Module R2 Overview

Process queues As processes enter the system and transition from state to state, they are stored queues. There may be many different types of queues:  Job queue: jobs waiting to be loaded as processes.  State queues: queues for each process state.  Device queues: processes waiting for access to a particular device.

Executing Processes Process scheduling and dispatching involves:  Creating processes as new jobs are admitted to the system.  Maintaining various queues, and moving processes from one queue to another.  Using some algorithm to select a process from the ready queue, and loading and giving control to the selected process.

R2: Implement the control structures to:  Implement 2 abstract data types  PCBs  Doubly linked queues  Operations  Create, initialize, and destroy PCB’s  Insert and remove PCB’s from queues  It will NOT load and dispatch processes

2 layers to R2  Internal layer  Defining abstract data types and the operations for objects of those types  These functions will be invoked, by operations implemented at the user interface level  User interface layer  Add new commands to COMHAN to allow the testing of your internal operations

Process Queues  Sample structure Typedef struct queue { PCB *front; PCB *rear; } Queue;  We suggest you implement a separate queue for each possible process state: ready, suspended ready, blocked, and suspended blocked.

Each queue is a doubly linked list!!!!! Ready queue Front Rear Process10 Previous Next = NULL Process3 Previous Next Process5 Previous Next Proess1 Previous=Null Next Priority = 50Priority = 10Priority = 50Priority=120

We implement the queues as a doubly linked list because:  Occasionally, a process to be terminated will be in the middle of the queue, it is more efficient to locate and “unlink it”, than to use traditional queue operations  Also, the ready queue is maintained in “priority” order, so new processes may be enqueued into the middle of the queue!

Queue Operations  Find PCB:  Searches for a specific PCB by process name.  It will “search” one or more queues  If the process is not found a “NULL” pointer will be returned.  If found the function will return a pointer to the PCB

 Insert PCB  Traditional FIFO insertion  For the blocked, suspended blocked, and suspended ready queue, we will implement the traditional FIFO enqueue.  New processes are added at the END of the queue.  Priority insertion  For the READY queue, we will implement a “priority” insertion algorithm  New processes are inserted into the queue IN FRONT of all processes with a lower priority

Ready queue Front Rear Process10 Previous Next = NULL Process3 Previous Next Process1 Previous=Null Next Process5 Previous Next Priority = 50 Priority = 10Priority = 50Priority=120 Process20 Previous Next Priority = 50

 Remove PCB  Receives a pointer to the PCB  A pointer to a queue  UNLINKS the PCB from the queue May be called after a “find PCB” request remove a specific PCB from a queue.  You may want to also implement a traditional “dequeue” operation.

Process Control Blocks (PCBs)  Suggested structure // PCB structure typedef struct PCB{ struct IOCB iocb; struct IOCB iocb; char pname[20]; //Process name at least 8 characters char pname[20]; //Process name at least 8 characters int pclass; //A code identifying the process as an int pclass; //A code identifying the process as an //application process or a system process //application process or a system process int priority; // range -128 ~ +127 int priority; // range -128 ~ +127 int state; // Running, Ready, Blocked, Suspended Ready, or int state; // Running, Ready, Blocked, Suspended Ready, or // Suspended blocked struct PCB * previous; // pointer of the previous PCB in the queue struct PCB * previous; // pointer of the previous PCB in the queue struct PCB * next; // pointer of the next PCB in the queue struct PCB * next; // pointer of the next PCB in the queue unsigned char * stack_base; // Stack pointer to the base unsigned char * stack_base; // Stack pointer to the base unsigned char *stack_p; //stackptr unsigned char *stack_p; //stackptr int memory; int memory; unsigned char *load; //pointer to where the process is loaded unsigned char *load; //pointer to where the process is loaded unsigned char *execution; //pointer to execution addr unsigned char *execution; //pointer to execution addr } PCB;

PCB operations  Allocate PCB This operation will dynamically allocate a “new” PCB and return a pointer to it. If the allocation fails, a NULL pointer should be returned

 Free PCB Given a pointer to a PCB, this function is responsible for:  Freeing all storage contained in the PCB  Freeing the space associated with the PCB  Setting the incoming pointer to NULL

 Setup PCB Initializes the contents of a PCB!!!  Receives a pointer to a PCB  Input parameters for Name, priority, class, and state (always “ready” for R2)  Allocate memory for the stack and initialize the stack pointer (not used in R2)  Allocate a memory area for the process. (this is not used in R2, so the memory size should be set to zero and all related pointers set to NULL!!!!!!)

User Interface operations!  Create_PCB  Allocates and sets-up a new PCB, it receives arguments for name, class and priority  Delete_PCB  Given a process name, this function will “find”, “remove”, and “free” a process!  Block  This function moves a process from the ready queue to the blocked queue (or from one suspended queue to the other.)  It receives the process name as an argument

 Unblock  This function moves a process from the blocked queue to the ready queue (or from one suspended queue to the other.)  It receives the process name as an argument  Suspend  This function moves a process from either the “ready” or “blocked” queue to the associated “suspended” queue.  It receives the process name as an argument

 Resume  This function moves a process from either of “suspended” queues back to either the “ready” or the “blocked” queues  It receives the process name as an argument Please note: These operations require that we “find” a PCB, “remove” it from one queue and “insert” it into another!

 Set Priority  This function is used to change the priority of a specific process.  It must “find” the process  Change the value of it’s priority  Show_PCB  Displays the contents of a single PCB  Receives the process name as an argument.

 Show All  This function prints information for ALL PCBs in the system!!!!! Displays the contents of EVERY queue!!!  Need to select an attractive display format!!!  Show Ready  This function displays information for all of the PCB in both the ready queue or the suspended ready queue.

 Show blocked  Shows information for all PCBs in either the blocked or suspended blocked queues.