On Mon, 20 Apr 2015 11:46:57 -0500, Paul Gilmartin <wrote: >Had bit 0 not been >pervasively exploited as a flag, 32-bit addressing could have been >accomplished easily.
If there had been no reason to run old code in AMODE 24, 32-bit addressing could have been accomplished easily. That was the hope when the System/360 Model 67 was designed. It proved to be difficult to eliminate all use of AMODE 24. It is easy for us to fault the designers of System/360 and OS/360 for not considering the future requirement for more than 16 MB. In 1964, that seemed like plenty of addressable memory. In their 1964 document, "Architecture of IBM System/360", Amdahl, Blaauw and Brooks wrote that the "design had to provide a dependable base for a decade of customer planning and customer programming...." and that "Storage capacities of more than the commonly available 32,000 words would be required." The System/360 was IBM's first "family" of computers, spanning a wide range of capabilities. -- Tom Marchant ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: INFO IBM-MAIN
