Even if they'd had the foresight to use a different way to indicate the end of a parameter list, I don't think they were consciously seeing not changing the extenal architecture of the hardware for 40 years. The 360 was designed to replace IBM and competitor systems that had different hardware instruction sets. Although the 360 supplied many different machine capacities in the same architecture, I don't know that IBM was already planning to always provide backward-compatible architecture forever. They may have considered providing emulators as they did for 1400 series in the early 360s, but expected conversions for major architectural changes, like 31/32 bit.
On 20-09-04 6:16 a.m., Leland Bond wrote:
Fullword bit 0 is used to mark the last entry in parameter lists since the beginning of OS/360. Using that bit for addresses would have broken too much software.
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