The next piece to support tasks in your operating system is per-task data structures, for example, the process control block (PCB). One bit of per-task state that needs to be saved is the file array, described earlier; another is the signal information. You may need to store some other things in the process control block; you must figure the rest out on your own. The final bit of per-task state that needs to be allocated is a kernel stack for each user-level program. Since you only need to support two tasks, you may simply place the first task’s kernel stack at the bottom of the 4 MB kernel
page that you have already allocated. The second task’s stack can then go 8 kB above it. This way, both tasks will have 8 kB kernel stacks to use when inside the kernel. Each process’s PCB should be stored at the top of its 8 kB stack, and the stack should grow towards them. Since you’ve put both stacks inside the 4 MB page, there is no need to “allocate” memory for the process control block. To get at each process’s PCB, you need only AND the process’s ESP register with an appropriate bit mask to reach the top of its 8 kB kernel stack, which is the start of its PCB. Finally, when a new task is started with the execute system call, you’ll need to store the parent task’s PCB pointer in the child task’s PCB so that when the child program calls halt, you are able to return control to the parent task.
The next piece to support tasks in your operating system is per-task data structures, for example, the process control block (PCB). One bit of per-task state that needs to be saved is the file array, described earlier; another is the signal information. You may need to store some other things in the process control block; you must figure the rest out on your own. The final bit of per-task state that needs to be allocated is a kernel stack for each user-level program. Since you only need to support two tasks, you may simply place the first task’s kernel stack at the bottom of the 4 MB kernel page that you have already allocated. The second task’s stack can then go 8 kB above it. This way, both tasks will have 8 kB kernel stacks to use when inside the kernel. Each process’s PCB should be stored at the top of its 8 kB stack, and the stack should grow towards them. Since you’ve put both stacks inside the 4 MB page, there is no need to “allocate” memory for the process control block. To get at each process’s PCB, you need only AND the process’s ESP register with an appropriate bit mask to reach the top of its 8 kB kernel stack, which is the start of its PCB. Finally, when a new task is started with the execute system call, you’ll need to store the parent task’s PCB pointer in the child task’s PCB so that when the child program calls halt, you are able to return control to the parent task.