John McKown writes: >The z does not have the concept of "data" versus "instruction" storage.
Well... There are nearly always exceptions. :-) Without going back in history I can think of two that are perhaps relevant today. In no particular order: 1. AMODE64. In certain languages on certain machines on certain releases of z/OS a 2GB executable program code limit is enforced while there is no corresponding restriction on data. That is, with such languages/machines/releases code must reside below the bar while data can reside anywhere within a 64-bit address space. (Note that IBM started supporting above-the-bar code execution particularly in z/OS 1.13 and particularly with Java, so the AMODE64 code/data distinction is getting less relevant over time.) 2. Flash Express, introduced with the zEnterprise EC12, is addressable -- it's "storage class memory" -- and z/OS can read and write select types of data to that memory. However, as far as I know it's not possible to execute code from Flash Express. Both of these exceptions strike me as "let's be sensible" sort of non-architectural safety restraints that IBM has either been progressively lifting (#1) or might hypothetically lift if some use case develops (#2), in my opinion, but there they are. My understanding is that #1 is a choice even if supported. That is, if above-the-bar execution is available, you can still choose whether to keep the restraint in place if you wish. -------------------------------------------------------------------------------------------------------- Timothy Sipples GMU VCT Architect Executive (Based in Singapore) E-Mail: [email protected] ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
