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Hex to Character Conversion Revisited

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About three years ago, I wrote an article exposing the MI instructions that allow you to convert text data containing hexadecimal characters, such as F1F2F0F0F0, into their character format, such as 12000.

The two MI instructions that perform conversion from hexadecimal to character and back again are cvthc and cvtch. I've seen dozens of hand-written routines that convert the hexadecimal strings to letters and letters to hexadecimal strings. Sounds like reinventing wheel to me.

All these routines became obsolete with the introduction of these two MI instructions back in the 1980s. It wasn't until IBM introduced System/C back in the mid 1990s that it became practical to use them outside of MI, but now with RPG IV's ability to call C and MI runtime functions, there is no excuse to ask "How do I...?" in this case.

The two MI instructions are as follows:

  • cvthc converts from character value to hexadecimal text (that is, from 'A' to X'C1').
  • cvtch converts from hexadecimal text to character value (that is, from X'C1' to 'A').

The interesting thing about these instructions is that they support conversion of a single character or multiple characters at one time. So you need only call these routines once per value rather than once per character.

The prototype for both MI instructions is as follows:

0001 H  BNDDIR('QC2LE')
.....D*ame+++++++++++EUDS.......Length+TDc.Functions+++++++++++++++
0002 D ToHex           PR                  EXTPROC('cvthc')
0003 D  szHexVal                  65534A   OPTIONS(*VARSIZE)
0004 D  szCharVal                 32767A   Const OPTIONS(*VARSIZE)
0005 D  nSrcLen                      10I 0 VALUE

0006 D FromHex         PR                  EXTPROC('cvtch')
0007 D  szCharVal                 32767A   OPTIONS(*VARSIZE)
0008 D  szHexVal                  65534A   Const OPTIONS(*VARSIZE)
0009 D  nSrcLen                      10I 0 VALUE

Note that I've named the prototypes differently than the MI instruction names. This is because I feel the MI names are too confusing. The TOHEX prototype converts normal character format into the two-character hexadecimal format. The FROMHEX prototype converts the two-character hexadecimal format to normal character format.

Hexadecimal representation of the value appears as pairs of characters. Consequently, the hexadecimal parameter must be at least twice the length of the character field. Also, the input value is specified on the second parameter, and the result is returned to the first parameter. Last, the length of the text to be converted (i.e., the length of the data in parameter 2) is specified on parameter 3.

To use these prototypes, simply store them in a separate source member and use /COPY or /INCLUDE to include them in your source member.

Note that on the first line in the example, the BNDDIR('QC2LE') keyword is specified. This is required when your program is taking advantage of any C or MI runtime functions. Do not include this line in the /COPY member; instead, put in the source member being compiled.

QC2LE is an IBM-provided binding directory that includes the names of the service programs the C runtime library uses. By using this binding directory, the RPG IV compiler binds to the C and MI runtime functions. No additional programming required!

Bob Cozzi is a programmer/consultant, writer/author, and software developer of the RPG xTools, a popular add-on subprocedure library for RPG IV. His book The Modern RPG Language has been the most widely used RPG programming book for nearly two decades. He, along with others, speaks at and runs the highly-popular RPG World conference for RPG programmers.

BOB COZZI

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.

MC Press books written by Robert Cozzi available now on the MC Press Bookstore.

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 Cozzi on everything RPG! What more could you want?
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