On 2023-09-25 20:28, Johnny Billquist wrote:
The way it was explained to me, is that if there is no memory cache, busy-waiting uses the bus, competing with DMA from, say, disk controllers. So a WAIT instruction improves performance in the common case of all processes waiting for I/O or timers.On 2023-09-25 20:17, Paul Koning wrote:On Sep 25, 2023, at 2:06 PM, Andrew Ball <asbatwrk%gmail.com@localhost> wrote:I wonder why VAX lacked that when the PDP-11 had it. My emulated VAX runs NetBSD/vax 9.3.The VAX architects apparently didn't see the need for WAIT, or they felt it was too much trouble to implement. In any case, it isn't part of the instruction set. Instead, when there is nothing else to do, you tend to be in a loop in the scheduler looking for action. What that looks like depends on the OS.At the time, a WAIT didn't seem like it would add any value on the VAX. I think they just simply thought that WAIT on the PDP-11 wasn't really that useful for anything more than making pretty patterns on the front panel, and the VAX don't have a front panel anyway...Of course, as soon as you started talking about virtual machines, it did make sense again. So I think there is something in VARM about this, but I don't think anything implemented, or used it. Hindsight and all that...
The VAX had cache from day one, so busy-waiting was simpler and never reached the SBI.
/Stellan