ASCII is an acronym for the American Standard Code for Information
Interchange. Pronounced ask-ee, ASCII is a code for representing English
characters as numbers, with each letter assigned a number from 0 to 127. For
example, the ASCII code for uppercase M is 77. Most computers use ASCII codes
to represent text, which makes it possible to transfer data from one computer
to another.

The 8-bit Atari computers use a modified version of the ASCII character set
called Atari ASCII, or ATASCII.

David Moeser produced this nice translation table.

        ASCII TRANSLATION TABLE -- IBM & ATARI 8-BIT (ATASCII)
        ======================================================          
             
        SECTION ONE: CONTROL CHARACTERS 
        ===============================  
          
 DECIMAL      ATARI    IBM <----> ATARI     ASCII
    -HEX  NAME KEY    GRAPHICS CHARACTER    FUNCTION
 =======  ==== ===  ======================  ========
   0  00  NUL  ^,   none        heart       Null
   1  01  SOH  ^A   smiley      |-          Start of header
   2  02  STX  ^B   [smiley]    right |     Start of text
   3  03  ETX  ^C   heart       (9:00)      End of last text
   4  04  EOT  ^D   diamond     -|          End of transmission
   5  05  ENQ  ^E   club        (9:30)      Enquiry
   6  06  ACK  ^F   spade       /           Acknowledge (handshake)
   7  07  BEL  ^G   rain dot    \           Bell
   8  08  BS   ^H   doorbell    L triangle  Backspace
   9  09  HT   ^I   o           low-R-sq.   Horizontal tab
  10  0A  LF   ^J   [doorbell]  R triangle  Line feed
  11  0B  VT   ^K   Mars        hi-R-sq.    Vertical tab
  12  0C  FF   ^L   Venus       hi-L-sq.    Form feed
  13  0D  CR   ^M   note        high bar    Carriage return
  14  0E  SO   ^N   2 notes     low bar     Shift out
  15  0F  SI   ^O   sun         low-L-sq.   Shift in
  16  10  DLE  ^P   R pennant   club        Data link escape (break)
  17  11  DC1  ^Q   L pennant   (3:30)      Device #1 (P:)
  18  12  DC2  ^R   V arrows    --          Device #2
  19  13  DC3  ^S   !!          cross       Device #3 (deselects P:)
  20  14  DC4  ^T   paragraph   cloudy      Device #4 (stop)
  21  15  NAK  ^U   section     low block   Negative acknowl. (error)
  22  16  SYN  ^V   short -     left |      Synchronous idle
  23  17  ETB  ^W   base-V-arrs.low T       End of block
  24  18  CAN  ^X   up arrow    hi perp.    Cancel memory (in buffer)
  25  19  EM   ^Y   DN arrow    left half   End medium (tape drive)
  26  1A  SUB  ^Z   R arrow     (3:00)      Substitute
  27  1B  ESC  EE   L arrow     escape      Escape
  28  1C  FS   E^-  (3:00)      up arrow    File separator
  29  1D  GS   E^=  ice needles DN arrow    Group separator
  30  1E  RS   E^+  up triangle L arrow     Record separator
  31  1F  US   E^*  DN triangle R arrow     Unit separator
  32  20  SPC  bar  space       space       Space    
     
  
        SECTION TWO: SPECIAL CHARACTERS
        ===============================
 127  7F  DEL  ETB  home plate  R pennant   Deleted
 155  9B  EOL  RETURN           box, etc.   ATASCII end of line (newline)
 13,10   CR/LF ENTER  ^M^J                  Windows,DOS,CP/M newline
 10   0A  LF   ENTER  ^J                    UNIX,Mac OS X,Amiga newline
 13   0D  CR   ENTER  ^M                    Apple II,MacOS (pre-X) newline
         
  
        KEY TO ABBREVIATIONS:
        ====================
 ^  = control key   L  = left          hi   = upper
 S  = shift key     R  = right         low  = lower
 E  = escape key    UP = points up     [  ] = inverse
 BS = backspace     DN = points down   V    = vertical
 TB = tab key       sq = square        perp = perpendicular
                    (time) = position of hands on a clockface
  
    Where possible, descriptions of graphics characters are taken 
 from standard symbols used in mathematics, weather, astronomy, etc.
 Note: Different computer platforms, operating systems, programs, 
 printers, etc. will produce different graphics characters.
  
  
        SECTION THREE: KEYBOARD CHARACTERS 
        ==================================   
           
 DECIMAL     IBM        ATARI      DECIMAL     IBM        ATARI 
    -HEX   KEY CHAR.  KEY CHAR.       -HEX   KEY CHAR.  KEY CHAR.
 =======   === ====   === ====     =======   === ====   === ====
  32  20   bar space  bar space    80  50    P     P    P     P
  33  21   S1    !    S1    !      81  51    Q     Q    Q     Q
  34  22   S'    "    S2    "      82  52    R     R    R     R
  35  23   S3    #    S3    #      83  53    S     S    S     S
  36  24   S4    $    S4    $      84  54    T     T    T     T
  37  25   S5    %    S5    %      85  55    U     U    U     U
  38  26   S7    &    S6    &      86  56    V     V    V     V
  39  27   '     '    S7    '      87  57    W     W    W     W
  40  28   S9    (    S9    (      88  58    X     X    X     X
  41  29   S0    )    S0    )      89  59    Y     Y    Y     Y
  42  2A   S8    *    *     *      90  5A    Z     Z    Z     Z
  43  2B   S=    +    +     +      91  5B    [     [    S,    [
  44  2C   ,     ,    ,     ,      92  5C    \     \    S+    \
  45  2D   -     -    -     -      93  5D    ]     ]    S.    ]
  46  2E   .     .    .     .      94  5E    S6    ^    S*    ^
  47  2F   /     /    /     /      95  5F    S-    _    S-    _
  48  30   0     0    0     0      96  60    `     `    ^.    `
  49  31   1     1    1     1      97  61    a     a    a     a
  50  32   2     2    2     2      98  62    b     b    b     b
  51  33   3     3    3     3      99  63    c     c    c     c
  52  34   4     4    4     4     100  64    d     d    d     d
  53  35   5     5    5     5     101  65    e     e    e     e
  54  36   6     6    6     6     102  66    f     f    f     f
  55  37   7     7    7     7     103  67    g     g    g     g
  56  38   8     8    8     8     104  68    h     h    h     h
  57  39   9     9    9     9     105  69    i     i    i     i
  58  3A   S;    :    S;    :     106  6A    j     j    j     j
  59  3B   ;     ;    ;     ;     107  6B    k     k    k     k
  60  3C   S,    <    <     <     108  6C    l     l    l     l
  61  3D   =     =    =     =     109  6D    m     m    m     m
  62  3E   S.    >    >     >     110  6E    n     n    n     n
  63  3F   S/    ?    S/    ?     111  6F    o     o    o     o
  64  40   S2    @    S8    @     112  70    p     p    p     p
  65  41   A     A    A     A     113  71    q     q    q     q
  66  42   B     B    B     B     114  72    r     r    r     r
  67  43   C     C    C     C     115  73    s     s    s     s
  68  44   D     D    D     D     116  74    t     t    t     t
  69  45   E     E    E     E     117  75    u     u    u     u
  70  46   F     F    F     F     118  76    v     v    v     v
  71  47   G     G    G     G     119  77    w     w    w     w
  72  48   H     H    H     H     120  78    x     x    x     x
  73  49   I     I    I     I     121  79    y     y    y     y
  74  4A   J     J    J     J     122  7A    z     z    z     z
  75  4B   K     K    K     K     123  7B    S[    {    ^;  spade
  76  4C   L     L    L     L     124  7C    S\    |    S=    |
  77  4D   M     M    M     M     125  7D    S]    }    E^< left-turn
  78  4E   N     N    N     N     126  7E    S`    ~    EBS L pennant
  79  4F   O     O    O     O     127  7F    none house ETB R pennant

A graphical ATARI / ASCII Table is available at:
http://www.akk.org/~flo/ATASCII.pdf