egos-2k+ syscall workflow
---

sys_send/recv (syscall.c)
  |                                                         ^
  +-> trap (syscall.c)                                    [ret]
        |                              USER SPACE           |
-----[trap]-------------------------------------------------|----
        |                               KERNEL              |
        +-> trap_entry (kernel.s)                         [mret]
            [switch to kernel stack]       [switch to user stack]
            [save context to stack]             [restore context]
            |                                               |
            +-> kernel_entry (kernel.c)                   [ret]
                [context: stack=>PCB]       [context: PCB=>stack]
                |                                           |
                ... (intr_entry? excp_entry?)               |
                |                                           |
                +-> proc_try_syscall (ipc.c)              [ret]
                    |                                       |
                    +-> proc_try_send/recv (ipc.c)          |
                    +-> sys_yield (scheduler.c)           [ret]
                            |                               |
                            +------>[schedule next proc]----+