CSE 221 - Operating Systems - Notes on "TENEX, a Paged Time Sharing System for the PDP-10"

Question:

What features in TENEX are reminiscent of features in Unix (a later system)?

Needed a better system which bested their previous systems, DEC 10/50 and the SDS 940.
Minimal change could be made to the processor, none to computation
Needed service within 6 months

Three broad goal categories

State of the art VM
- paged virtual address space
- multiple processes
- File system integrated into virtual address space
- extended instructions
Good human engineering
- command language interpreter
- terminal interface design to facilitate interaction
- VM functions providing reasonable defaults
- encourage cooperation among users
Implementable, maintainable, modifiable
- modular software
- debuggable
- instrumentation for performance

Created a “paging device” capable of translating virtual pages into physical ones.
may have multiple virtual addresses, but only one physical stored.
copy-on-write feature to permit read/write sharing of pages.
per-page status bit
copied to private area once written
“core status” table noting when a page was referenced and which processes use a page

Add a JSYS instruction providing an independent transfer mechanism into a monitor
transfer from user program to monitor routine in one instruction using the “JSYS Transfer Vector”
JSYS address provides the index to the proper transfer vector entry

Is this the beginnings of “root” privilege vs user-level?

Operates with 256k of Vmem, 512 word page
paging hardware traps data not in core memory and transfers it to memory transparently from user process.
does not make processor IO instructions available directly
virtual memory map of 512 slots

process to receive asynchronous signals from other processes or terminals
detect sets of unusual conditions, including illegal memory reference, process overflow, EOF, and data errors.

Interestingly, made compatible with DEC 10/50 programs
Held on the system ready whenever a 10/50 programs makes a syscall -> load into core
with paging, only one copy on the system

uniform access of I/O function
name as symbolic pointer to data
file access proection
tree of max depth 5
automatic version control
file extensions
file access of list, read, write, append, execute
permissions based on file location relative to directory where it is stored (15 bits)
directory attached to job of the program is same as owning
directory attached to job that is running is same group
neither of the above cases
concurrent access via “thawed” or “unthawed”

Equitable distribution of CPU time - 1/N per job where N is the total number of jobs
TENEX is interactive - give prompt service to interative requests
Balance set is the set of jobs with highest priority whose total working sets fit in core mem.
interaction causes placement onto high priority scheduling queue
- For tenex, priority is based on long-term average ratio of CPU use to real time.
- new priority determined by priority before interaction and length of interaction
- priority decreased at constant rate, increased while blocked.
short burst of max priority and queue position determined by long term average
- hopeful to get quick service to very short interactions.
Admin can increase load by increasing priority based on system fraction F \(C/T > F\)

information provided by status table in page hardware is essential to memory operation in tenex
process reference a page not in core, page trap occurs and core management routine is invoked to retrieve the page
- if a process page fault time is > P_av, a system paramter, then it is below its working set size, and a page is pulled in.