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Code: Select all

  // Here we:
   // * Stop interrupts so we don't get interrupted.
   // * Temporarily put the new EIP location in ECX.
   // * Load the stack and base pointers from the new task struct.
   // * Change page directory to the physical address (physicalAddr) of the new directory.
   // * Put a dummy value (0x12345) in EAX so that above we can recognise that we've just
   // switched task.
   // * Restart interrupts. The STI instruction has a delay - it doesn't take effect until after
   // the next instruction.
   // * Jump to the location in ECX (remember we put the new EIP in there).
   asm volatile("         \
     cli;                 \
     mov %0, %%ecx;       \
     mov %1, %%esp;       \
     mov %2, %%ebp;       \
     mov %3, %%cr3;       \
     mov $0x12345, %%eax; \
     sti;                 \
     jmp *%%ecx           "
                : : "r"(eip), "r"(esp), "r"(ebp), "r"(current_directory->physicalAddr));
}

Code: Select all

void switchcontext(context * current, context * next)
{
  if (0==setjmp(current->jmpbuf)) {
    /* Context saved, switch to next */
    longjmp(next->jmpbuf, 1);
  }
}

Code: Select all

typedef unsigned long jmp_buf[23];
int setjmp(jmp_buf*);
int longjmp(jmp_buf*, int);

Code: Select all

setjmp:
  mflr r0  #load link register into r0
  mfcr r4  #load condition register into r4
  stw r1,0(r3) #save r1
  stw r2,4(r3) #save r2
  stmw r13,8(r3) #save r13-r31
  stw r0, 21*4(r3) #save link register
  stw r4, 22*4(r3) #save condition register
  li r3,0 #set return value to 0
  blr #return

longjmp:
  lwz r1,0(r3) #load r1 back
  lwz r2,4(r3) #load r2 back
  lmw r13,8(r3) #load r13-r31 back
  lwz r0, 21*4(r3) #load old LR into r0
  lwz r5, 22*4(r3) #load old CR into r5
  mtlr r0 #restore LR
  mtcr r5 #restore CR
  mr. r3,r4 #move return value over to R3, testing for equality with 0
  bne 1f #if it is:
  li r3,1 #set return value to 1
1:
  blr #return