What's the rationale behind this EBCDIC to ASCII conversion?

Question

I would like to understand how exactly this conversion happens.

http://www8.cs.umu.se/~isak/Snippets/a2e.c

/*
**  ASCII <=> EBCDIC conversion functions
*/

static unsigned char a2e[256] = {
          0,  1,  2,  3, 55, 45, 46, 47, 22,  5, 37, 11, 12, 13, 14, 15,
         16, 17, 18, 19, 60, 61, 50, 38, 24, 25, 63, 39, 28, 29, 30, 31,
         64, 79,127,123, 91,108, 80,125, 77, 93, 92, 78,107, 96, 75, 97,
        240,241,242,243,244,245,246,247,248,249,122, 94, 76,126,110,111,
        124,193,194,195,196,197,198,199,200,201,209,210,211,212,213,214,
        215,216,217,226,227,228,229,230,231,232,233, 74,224, 90, 95,109,
        121,129,130,131,132,133,134,135,136,137,145,146,147,148,149,150,
        151,152,153,162,163,164,165,166,167,168,169,192,106,208,161,  7,
         32, 33, 34, 35, 36, 21,  6, 23, 40, 41, 42, 43, 44,  9, 10, 27,
         48, 49, 26, 51, 52, 53, 54,  8, 56, 57, 58, 59,  4, 20, 62,225,
         65, 66, 67, 68, 69, 70, 71, 72, 73, 81, 82, 83, 84, 85, 86, 87,
         88, 89, 98, 99,100,101,102,103,104,105,112,113,114,115,116,117,
        118,119,120,128,138,139,140,141,142,143,144,154,155,156,157,158,
        159,160,170,171,172,173,174,175,176,177,178,179,180,181,182,183,
        184,185,186,187,188,189,190,191,202,203,204,205,206,207,218,219,
        220,221,222,223,234,235,236,237,238,239,250,251,252,253,254,255
};

static unsigned char e2a[256] = {
          0,  1,  2,  3,156,  9,134,127,151,141,142, 11, 12, 13, 14, 15,
         16, 17, 18, 19,157,133,  8,135, 24, 25,146,143, 28, 29, 30, 31,
        128,129,130,131,132, 10, 23, 27,136,137,138,139,140,  5,  6,  7,
        144,145, 22,147,148,149,150,  4,152,153,154,155, 20, 21,158, 26,
         32,160,161,162,163,164,165,166,167,168, 91, 46, 60, 40, 43, 33,
         38,169,170,171,172,173,174,175,176,177, 93, 36, 42, 41, 59, 94,
         45, 47,178,179,180,181,182,183,184,185,124, 44, 37, 95, 62, 63,
        186,187,188,189,190,191,192,193,194, 96, 58, 35, 64, 39, 61, 34,
        195, 97, 98, 99,100,101,102,103,104,105,196,197,198,199,200,201,
        202,106,107,108,109,110,111,112,113,114,203,204,205,206,207,208,
        209,126,115,116,117,118,119,120,121,122,210,211,212,213,214,215,
        216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,
        123, 65, 66, 67, 68, 69, 70, 71, 72, 73,232,233,234,235,236,237,
        125, 74, 75, 76, 77, 78, 79, 80, 81, 82,238,239,240,241,242,243,
         92,159, 83, 84, 85, 86, 87, 88, 89, 90,244,245,246,247,248,249,
         48, 49, 50, 51, 52, 53, 54, 55, 56, 57,250,251,252,253,254,255
};

char ASCIItoEBCDIC(const unsigned char c)
{
        return a2e[c];
}

char EBCDICtoASCII(const unsigned char c)
{
        return e2a[c];
}

In case of a sample text char sample[6] = "hello";, how exactly h would be converted from ascii to ebcdic and vice versa?

EDIT: I really have no need of converting a string. I just want to know how these tables were constructed.

It is simply a lookup table. If you have a char c then a2e[(unsigned)c] produces the ebcdic code. One character at a time. — Hans Passant, Feb 18 '16 at 20:42
I'm fine with a[x]; I just wanted to know how the arrays were formed. — San, Feb 18 '16 at 20:49
@san the arrays are initialized with the values that define the mapping from ASCII to EBCDIC and vice versa. — Jabberwocky, Feb 18 '16 at 21:02
someone looked at an ASCII table and an EBCDIC table and found where the corresponding characters were — M.M, Feb 18 '16 at 21:11

chux - Reinstate Monica · Accepted Answer · 2016-02-18T22:07:26.957

I just want to know how these tables were constructed.

'0' in ASCII has code value of 48.
'0' in EBCDIC has code value of 240.

So we need tables that map 48 to 240 and 240 to 48.

Take converting '0' with an ASCII code value of 48. a2e[48] returns 240, the EBCDIC code for the character '0'.

static unsigned char a2e[256] = {
    ...
    ...
    ...
    240,241,242,243,244,245,246,247,248,249,122, 94, 76,126,110,111,
    ...

Now use 240, the EBCDIC code for the character '0'. e2a[240] which returns 48, the ASCII code for the character '0'.

static unsigned char e2a[256] = {
     ...
     13 more rows
     ...
     48, 49, 50, 51, 52, 53, 54, 55, 56, 57,250,251,252,253,254,255
};

Do this for all the other common characters. Define mappings as able for the rest. Recall ASCII only defines 128 characters while EBCDIC defines 256.

Miscellanea

The EBCDIC layout has some relationship to some punch cards.

In both character sets, the characters 0, 1, ... 9 are sequential - a requirement for coding in C.

In both sets, upper case and lower case letters differ by 1 bit, so one can code the following (still better to use C's tolower())

int from_upper_to_lower(int code) {
  return code - 'A' + 'a`;
}

Vlad from Moscow · Answer 2 · 2016-02-19T00:25:40.637

The functions that convert from one character set to another can be written in assembler using the tables.

In C you can convert this string from ASCII to EBCDIC

char sample[6] = "hello";

the following way

char * AsciiToEbcdic( char *s )
{
    for ( char *p = s; *p; ++p ) *p = a2e[( unsigned char )*p];

    return s;
}

provided that the table is accessible in the function.

Here is a demonstrative program.

#include <stdio.h>

char * AsciiToEbcdic( char *s )
{
    static unsigned char a2e[256] = 
    {
          0,  1,  2,  3, 55, 45, 46, 47, 22,  5, 37, 11, 12, 13, 14, 15,
         16, 17, 18, 19, 60, 61, 50, 38, 24, 25, 63, 39, 28, 29, 30, 31,
         64, 79,127,123, 91,108, 80,125, 77, 93, 92, 78,107, 96, 75, 97,
        240,241,242,243,244,245,246,247,248,249,122, 94, 76,126,110,111,
        124,193,194,195,196,197,198,199,200,201,209,210,211,212,213,214,
        215,216,217,226,227,228,229,230,231,232,233, 74,224, 90, 95,109,
        121,129,130,131,132,133,134,135,136,137,145,146,147,148,149,150,
        151,152,153,162,163,164,165,166,167,168,169,192,106,208,161,  7,
         32, 33, 34, 35, 36, 21,  6, 23, 40, 41, 42, 43, 44,  9, 10, 27,
         48, 49, 26, 51, 52, 53, 54,  8, 56, 57, 58, 59,  4, 20, 62,225,
         65, 66, 67, 68, 69, 70, 71, 72, 73, 81, 82, 83, 84, 85, 86, 87,
         88, 89, 98, 99,100,101,102,103,104,105,112,113,114,115,116,117,
        118,119,120,128,138,139,140,141,142,143,144,154,155,156,157,158,
        159,160,170,171,172,173,174,175,176,177,178,179,180,181,182,183,
        184,185,186,187,188,189,190,191,202,203,204,205,206,207,218,219,
        220,221,222,223,234,235,236,237,238,239,250,251,252,253,254,255
    };

    for ( char *p = s; *p; ++p ) *p = a2e[( unsigned char )*p];

    return s;
} 

int main( void ) 
{
    char s[] = "@@@@@";

    printf( "\"%s\"\n", s );
    printf( "\"%s\"\n", AsciiToEbcdic( s ) );

    return 0;
}

Its output is

"@@@@@"
"|||||"

What's the rationale behind this EBCDIC to ASCII conversion?

2 Answers2