On Jan 28, 2021, at 3:27 PM, Johnny Billquist <bqt%Update.UU.SE@localhost> wrote:
On 2021-01-26 14:06, Nigel Johnson wrote:
Thanks, Mouse. Yes, as an FE I used to have to figure out those floating assignment from charts - some DEC systems (RSTS, RT11) could not be told where to look - they had to be in the right place. ...
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I think RSTS/E made more use of the probing at runtime. Not sure about RT-11.
On RSTS/E:
1. At boot time it would scan the bus looking for addresses that respond, and assign them to devices according to the floating address rules.
2. You could always explicitly set the address, overriding the float handling for that device. So no, it is not correct that RSTS "could not be told where to look".
3. There also exist "floating vector" rules, which in a similar fashion would guide what interrupt vector to assign to a device. I don't know if any OS actually used those. RSTS definitely did not. Instead, at boot time it would "poke" each identified device to cause it to interrupt, which would tell it what the vector is rather than having to assume. Two exceptions: programmable interrupt devices would simply be assigned a vector that was available, and the CR11 card reader would be assumed to be at its standard vector since there is no way to make it interrupt (short of attempting to read a card, which isn't a good answer).