Linux 2.4 box using gcc 3.4.6 don't seem to follow the same conventions as the i386-jos-elf-gcc compilers. Can run make 'TOOLPREFIX=' or edit the Makefile. curproc[cpu()] can now be NULL, indicating that no proc is running. This seemed safer to me than having curproc[0] and curproc[1] both pointing at proc[0] potentially. The old implementation of swtch depended on the stack frame layout used inside swtch being okay to return from on the other stack (exactly the V6 you are not expected to understand this). It also could be called in two contexts: at boot time, to schedule the very first process, and later, on behalf of a process, to sleep or schedule some other process. I split this into two functions: scheduler and swtch. The scheduler is now a separate never-returning function, invoked by each cpu once set up. The scheduler looks like: scheduler() { setjmp(cpu.context); pick proc to schedule blah blah blah longjmp(proc.context) } The new swtch is intended to be called only when curproc[cpu()] is not NULL, that is, only on behalf of a user proc. It does: swtch() { if(setjmp(proc.context) == 0) longjmp(cpu.context) } to save the current proc context and then jump over to the scheduler, running on the cpu stack. Similarly the system call stubs are now in assembly in usys.S to avoid needing to know the details of stack frame layout used by the compiler. Also various changes in the debugging prints.