Dean Earley asks, “Why is there a ‘current directory’ AND an current drive? Why not merge them?” Pithy answer: Originally, each drive had its own current directory, but now they don’t, but it looks like they do. Okay, let’s unwrap that sentence. You actually know enough to answer the question yourself; you just have […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[2],"class_list":["post-12563","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-history"],"acf":[],"blog_post_summary":"
Commenter Dean Earley asks, “Why is there a ‘current directory’ AND an current drive? Why not merge them?” Pithy answer: Originally, each drive had its own current directory, but now they don’t, but it looks like they do. Okay, let’s unwrap that sentence. You actually know enough to answer the question yourself; you just have […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/12563","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=12563"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/12563\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=12563"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=12563"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=12563"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}