MULTIPROGRAMMING

abstraction (service) provided by the OS: you want to run a program, so the OS provides you with a process, address space (space-multiplex RAM), execution (time-multiplex CPU), and files (space-multiplex disks)

protection

• base and limit registers in the CPU protect the OS and hardware from your process, and protect processes from each other
• kernel mode for executing privileged instructions protects the hardware from your process
• clock interrupts protect the system from your infinite loops and allocate CPU cycles fairly

multiprogramming implemented with process table, ready queue, clock chip interrupt handler

process table

• array of PCBs (process control blocks) inside OS (in RAM)
• each executing process has a PCB (entry) in the process table
• each PCB contains
  • state: running, ready, blocked, suspended, exited
  • CPU register save area: PC, PSW, SP, base, limit, etc.
  • owner, priority
  • list of open files and RW positions therein
  • clock ticks used since last being given the CPU

ready queue

• doubly linked list inside OS (in RAM) of all processes in the ready state (the process at the head is in the running state)
• head and tail pointers inside the OS
• when a process changes state from running to blocked or suspended or exited, it is taken out of the ready queue
• when a process changes state to ready (from blocked or suspended), is it linked into the ready queue, usually at the rear or front

clock chip interrupts

• clock chip raises system bus interrupt line and places its device ID onto the system bus
• causes switch to CPU kernel mode and save PC register contents in PCB pointed to by ready queue head pointer
• load PC from clock chip's entry in the interrupt vector (the entry indexed by the device ID on the system bus)
• execute the clock chip (hardware) interrupt handler in kernel mode
• disable interrupts
• save all CPU registers in PCB pointed to by ready queue head pointer
• increment the clock ticks used field in the PCB pointed to by ready queue head pointer
• if clock ticks used >= MAX_TICKS_ALLOWED then
  • move the PCB pointed to by ready queue head pointer to the rear of the ready queue
  • change head and tail pointers, next pointers, etc.
  • zero out clock ticks used in the (now different) PCB pointed to by ready queue head pointer
• restore CPU registers from PCB pointed to by ready queue head pointer
• enable interrupts
• return from TRAP/INTERRUPT
• load PC register from PCB pointed to by ready queue head pointer
• switch to CPU user mode
• continue executing some process from where it left off after last having the CPU

MAX_TICKS_ALLOWED is usually 7 so that each process gets a time slice of about 100 milliseconds of CPU time (the clock chip interrupts about 60 times a second or every 16.67 milliseconds)

home page: http://elvis.rowan.edu/~hartley/index.html
e-mail: hartley@elvis.rowan.edu