Re: Documentation of OS/400 forward compatibility

Only alluded to at:
http://www.ibm.com/servers/enable/site/porting/iseries/overview/overview.html
"Applications in this [programming] model benefit from the TIMI for complete protection and benefit from future hardware changes."

A program with observability can run [assuming the run-time references are available there] on whatever release on or to which it was compiled, and any later release. There is an N-2 limit for providing backwards compatible compile and binary [with template/observability], but upwards compatible is not limited. A program compiled on v1r1 [and even those compiled on s/38] can be restored to and run on any release including v5r4m0.

How? ...
When compiling on v5r2m0 TGTRLS(V4R5M0) the compiler being used is actually _the_ v4r5m0 compiler. Therefore the program template that is created is the same as that which would be created on an actual v4r5m0 installed system with the same PTF [fix level]. The instruction stream is then /translated/ from the template. The instruction stream created is always for the current release. That program can be saved and restored to the lower release level, but the template may need to be re-translated to effect an instruction stream compatible with the release on which it has been restored. Refer to the FRCOBJCVN parameter help text on RSTLIB & RSTOBJ commands [also a system value QFRCCVNRST may be of interest]; whether the retranslation occurs during restore or the first time the program is accessed may be an issue. Just as this transpires going back releases, the same template can be translated again into whatever instruction stream for the current release on which the program /finds itself/ irrespective of whether by restore or having the hardware and/or LIC & OS release upgraded; when due to upgrade vs restore, there is a STROBJCVN command. Because of this feature of the Technology-Independent Machine Interface [TIMI], the old 48-bit AS/400 programs can be restored onto the 64-bit System i5 and run. And because the translator is the only one on that release, the instruction stream that is generated will use the 64-bit instructions; no 'thunking'.

Compiling to a previous release and restoring to that target and any follow-on release is an oft utilized feature by some. It is somewhat typical after a single system is divided into two or more [partitions] to separate development from both production and testing partitions, in preparation to move to the newer release.