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249 lines
8.3 KiB
Markdown
249 lines
8.3 KiB
Markdown
# Reading Directories
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In this project you will write your non-trivial first assembly language
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program from scratch. The program will use library calls to open the
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given Linux directory (or the current directory if no command line
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argument is given) for reading. It will read every directory entry,
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printing out each file's inode number, type and name.
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## Samples
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### Output when no command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out
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40905818 0x04 .
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7943328 0x04 ..
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40905889 0x08 README.md
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40908053 0x08 a.out
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40906048 0x08 main.c
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40905888 0x08 .gitignore
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40905819 0x04 .git
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40907182 0x04 .vscode
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perryk@ROCI pk_dirent %
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```
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The first column is the named file's inode number. Think of an inode
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number as a file's unique (per file system) serial number. You must
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print it left justified in a field of 20 digits using `printf` - since
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it is possible you've never used `printf` before, I will supply the
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correct (for `C`) statement:
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```c
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printf("%-20llu 0x%02x %s\n", de->d_ino, de->d_type, de->d_name);
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```
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In my code, `de` is defined:
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```c
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struct dirent * de;
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```
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The second column is the file's type printed as a single byte's worth of
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hex.
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The third column is the file's name.
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### Output when a command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out /
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2 0x04 .
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1 0x04 ..
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1152921504606781440 0x0a home
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1152921500312809862 0x04 usr
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1152921500312809678 0x04 bin
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1152921500312809756 0x04 sbin
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1152921500311879697 0x08 .file
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1152921500312809753 0x0a etc
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1152921500312845202 0x0a var
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1152921500311879700 0x04 Library
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1152921500311879701 0x04 System
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1152921500312809755 0x04 private
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1152921500311879698 0x04 .vol
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1152921500312809676 0x04 Users
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1152921500311879699 0x04 Applications
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1152921500312809754 0x04 opt
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1152921500312809752 0x04 dev
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1152921500312809677 0x04 Volumes
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1152921500312809861 0x0a tmp
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1152921500312809751 0x04 cores
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perryk@ROCI pk_dirent %
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```
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### Output when a bad command line argument is given
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```text
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perryk@ROCI pk_dirent % ./a.out does_not_exist
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does_not_exist: No such file or directory
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perryk@ROCI pk_dirent %
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```
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The error string is produced by `perror()`.
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## man
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Here is where you will get documentation for `perror()`, `opendir()`,
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`closedir()`, and `readdir()`. The man page for `readdir()` also
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describes `struct dirent`.
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```text
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man perror
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man opendir
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man closedir
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man readdir
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```
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`man` is your friend, though of course in the 21st century it should be
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called `person`. To learn more about `man`, do the obvious thing:
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```text
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man man
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```
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"Just" 439 lines.
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**APPLE MAN DIFFERS FROM LINUX MAN -- WHY? APPLE.**
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It will be pointless to try the above Linux shell commands from a
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Windows command prompt but hey - give it a try. So where should you read
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these `man` pages? In your ARM Linux VM, of course.
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## `opendir()`
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This function takes a NULL terminated `C-string` and attempts to open it
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as a directory. Get the details from the `man` page. If you get an error
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return, pass the attempted directory name to `perror()` to get the right
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error message.
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## `closedir()`
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Call this function to close a successfully opened directory. Get the
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details from the `man` page.
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## `readdir()`
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Call this function to be given a pointer to the next `dirent` or `NULL`
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if there are no more (or there is an error). Pay attention to the `man`
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page to distinguish between no more `dirent` structures and an error. In
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short, `errno` should be initialized to 0 then checked once you've
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gotten a `NULL` back from `readdir()`.
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## Source code to a `C` version
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At the beginning of this document I said:
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*In this project you will write your first assembly language program
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from scratch.*
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but here's the source code to my `C` version because you may be just
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getting started with `C` and Linux programming. And because I'm a
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wonderful pushover of a professor.
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```c
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#include <stdio.h> /* 1 */
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#include <errno.h> /* 2 */
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#include <dirent.h> /* 3 */
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/* 4 */
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int main(int argc, char ** argv) { /* 5 */
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int retval = 1; /* 6 */
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char * dirname = "."; /* 7 */
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/* 8 */
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if (argc > 1) /* 9 */
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dirname = argv[1]; /* 10 */
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/* 11 */
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DIR * dir = opendir(dirname); /* 12 */
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if (dir) { /* 13 */
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struct dirent * de; /* 14 */
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errno = 0; /* 15 */
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while ((de = readdir(dir)) != NULL) /* 16 */
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printf("%-20llu 0x%02x %s\n", de->d_ino, de->d_type, de->d_name); /* 17 */
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if (errno != 0) /* 18 */
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perror("readdir() failed"); /* 19 */
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closedir(dir); /* 20 */
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retval = (errno != 0); // force error return to be 1 /* 21 */
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} /* 22 */
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else /* 23 */
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perror(dirname); /* 24 */
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return retval; /* 25 */
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} /* 26 */
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```
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### Line 6 and Line 21
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Command line programs return 0 to who called them when all is well. A
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non-zero return value signifies and error.
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### Lines 7, 9 and 10
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Notice how the program is made to default to the current directory
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(`"."`) which can be overridden if a command line argument is supplied.
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### Line 15
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`errno` is initialized to 0 and then quizzed to see if it turned
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non-zero when `readdir()` finally returns `NULL`.
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### Line 17
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Implementing this line is where you will need to calculate the correct
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offsets to each data member.
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See the book chapter on `struct`.
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### Line 18
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The error condition is distinguished from the end of the directory by
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looking at `errno`.
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## Getting the address of `errno`
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`errno` is an `extern`. To store anything into it (or query its
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contents), you must have its address. For reasons which will be
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explained, getting its address is accomplished by calling a [library
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function](http://refspecs.linux-foundation.org/LSB_4.0.0/LSB-Core-generic/LSB-Core-generic/baselib---errno-location.html).
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## Remember to properly set the return value of `main()`
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If all ends well, zero should be returned from `main()`. If any error is
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found, a value of 1 should be returned.
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Check in this way:
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```text
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pk_dirent > ./a.out
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4327 0x04 .
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4328 0x04 ..
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4329 0x08 main.s
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4330 0x08 README.md
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4331 0x08 a.out
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4332 0x08 main.c
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4333 0x08 .gitignore
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4334 0x08 project.s
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4335 0x04 .git
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4336 0x04 .vscode
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pk_dirent > echo $?
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0
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pk_dirent > ./a.out main.s
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main.s: Not a directory
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pk_dirent > echo $?
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1
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pk_dirent >
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```
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`$?` is a shell variable that contains the value returned from the last
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program run by the shell.
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## Likely source of error
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If you're printing garbage, double check your calculations of offsets
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within the `dirent`. While this isn't the only explanation, it is a
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likely explanation.
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## Setting expectations
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I provide the following not as a challenge, but to set your
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expectations.
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My assembly language solution is about 60 lines plus comments. If you
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find yourself writing much more than this, you're doing it wrong.
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