Editing Compatible Time-Sharing System (section)

==Implementation==

===Kernel===
CTSS used a modified IBM 7090 mainframe computer<ref>{{cite web |url=https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/RPQs.html |title=Documents describing special hardware for CTSS |access-date=2024-04-21 |archive-date=2024-06-25 |archive-url=https://web.archive.org/web/20240625150843/https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/RPQs.html |url-status=live }}</ref> that had two 32,768 (32K) 36-bit-[[Word (computer architecture)|word]] banks of [[core memory]] instead of the default configuration which provides only one.<ref name="osc">{{cite book |title=Operating System Concepts |chapter=13: Historical Perspective |page=514 |first1=Abraham |last1=Silberschatz |first2=James L. |last2=Peterson |date=June 1988 |publisher=Addison-Wesley |isbn=0-201-18760-4}}</ref><ref>{{cite book |url=https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/L22-6636-1.pdf |title=IBM 7090 and 7094 Data Processing Systems Additional Core Storage - RPO E02120 (7090) Dr RPO E15724 (7094) |series=Special Systems Features Bulletin |publisher=IBM |id=L22-6636-1}}</ref> One bank was reserved for the time-sharing supervisory program, the other for user programs. CTSS had a protected-mode kernel; the supervisor's functions in the A-core (memory bank A) could be called only by software interrupts, as in modern operating systems. Causing memory-protection interrupts were used for software interrupts.<ref name="ctsspg63" /> [[Central processing unit|Processor]] allocation [[Scheduling (computing)|scheduling]] with a quantum time unit 200 ms, was controlled by a [[multilevel feedback queue]].<ref name="osc"/> It also had some special memory-management hardware,<ref name="multiprogramming-package">{{cite book |url=https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/L22-6641-3.pdf |title=IBM 7090-7094 Multiprogramming Package RPO E07291 (7090) or RPO 880287 (7094) |series=Special Systems Features Bulletin |publisher=IBM |id=L22-6641-3}}</ref> a clock interrupt,<ref>{{cite book |url=https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/L22-6554-1.pdf |title=Core Storage Clock and Interval Timer - RPO FB9349 |series=Special Systems Features Bulletin |publisher=IBM |id=L22-6554-1 |access-date=2024-04-21 |archive-date=2023-04-08 |archive-url=https://web.archive.org/web/20230408045855/https://people.csail.mit.edu/saltzer/Multics/CTSS-Documents/RPQs/L22-6554-1.pdf |url-status=live }}</ref> and the ability to trap certain instructions.<ref name="multiprogramming-package" />

===Supervisor subroutines===
* RDFLXA – Read an input line from console
* WRFLX – Write an output line to console
* DEAD – Put the user into dead status, with no program in memory
* DORMNT – Put the user into dormant status, with program in memory
* GETMEM – Get the size of the memory allocation
* SETMEM – Set the size of the memory allocation
* TSSFIL – Get access to the CTSS system files on the disk
* USRFIL – Change back to user's own directory
* GETBRK – Get the instruction location counter at quit

===Programming languages===
CTSS at first had only an assembler, [[FORTRAN Assembly Program|FAP]], and a compiler, MAD. Also, [[Fortran#FORTRAN II|Fortran II]] code could be translated into MAD code by using MADTRN. Later half of the system was written in MAD. Later there were other programming languages including [[COMIT|COMIT II]], [[Lisp (programming language)|LISP 1.5]] and a version of [[ALGOL]].

===File system===
Each user had their own [[Directory (computing)|directory]], and there were also shared directories for groups of people with the same "problem number". Each [[Computer file|file]] had two names, the second indicating its type as did the [[Filename extension|extension]] in later systems. At first, each file could have one of four modes: temporary, permanent, read-only class 1, and read-only class 2. Read-only class 1 allowed the user to change the mode of the file. Files could also be symbolically linked between directories. A directory listing by ''listf'':
 	10 FILES	20 TRACKS USED
 DATE		NAME		MODE	NO. TRACKS
 5/20/63		MAIN	MAD	P	15
 5/17/63 	DPFA	SYMTB	P	1
 5/17/63 	DPFA	BSS	P	1
 5/17/63 	DPFA	FAP	P	2

===Peripherals===
Input-output hardware was mostly standard IBM [[peripheral]]s. These included six data channels connecting to:
* Printers, [[punched card]] readers and punches
* [[IBM 729]] tape drives, an [[IBM 1301]] disk storage, later upgraded to an [[IBM 1302]], with 38 million word capacity
* An [[IBM 7320]] drum memory with 186K words that could load a 32K-word memory bank in one second (later upgraded to 0.25 seconds)
* Two custom high-speed vector graphics displays<ref>{{cite web |url=http://www.bitsavers.org/pdf/mit/lcs/tr/MIT-LCS-TR-056.pdf |title=An Integrated Hardware-Software System for Computer Graphics in Time Sharing |first1=D. E. |last1=Thornhill |first2=R. H. |last2=Stoz |first3=T. T. |last3=Ross |first4=J. E. |last4=Ward |date=1968 |publisher=MIT Project MAC |access-date=February 20, 2022 |id=ESL-R-356 and MAC-TR-56 |quote=Console facilities include hardware projection of three-dimensional line drawings, automatic light pen tracking, and a flexible set of knob, switch, and push-button inputs. The console is attached to the Project MAC IBM 7094 Compatible Time-Sharing System either directly or through a PDP-7 Computer.}}</ref>
* An [[IBM 7750]] transmission control unit capable of supporting up to 112 [[teleprinter]] terminals, including [[IBM 1050]] [[Selectric]]s and [[Teletype Corporation#Teletype Corporation|Model 35]]s. Some of the terminals were located remotely, and the system could be accessed using the public [[Telex|Telex and TWX]] networks.