Gotcha: Differences With DO Between S/36 and AS/4

RPG
Typography
  • Smaller Small Medium Big Bigger
  • Default Helvetica Segoe Georgia Times

Duty calls for ninety-nine repetitions of the loop in the program in 1.

Duty calls for ninety-nine repetitions of the loop in the program in Figure 1.

The S/36 runs this program without trouble, and so does the AS/400 in the S/36E. Both understand very well their orders: repeat the loop ninety-nine times, then end the program.

The AS/400 native environment, on the other hand, tries to go beyond the call of duty, and this gets it into trouble very quickly: RPG0121 is issued, which reads "The array index is invalid (C G S D)."

Taking option D (for Dump) generates a printout that lets you examine the values of all variables. Variable X has a value of... zero! Obviously, the program tried to loop for the hundredth time: this zero is a number 100 truncated to two digits.

If you are converting programs to native, beware.

Bill Robins Riverside, California

Editor's Note:

What is really disturbing about this bug is that if you remove the array reference from within the loop (ARR,X), the program will loop forever, even though it goes against common sense. This problem was reported to IBM Level 2 as a compiler bug on January 21, 1991. Their reply was FAXed to us the next day:

"The user obtains different results because of the differences between RPG II and RPG III compilers. In the user's program, the limit value (Factor 2) of the DO op-code is 99 while the length of the index variable (Result Field) is only 2. In the S/36 RPG II compiler, the DO-END statement will loop from the starting value (Factor 1) to 99 (Factor 2) and then stop. After the END statement, the index variable is set to 00. In the RPG III compiler, since the index variable will never be greater than the limit value, the DO-END statement will loop forever or issue a message RPG0121.

"In the AS/400, the RPG/36 compatible compiler behaves the same as the S/36 RPG II compiler while RPG/400 and RPG/38 behave the same as the S/38 RPG III compiler. As a result, no changes will be made to the compiler.

"This difference will be documented in Appendix B, 'AS/400 RPG/400 and AS/400 RPG II System/36-Compatible Functions,' in the RPG/400 User's Guide (SC09-1161)."

For the record, we must refer the reader to page 11-53 of the RPG/400 Reference Guide, which states the following near the bottom of the page:

"3. If the index value is greater than the limit value, control passes to the calculation following the associated END statement (step 7). Otherwise, control passes to the first operation after the DO statement (step 4)."

The implication from IBM's "solution" to this bug seems to be that it is not really a bug, only a documentation issue, so they have nothing to fix. This is the stuff of which "Gotchas" are made. This one may remain a "Gotcha!" until the end of time.


Gotcha: Differences With DO Between S/36 and AS/4

Figure 1 Program to perform 99 repetitions

 Figure 1: Program to Perform Ninety-Nine Repetitions ....1.... ....2.... ....3.... ....4.... ....5.... ....6.... ....7 E ARR 99 7 2 * C Z-ADD0 TOTAL 92 C 1 DO 99 X 20 C ADD ARR,X TOTAL C END * C SETON LR ....1.... ....2.... ....3.... ....4.... ....5.... ....6.... ....7 
BLOG COMMENTS POWERED BY DISQUS

LATEST COMMENTS

Support MC Press Online

$

Book Reviews

Resource Center

  •  

  • LANSA Business users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.

  • The MC Resource Centers bring you the widest selection of white papers, trial software, and on-demand webcasts for you to choose from. >> Review the list of White Papers, Trial Software or On-Demand Webcast at the MC Press Resource Center. >> Add the items to yru Cart and complet he checkout process and submit

  • SB Profound WC 5536Join us for this hour-long webcast that will explore:

  • Fortra IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn: