Working with the Unix OS

Introduction to kernel
Fields in the process table:
- a state field
- identifiers indicating the user who owns the
process
- an event descriptor set when a process is
suspended
The u area contains:
- a pointer to the process table slot of the
currently executing process
- parameters of the current system call, return
values and error codes
- file descriptors for all open files
- internal I/O parameters
- current directory and current root
- process and file size limits
Figure 1. Data Structures for Processes
Context of a Process
The context of a process is it state, as defined by its text, the values of its global user variables and data structures,
the values of machine registers it uses, the values stored in its process table slot and u area, and the contents of its
user and kernel stacks.
When the kernel decides that it should execute another process, it does a context switch.
Moving between user and kernel mode is a
change in mode.
! Process states
- executing in user mode
- executing in kernel mode
- not executing, ready to run
- sleeping, e.g. waiting for I/O to
complete
" Process States and Transitions
Directed graph
- nodes - states a process can enter
- edges - events that move from
one state to another
Figure 2. Process States and Transitions
The kernel allows a context switch only when a process moves from state "kernel running" to "asleep in memory".
Critical sections of code are executed by at most one process at a time.
Figure 3. Sample Code Creating Doubly linked List
struct queue {
…
}*bp, *bpl;
bpl->forp = bp->forp;
bpl->backp = bp;
bp->forp = bpl;
/*consider possible context switch here */
bpl->forp->backp = bpl;
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