Hi, While studying the V6 kernel, I search for some rules as when to raise the interrupt priority level (IPL). I came up with something like this: A kernel process needs to raise the IPL if a change of data must be atomic and might be accessed (not necessarily changed) by an IRS. In light of this rule, some raising of interrupts seem unnecessary: - In m40.s changes of the user space segmentation registers are bracketed by raising and lowering the IPL, although these registers are never accessed by an ISR. - Likewise, in m40.s, just before the trap routine checks the runrun flag and restores r0, r1 and the user stackpointer, it raises the IPL. I can't see what possibly should go wrong, if being interrupted while restoring the values. I'd appreciate any help from someone with more experience in kernel programming. Greetings, Wolfgang Helbig