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A very simple kernel

A very simple kernel. Anders P. Ravn April 2005. A kernel specification. /* kernel.h Interface to a lightweight kernel that implements concurrent processes and a release primitive `pause'. Anders P. Ravn, DTU, Denmark 24 April 1998 apr@it.dtu.dk */ typedef void (* Program )(void);

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A very simple kernel

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  1. A very simple kernel Anders P. Ravn April 2005

  2. A kernel specification /* kernel.h Interface to a lightweight kernel that implements concurrent processes and a release primitive `pause'. Anders P. Ravn, DTU, Denmark 24 April 1998 apr@it.dtu.dk */ typedef void (*Program)(void); /* A program text is a function without parameters*/ typedef void * Thread; /* identifier for a process */ extern Thread create(Program p,unsigned int stacksize); /* creates a process with a stackof the specified size and starts it executing theprogram. If there is insufficientmemory, the result is NULL */ extern void pause(void); /* release the processor */

  3. Multiprogramming #include ”kernel.h” void process() { /* do something */ pause(); /* do something */ } void main() { Thread p1, p2; p1 = create(&process,2000); /* p1 is started */ p2 = create(&process,1000); /* p2 is started */ /* the kernel will see to it that main is left when both p1 and p2 has exited */ }

  4. A kernel implementation I typedef unsigned long Register; typedef struct x {struct x* next; Register esp;} Threaddescriptor; static Threaddesriptor* ready; /* queue of threads linked cyclically; ready points to last, the first is current */ #define current ready->next ready: current esp1 esp2 esp3

  5. A kernel implementation II void pause() { Register sp; __asm__(”pushal movl %%esp,%sp"); /* sp -> |saved registers ... | eip return from call */ DISABLE; /* scheduling */ current->esp = sp; ready = current; sp = current->esp; __asm__(" movl %sp,%%esp popal" ); ENABLE; } stack1 stack2 ready: current esp1 esp2 esp3

  6. A kernel implementation III pause: pushl %ebp movl %esp,%ebp pushal movl %esp,%ecx sp = esp movl ready,%eax movl (%eax),%edx current->esp movl %ecx,4(%edx) = sp movl %edx,ready ready = current movl (%edx),%edx movl 4(%edx),%ecx sp = current->esp movl %ecx,%esp popal leave ret

  7. A kernel implementation IV pause: pushal movl ready,%eax movl (%eax),%edx current->esp movl %esp,4(%edx) = esp movl %edx,ready ready = current movl (%edx),%edx movl 4(%edx),%esp esp = current->esp popal ret

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