1. Implementing Processes and Threads
© 2004, D. J. Foreman

2. Required Software for Threads
UNIX (Linux, OpenBSD, FreeBSD, Solaris)
Exported POSIX API or use "Pthreads" API
gcc or g++ with -lpthread -lposix4 -lthread
Windows (98/ME/NT/XP)
WIN32 API – not POSIX compliant
Pthreads.DLL – freeware
sources.redhat.com/pthreads-win32
Copy pthread.dll to C:\windows
Keep .h files wherever you want them
© 2004, D. J. Foreman

3. Exploring the Abstraction
Loc 0 Loc n
Loc 0 Loc n
Loc 0 Loc n
User i
Processes & RAM
User j
Processes & RAM
User k
Processes & RAM
CPU i
CPU j
CPU k
Abstract
Actual
RAM
User i
Processes
User j
Processes
User k
Processes
Loc 0
Loc n
CPU
Page space
© 2004, D. J. Foreman

4. Process Manager Responsibilities
Define & implement the essential characteristics of a process and thread
Algorithms for behavior
Process state data
Define the address space (and thus available content)
Manage the resources (I/O, RAM, CPU)
Tools to manipulate processes & threads
Tools for scheduling the CPU (Ch 7)
Tools for inter-thread synchronization (Ch 8,9)
Handling deadlock (Ch 10)
Handling protection (Chapter 14) (if time permits)
© 2004, D. J. Foreman

5. Resources What is a "resource" R={Rj|0<=j<=m}
Requestable blocking object or service
Reusable – CPU, RAM, disk space, etc
Non-reusable (consumable)
Data within a reusable resource
R={Rj|0<=j<=m}
Rj is one type of resource, such as RAM
C={cj>=0| Rj R(0<=j<m)}
cj is the # of available units of Rj
© 2004, D. J. Foreman

6. Resource Mgmt Model
 {Mgr} : Rj (Mgr(Rj) gives ki<=ci units of Rj to Pn)
Pn may only request i units of Rr
Pn may only request unlimited units of Rn
Why do we need the set notation?
Formalized descriptions can lead to deadlock detection and prevention algorithms
© 2004, D. J. Foreman

7. Windows NT/2K/XP Process Mgmt
Split into 2 facilities:
NT Kernel
Object mgmt
Interrupt handling
Thread scheduling
NT Executive
All other Process aspects
See "Inside Windows 2000", 3e, Solomon & Russinovich, Ch. 6, MS Press, 2000
© 2004, D. J. Foreman

8. The Address Space Boundaries of memory access
H/W can help (DAT) (more later)
Multiprogramming possible without H/W!!!!
Self-relocation
Pre-load relocation
Both use true addresses FIXED at load time
NO paging, but MAY have swapping
Windows 3.1
IBM OS/VS1
© 2004, D. J. Foreman

9. Address Binding Given: Where are X, y, z and q??
int function X(y,z)
{Int q; return ff(y,z)}
Void function M {X(3,4);}
Where are X, y, z and q??
How does X get control from M?
What happens if there is an interrupt BETWEEN M's call to X and X starting?
© 2004, D. J. Foreman

10. Address Binding-2-fixed
Gather all files of the program
Arrange them in RAM in linear fashion
Determine runloc for the executable
Find all address constants (functions and External data)
Find all references to those constants
Modify the references in RAM
Store as an executable file
Run at the pre-determined location in RAM
© 2004, D. J. Foreman

11. Address Binding-3-dynamic
Perform "fixed binding", but in step 3, use a value of "zero"
In step 6, mark as "relocatable"
For step 8, before actually transferring control, REPEAT 4-6 using the actual runloc determined by the loader
Same for DLL members
© 2004, D. J. Foreman

12. Address Binding-4 DLL's
How does a program find a DLL it didn't create?
Each DLL member has a specific name
System has list of DLL member names
When DLL is requested, system fetches module and dynamically binds it to memory, but NOT to the caller!
System transfers control to DLL member
© 2004, D. J. Foreman

13. Address Binding - 5
How does the system make it look as if each abstract machine starts at 0?
How does the system keep user spaces apart
How does the system protect address spaces
© 2004, D. J. Foreman

14. Context Switching
Power on, ROM reads bootstrap program from head 0 of device
ROM transfers control to the program
Bootstrap program reads the loader
Loader reads the kernel
Kernel gets control and initializes itself
Kernel loads User Interface
Kernel waits for an interrupt
Kernel starts a process, then waits again
© 2004, D. J. Foreman

15. Context Switching - 2 Device requests interrupt
ROM inspects system for ability to accept
If interrupts are masked off, exit
Future interrupts may be queued by hardware
Or devices may be informed to re-try
If interrupts are allowed:
set status (in RAM, control store, etc)
Atomically: load new IC, privileged mode, interrupts masked off
Kernel processes the interrupt
© 2004, D. J. Foreman

16. Context Switching - 3 The actual Context Switch:
Save all user state info:
Registers, IC, stack pointer, security codes, etc
Load kernel registers
Access to control data structures
Locate the interrupt handler for this device
Transfer control to handler, then:
Restore user state values
Atomically: set IC to user location in user mode, interrupts allowed again
© 2004, D. J. Foreman

17. Questions to ponder Why must certain operations be done atomically?
What restrictions are there during context switching?
What happens if the interrupt handler runs too long?
Why must interrupts be masked off during interrupt handling?
© 2004, D. J. Foreman

18. What is a "Handle"? Application requests an object
A window, a chunk of RAM, a file, etc.
Must give application a way to access it
Done via a "handle"
A counter (file handles)
An address in user RAM (structures)
Always a "typed" variable
Helps insure correct usage (except "C" doesn't enforce typed usage)
© 2004, D. J. Foreman