There's a fundamental difference between B * and EX reg,*; the EX gives you a single instruction that never terminates, while the branch gives you multiple instructions each of which completes quickly. So to allow the execute you'd have to make it re startable.
BTW, IBM had earlier machines in which n execute was allowed to address another execute; I never saw an code that exploited chained executes. -- Shmuel (Seymour J.) Metz http://mason.gmu.edu/~smetz3 ________________________________________ From: IBM Mainframe Assembler List <[email protected]> on behalf of Steve Smith <[email protected]> Sent: Monday, August 6, 2018 7:49 PM To: [email protected] Subject: Re: EX On 8/6/2018 18:12, Dan Greiner wrote: > I was once asked why the execute exception existed. That is, why not just let > the hardware — or, in this odd case, the firmware — cascade down a chain of > multiple EX instructions, ORing the bits of the R1 field with the subsequent > target instruction, whatever instruction that might be. Aside from there > being absolutely no practical reason for wasting circuits on such folly, the > answer is obvious ... the EX instruction could target itself, and the CPU > would get its knickers tied into a knot without an exception. Obvious, maybe, but maybe not. B * isn't prevented from spinning the CPU. It has no knickers, and cares not if you carelessly make it work furiously at nothing. sas
