Binary (number system): How do you convert "P" from ASCII to hexadecimal?

You can convert directly to hex by dividing for 16. 112/16 = 7 ===> 07

Hexadecimal is simply a notation used to express an integer quantity. "p" is another representation, and is implied in a certain context, such as a terminal or text editor. Each is merely the representation of a byte, which has exactly the same arrangement of binary digits in either representation, as well as other representations. It makes more sense to invert your statement 'ASCII code for p is 112'. Actually, the byte is 112 (in decimal), and its ASCII representation is 'p'. Expressed in hexadecimal, the very same arrangement of binary digits is 70. It's the very same byte, just represented differently for different purposes depending on context. The following bit of C code demonstrates this: // A single byte with a value unsigned char myByte = 112; // Three ways to print the value of the same byte printf( "%c %02X %d\n", myByte, myByte, myByte ); What gets confusing is when people want a printable, human-readable representation of a byte. If you just print the byte, it's default representation will be the ASCII version. In order for you to see the hexadecimal (or other radix) version, it needs to be represented by a string of two ASCII characters which can be printed. Each of those characters, of course, is a byte which has an appropriate ASCII representation.

If you wish to covert the letter "p"  in lowercase ASCII (= 112 decimal) to two characters representing this into a displayable ASCII format "70" you can do it using a 256 byte table, indexed by the value of the character in question. (In case the value exceeded its ASCII value range, it would be safer to use a 512 byte table to cater for all possible values up to X"FF"). The table below only shows the first 66 characters of this table (for brevity). However, you should be able to see that decimal 14 ('decimal shift out') will convert to characters "0E"; decimal 27 ('escape') to "1B" ; decimal 32 ('space') to "   " and so on. Here is a link to an ASCII table [ http://simple.wikipedia.org/wiki/ASCII ] For [ http://en.wikipedia.org/wiki/EBCDIC ] characters you would need a 512 byte table, but still only a single assignment to extract the two characters. In low level languages (Assembler), you would convert the value of the ASCII or EBCDIC character to an offset into the table by left shifting by 1 bit first) (In all cases you can get the two byte hex string using just a 16 byte table but it requires about 4 or 5 instructions to convert, rather than a single instruction). 'C' example /* ASCII value to HEX character array  (Hex 00-7F) */ static const char  ASCII_HEX[] = { "00", "01", "02", "03", "04","05","06","07","08","09",0A","0B","0C,"0D","0E",0F" , "10", "11","12","13","14","15","16","17","18","19","1A","1B","1C","1D","1E","1F" ,"   "...etc to "7F"};