{"id":9484,"date":"2025-02-05T03:00:00","date_gmt":"2025-02-05T08:00:00","guid":{"rendered":"https:\/\/www.both.org\/?p=9484"},"modified":"2025-02-01T14:41:21","modified_gmt":"2025-02-01T19:41:21","slug":"draw-a-cylon-eye-in-dos","status":"publish","type":"post","link":"https:\/\/www.both.org\/?p=9484","title":{"rendered":"Draw a Cylon eye in DOS"},"content":{"rendered":"
\r\n
\r\n \r\n <\/i>\r\n <\/a>\r\n 1 <\/span>\r\n<\/div><\/div>\n

I first got \u201cinto\u201d programming because it was fun. And while I also write programs to solve problems and make useful tools, that desire to find fun things to do when programming has stuck with me to this day.<\/p>\n\n\n\n

I wanted to share a fun program you can write for yourself. If you remember the original Battlestar Galactica<\/em> TV show from the late 1970s, or the rebooted show from the early 2000s, you may recall the Cylons. (If you don\u2019t know the show, it was a low-budget sci-fi TV series that came out right after the first Star Wars<\/em> movie.) The \u201ccombat\u201d Cylon had a distinguishing feature: its sweeping red eye.<\/p>\n\n\n\n

Let\u2019s write a program to emulate the sweeping \u201ceye\u201d of a Cylon:<\/p>\n\n\n\n

Using DOS conio<\/h2>\n\n\n\n

I wrote this program under FreeDOS because DOS makes it really easy to draw extended<\/em> characters to the screen, beyond the 7-bit ASCII characters. The DOS \u201cextended ASCII\u201d character set includes a filled-in block (0xdb<\/code>) and a series of partially \u201cshaded\u201d boxes (0xb0<\/code>, 0xb1<\/code>, and 0xb2<\/code>). We can use these extended characters to draw an eye (filled-in block) and simulate a \u201ctrail\u201d behind the movement (with the shaded boxes).<\/p>\n\n\n\n

I\u2019ll use the OpenWatcom C compiler on FreeDOS, which includes a few useful libraries. The conio.h<\/code> library provides direct access to the video console<\/em>, and the graph.h<\/code> functions let us set the text colors and define regions on the screen called text \u201cwindows.\u201d<\/p>\n\n\n\n

To write this program, we\u2019ll need only a few functions:<\/p>\n\n\n\n

    \n
  • _setvideomode<\/code> clears the screen and puts the display into a specific mode, like 80×25 color<\/li>\n\n\n\n
  • _settextwindow<\/code> defines a region of the display as a text \u201cwindow\u201d<\/li>\n\n\n\n
  • _settextposition<\/code> will move the cursor to a row,col<\/em> coordinate on the screen<\/li>\n\n\n\n
  • _outmem<\/code> will print characters from an array to the console<\/li>\n<\/ul>\n\n\n\n

    Drawing an \u201ceye\u201d<\/h2>\n\n\n\n

    We can use these functions to draw an \u201ceye\u201d that moves from left to right across the screen. First, define an array of five elements, called right<\/code>. The array should contain a space, plus the shaded boxes (light to dark), then the filled-in block.<\/p>\n\n\n\n

        char right[] = { 0x20, 0xb0, 0xb1, 0xb2, 0xdb };<\/code><\/pre>\n\n\n\n
    The 0x20<\/code> character is the ASCII space; the 0xdb<\/code> character is a filled-in block. The middle characters are shaded boxes.<\/p>\n\n\n\n
    To \u201canimate\u201d this block moving across the screen, draw successive iterations of the array from columns 1 to 75. The leading space in the array effectively erases<\/em> the previous iteration of the \u201ceye.\u201d<\/p>\n\n\n\n
        for (col = 1; col <= 75; col++) {\n        _settextposition(1, col);\n        _outmem(right, 5);\n        delay(10);                 \/* ms *\/\n    }<\/code><\/pre>\n\n\n\n
    This is a short loop that contains only a few statements: for each value from 1 to 75, set the cursor position to a specific column on line 1, then print the 5 elements from the right<\/code> array. The delay<\/code> function (defined in i86.h<\/code>) delays program execution for a specified number of milliseconds. In this case, a very short delay of 10ms is fast enough to look like the \u201ceye\u201d is moving but slow enough that a human can see it.<\/p>\n\n\n\n
    We can also write a similar loop to move the \u201ceye\u201d from the right to left, using another array called left<\/code>. If we put both loops together, we have a function that continuously draws an eye moving to the right, then to the left, and back again.<\/p>\n\n\n\n
    void cylon(void)\n{\n    char left[] = { 0xdb, 0xb2, 0xb1, 0xb0, 0x20 };\n    char right[] = { 0x20, 0xb0, 0xb1, 0xb2, 0xdb };\n    short col;\n\n    while (1) {\n        for (col = 1; col <= 75; col++) {\n            _settextposition(1, col);\n            _outmem(right, 5);\n            delay(10);                 \/* ms *\/\n\n            if (kbhit()) {\n                return;\n            }\n        }\n\n        for (col = 75; col >= 1; col--) {\n            _settextposition(1, col);\n            _outmem(left, 5);\n            delay(10);                 \/* ms *\/\n\n            if (kbhit()) {\n                return;\n            }\n        }\n    }\n}<\/code><\/pre>\n\n\n\n
    Because the function runs forever, I\u2019ve added the kbhit<\/code> test to each iteration of both loops. This returns a true value only if the user has pressed a key on the keyboard. Later, you can retrieve that key value using the getch<\/code> function. But for this function, I\u2019m only using kbhit<\/code> to detect when the user is ready to end the program.<\/p>\n\n\n\n
    The \u2018main\u2019 program<\/h2>\n\n\n\n
    We can use this function as the core part of our program to draw the Cylon \u201ceye.\u201d The main<\/code> program only needs to set up the screen, then call the cylon<\/code> function to draw the sweeping eye. When the cylon<\/code> function returns, the program can clear the keyboard input buffer using getch<\/code>, then exit back to DOS.<\/p>\n\n\n\n
    In OpenWatcom, the _setvideomode<\/code> function sets a specific video mode, like _TEXTC80<\/code> for color text at 80 columns and 25 lines. The function returns the number of lines, or zero if there was an error.<\/p>\n\n\n\n
    After that, we can use _settextwindow<\/code> to define a region on the screen as a text \u201cwindow,\u201d and _clearscreen<\/code> to erase either the whole screen (with _GCLEARSCREEN<\/code>) or just the window (with _GWINDOW<\/code>). If we set the background color with _setbkcolor<\/code> before clearing, we can effectively set a background color across the full screen or the window.<\/p>\n\n\n\n
        \/* clear screen *\/\n\n    if (_setvideomode(_TEXTC80) == 0) {\n        puts(\"cannot set video mode\");\n        return 1;\n    }\n\n    \/* draw the eye *\/\n\n    _setbkcolor(7);                    \/* white *\/\n    _clearscreen(_GCLEARSCREEN);\n\n    _setbkcolor(0);                    \/* black *\/\n    _settextwindow(10, 1, 10, 80);     \/* from r,c=10,1 to r,c=10,80 *\/\n    _clearscreen(_GWINDOW);\n\n    _settextcolor(12);                 \/* br red *\/\n    _settextcursor(0x2000);            \/* no cursor *\/<\/code><\/pre>\n\n\n\n
    Putting it all together<\/h2>\n\n\n\n
    We can assemble these pieces into a DOS program that erases the screen with a white background, then defines a black 1-line \u201cwindow\u201d across the width of the screen on line 10. This is an attempt to emulate the Cylons from the TV show, which were actually shiny silver with a black visor that contained the bright red \u201ceye\u201d that swept ominously from side to side.<\/p>\n\n\n\n
    It\u2019s important to know that after you\u2019ve defined a text window, the _settextposition<\/code> function uses coordinates relative to the window<\/em>. So drawing on \u201cline 1\u201d in the window just prints text on the first line of the window.<\/p>\n\n\n\n
    #include <stdio.h>\n#include <conio.h>                     \/* console *\/\n#include <graph.h>                     \/* colors *\/\n#include <i86.h>                       \/* delay *\/\n\nvoid cylon(void)\n{\n    char left[] = { 0xdb, 0xb2, 0xb1, 0xb0, 0x20 };\n    char right[] = { 0x20, 0xb0, 0xb1, 0xb2, 0xdb };\n    short col;\n\n    \/* text window already defined, so use \"row\" = 1 *\/\n\n    while (1) {\n        for (col = 1; col <= 75; col++) {\n            _settextposition(1, col);\n            _outmem(right, 5);\n            delay(10);                 \/* ms *\/\n\n            if (kbhit()) {\n                return;\n            }\n        }\n\n        for (col = 75; col >= 1; col--) {\n            _settextposition(1, col);\n            _outmem(left, 5);\n            delay(10);                 \/* ms *\/\n\n            if (kbhit()) {\n                return;\n            }\n        }\n    }\n}\n\nint main()\n{\n    \/* clear screen *\/\n\n    if (_setvideomode(_TEXTC80) == 0) {\n        puts(\"cannot set video mode\");\n        return 1;\n    }\n\n    \/* draw the eye *\/\n\n    _setbkcolor(7);                    \/* white *\/\n    _clearscreen(_GCLEARSCREEN);\n\n    _setbkcolor(0);                    \/* black *\/\n    _settextwindow(10, 1, 10, 80);     \/* from r,c=10,1 to r,c=10,80 *\/\n    _clearscreen(_GWINDOW);\n\n    _settextcolor(12);                 \/* br red *\/\n    _settextcursor(0x2000);            \/* no cursor *\/\n\n    cylon();\n\n    \/* done *\/\n\n    if (getch() == 0) { getch(); }\n\n    _setvideomode(_DEFAULTMODE);\n    return 0;\n}\n<\/code><\/pre>\n\n\n\n
    Save this as cylon.c<\/code> on your FreeDOS system, and compile it with OpenWatcom (WCL<\/code> is the W<\/strong>atcom C<\/strong>ompiler and L<\/strong>inker) like this:<\/p>\n\n\n\n
    > WCL cylon.c<\/code><\/pre>\n\n\n\n
    Now you can run the cylon.exe<\/code> program whenever you want to experience the roving red eye of a Cylon moving from left to right\u2026<\/p>\n\n\n\n
    \"\"
    Cylon eye moving left to right<\/figcaption><\/figure>\n\n\n\n
    And from right to left\u2026<\/p>\n\n\n\n
    \"\"
    Cylon eye moving right to left<\/figcaption><\/figure>\n","protected":false},"excerpt":{"rendered":"
    Have fun by writing a program to simulate a Cylon’s sweeping red eye<\/p>\n","protected":false},"author":33,"featured_media":2725,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_lmt_disableupdate":"","_lmt_disable":"","footnotes":""},"categories":[340,150],"tags":[267,147,152],"class_list":["post-9484","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-freedos","category-programming","tag-freedos","tag-fun","tag-programming"],"modified_by":"Jim Hall","_links":{"self":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/9484","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/users\/33"}],"replies":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=9484"}],"version-history":[{"count":1,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/9484\/revisions"}],"predecessor-version":[{"id":9487,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/posts\/9484\/revisions\/9487"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=\/wp\/v2\/media\/2725"}],"wp:attachment":[{"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=9484"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=9484"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.both.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=9484"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}