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DTSS – (1963 to 1965)
page 4, paragraph 2
DTSS – (1963 to 1965)
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Hardware components
Time-line
Software
Anecdotes
By John S. McGeachie ‘65
DTSS Component Overview
GE-235
DN-30
(40)
DISC
Teletypes – slow but reliable
• Photo is 33 ASR but we had model
35 KSR units – larger and rock-solid
• Upper case only
• 10 characters per second
• 11 bits per character:
Start bit – zero
Eight data bits (7 + parity) (ASCII)
Two stop bits – ones
DN-30 Communications Processor
Extraordinarily elegant design, built for multi-terminal I/O
• User I/O – bit-by-bit!
• Master scheduler
• Storage manager
• 16K 18-bit words
• 7 µsec cycle time
GE-235 processing unit
Standard batch-oriented computing workhorse …
with a priority interrupt mechanism
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Compile and run user programs
Background tasks
16k 20-bit words
6 µsec cycle time
Separate floating point arithmetic unit
Disc Storage Unit
Physically massive dual-access disc subsystem
• Three boxes:
- Access controller
- File electronics
- Storage unit
• 64-word records
• 225 ms average access time
• 238 & 476 kbps xfer rates
• 14 MB storage capacity …
for all users
16 coffee-table-size platters
Other stuff
Magnetic tape
Card reader
Card punch
Printer
Dual-access
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Cooperative access – each machine “relinquished”
access to the other
DN-30 read/write: 18 bits of each 20-bit word
DN-30 could override and seize control
Special modification: 96-record read-write (6144
words) with one seek (normally 16 records max)
Computer-to-computer
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Key to system operation
DN-30 reads/writes GE-235 memory via
Computer Interface Unit (“CIU-930”)
18-bit DN-30 word to bits 2-19 of GE-235 word;
bits 0, 1, and parity bit sent from “control flipflops” – essentially, three extra bits available
for certain I/O operations.
DTSS – 1963 to 1965
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Hardware components
Time-line
Operating systems software
Anecdotes
Time-line – we beat the estimates!
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1963 Fall – learn to program, grasp strategy
1964:
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Winter – code execs, debug Basic
Spring – success! Three teletypes.
Summer – 20 teletypes, Algol
Fall – background; system replicated at GE
1965:
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Summer – GE begins commercial service in Phoenix & NYC
Fall – 40 teletypes
The early players
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Kemeny & Kurtz – select hardware, develop strategy, create
Basic
Busch ’66 (DN-30) & McGeachie ’65 (GE-235) – design
and code execs, devise overall tactics, be impatient
Bellairs ’65 – Basic (JGK on sabbatical)
Moore ’65 – Symmaint, Fortran
Froehbose ’65 – Edit
Garland ’63, O’Gorman ’64 (?) – Algol
Jim Brackett (GE) – keep hdwe running, get modifications
approved
The later players – some errors and
omissions (after my time)
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Dick Lacey ‘67
Ron Martin ’67
Steve Hobbs ’67
Dave McGill ’67
Gary Broughton ‘?
At GE – Larry Hittel, George Freund, George
Friend, Walt Kozinski & others
DTSS – 1963 to 1965
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Hardware components
Time-line
Operating systems software
Anecdotes
DN-30 – operating system (“exec”)
D-30 exec had two modes of operation:
• Real-time – not interruptible
•An interrupt every bit-time for TTY I/O –
10 cps x 11 bits/char => 110 interrupts/sec => 9 ms
•Five TTYs serviced during each of eight interrupts
•CIU (to/from GE-235) serviced on cycles 4, 8, and 11
• Spare-time – interruptible
•Disc I/O (19 seconds to output 64 words to TTY)
•Hello, OLD, LIST, SAVE, RUN, BYE, …
•Schedule jobs to run on GE-235
•Assign disk space for saving programs
DN-30 – memory usage
Each terminal has 48 words of character and control storage,
and two 64-word disc I/O buffers. Addressing idiosyncrasies
require that these all start on 64-word boundaries.
 Exec
 Per
data & tables
TTY data
1024 words
3072
 Code
5120 !!!
 I/O
7168
Total
buffers
16384 words
DN-30 – scheduler
Six priority levels:
0 – OLD
1 – LIST
2 – SAVE
3 – Initial RUN
4 – Program request for input
5 – Continued RUN, background
“As a given problem continues to run, it is given progressively longer
running times in the GE-235, but is eligible for scheduling progressively less
often. The effect is to allow long problems to run with fewer swaps, while
short problems receive rapid response.” [Comments in the code!]
Computer-to-computer (cont.)
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DN-30 sends commands to a mailbox area:
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List or Save (implies Edit)
Compile and run (Basic, Algol, etc.)
Suspend and swap
Continue previously suspended program
GE-235 responds with acknowledgment and other info
Failure to respond in ~20 ms results in re-boot
DTSS innovation – use the CIU for commands, and the
disc for data exchange.
GE-235 executive
• Edit, compile, and execute user programs
• Suspend and resume user programs on
instructions from DN-30
• Run background tasks when time available:
– Run software maintenance and assembly
– Billing
– List card decks (on high-speed printer)
– etc.
GE-235 memory usage
 Data
& buffers
2048 words
 User
program area
6144
 Compiler
& run-time
 Resident code
 Overlay code
Total
6144
1536
512
16384 words
GE-235 overlay area
Small (512-word) area into which blocks of code
were loaded as needed for background tasks:
•19 overlays by August 1965
•Peripheral operations
•Billing updates
•Disc maintenance
•Assemblers (for GE-235 and DN-30
programs)
GE-235 – application programs
• Application programs =
• Languages (Basic, Algol, Fortran)
• Special editors (document formatting)
• “Background” maintenance aids (e.g.,
assemblers, assembly-language editors)
• 6,144 words in memory at one time
• Application “overlays” allowed – application
could ask 235-exec to overwrite itself with all
or part of another 6,144-word block.
Efficiency issues
• Never reload a language system if it is already in
memory
• Within given priority and run queue, sequence user
programs using same compiler (see above)
• Alternate compilers on tape, read tape backwards or
forwards according to system required (eventually
moved languages to disc for reliability)
• Locate directories in middle of disc platters
• Background software maintenance (Symmaint)
Implications
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Good protection against errant (but not
malicious) application software.
Subscript-checking for all user programs.
Applications (compilers, run-time packages,
whatever) re-entrant.
GE-235 exec required small footprint.
Disc I/O significant (so what’s new?)
Fail-safe & unattended operation
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GE-235 – DN-30 forces boostrap if:
–Failure
to respond to DN-30 in 20 ms
–Too long to complete certain operations
–Illegal response in mailbox area
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DN-30
–Q-ctr
going to zero creates interrupt
–If two reloads of Q-ctr w/o intervening interrupt,
–If Q-ctr drops to –32,
–If Halt instruction executed,
DN-30 “hardware loads” (boostrap DN-30 from embedded paper
tape reader)
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Hardware modifications
DTSS – 1963 to 1965
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Hardware components
Time-line
Operating systems software
Anecdotes
Anecdotes
Busch
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McGeachie
“He did it” … “I did it”
Landers Restaurant
Hide & seek
The night is ours
Kemeny goes on sabatical
Kevin O’Gorman & “Dartmouth Algol”
DTSS Component Overview

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