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What is the command for finding the number of bit of an OS/390 or a z/OS?

romil gaurav
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3 Answers3

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Since there didn't seem to be a "real" answer on this thread, I thought I'd provide one just in case anyone needs the information...

The definitive source of whether you're running in 64-bit mode is the STORE FACILITY LIST (STFL, or STFLE) hardware instruction. It sets two different bits - one to indicate that the 64-bit zArchitecture facility is installed, and one to indicate that the 64-bit zArchitecture facility is active (it was once possible to run in 31-bit mode on 64-bit hardware, so this would give you the "installed, but not active" case).

The operating system generously issues STFL/STFLE during IPL, saving the response in the PSA (that's low memory, starting at location 0). This is handy, since STFL/STFLE are privileged instructions, but testing low storage doesn't require anything special. You can check the value at absolute address 0xc8 (decimal 200) for the 0x20 bit to tell that the system is active in 64-bit mode, otherwise it's 31-bit mode.

Although I doubt there are any pre-MVS/XA systems alive anymore (that is, 24-bit), for completeness you can also test CVTDCB.CVTMVSE bit - if this bit is not set, then you have a pre-MVS/XA 24-bit mode system. Finding this bit is simple - but left as an exercise for the reader... :)

If you're not able to write a program to test the above, then there are a variety of ways to display storage, such as TSO TEST or any mainframe debugger, as well as by looking at a dump, etc.

Valerie R
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While I was not able to find commands to give this information, I think below is what you're looking for:

According to this: https://en.wikipedia.org/wiki/OS/390

z/OS is OS/390 with various extensions including support for 64-bit architecture.

So if you're on a zSeries processor with z/OS, you're on 64-bit.

According to this: https://en.wikipedia.org/wiki/IBM_ESA/390

OS/390 was installed on ESA/390 computers, which were 32-bit computers, but were 31-bit addressable.

Almo
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  • No, ESA/390 computers were 31-bit addressing. – Kevin McKenzie Jan 26 '17 at 23:03
  • "The architecture maintains problem state backward compatibility with the 24-bit-address/32-bit-data System/360 (1964) and all intermediate large system 24/31-bit-address/32-bit-data architectures (System/370, System/370-XA, and ESA/370)." The wikipedia article says the 390 is 32 bit. Am I missing something? – Almo Jan 27 '17 at 01:49
  • It's complicated. The general purpose registers were 32-bit, and had, in fact, been 32-bit since the beginning of OS/360. However, you could only address 24-bits of memory, even though addresses were stored in 32-bit words. But, since memory was a very scare commodity in those days, programmers would often use the other 8 bits for status flags and the like. – Kevin McKenzie Jan 27 '17 at 02:35
  • So when IBM decided to increase the amount of addressable storage, they had a serious problem, because one of the promises that was made when the System 360 was introduced was that programs would always work, going forward, without recompiling/rewriting. So IBM decided to make the high order address bit a flag bit that would determine what addressing mode you were in. So System 390 was a 31-bit architecture, not a 32-bit architecture, since the architecture is generally used to signify the amount of addressable storage. – Kevin McKenzie Jan 27 '17 at 02:36
  • Adjusted answer. Is it better I hope? Very interesting, thanks. – Almo Jan 27 '17 at 03:41
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    There are 3 mainframe architectures, and the cool thing about the hardware is that it supports all three concurrently. The original S/370 had 32-bit registers but 24-bit addressing (address space of 16MB). MVS/XA added the second mode, 31-bit/2GB address spaces, and "zArchitecture" added the full 64-bit architecture. Even the latest systems run mixes of 24- 31- and 64-bit software in parallel - it's easy for any app to switch between these modes. Quite unusual compared to other architectures. – Valerie R Feb 09 '17 at 16:59
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For either z/OS or OS/390 I believe you can do a D IPLINFO and look for ARCHLEVEL. ARCHLEVEL 1 = 31 bit, ARCHLEVEL 2 = 64 bit. But it's been a very long time since I've been on an OS/390 system.

randomScott
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