Editing UNIVAC I (section)

==Technical description==
{{more citations needed section|date=March 2015}}

===Major physical features===
[[File:7AK7 vacuum tubes.jpg|thumb|[[7AK7]] vacuum tubes in a 1956 UNIVAC I computer]]
UNIVAC I used 6,103 [[vacuum tube]]s,<ref>The vacuum tubes used in the UNIVAC I were mostly of type [[25L6]] (3,947), but the machine also used tubes of type 6AK5 (412), [[7AK7]] (363), 6AU6 (57), 6BE6 (?), 6SN7 (264), 6X5 (?), 28D7 (274), 807 (27), 829B (47), 2050 (151), 5545 (?), 5651 (?), 5687 (14), 5915 (96), 6AL5 (199), 6AN5 (136), 6AH6 (?), 5V4 (?), 5R4 (?), 4D32 (?), 3C23 (64), and 8008 (?).</ref><ref>{{Cite book |url=https://archive.org/details/sim_electronic-industries-tele-tech_1954-03_13_3/page/n4/ |title=Tele-tech & Electronic Industries |date=1954-03-01 |publisher=Chilton Co.  |volume=13 |pages=3 |language=English |chapter=Totals: Germanium Diode Sales; Supply of Engineering Graduates - Computer Components |issue=3}}</ref> weighed {{Convert|16686|lb|ST MT|1}}, consumed 125 [[kilowatt|kW]],<ref>{{Cite web|url=http://www.ed-thelen.org/comp-hist/BRL61-u3.html#UNIVAC-I|title=UNIVAC I|last=Weik|first=Martin H.|date=March 1961|website=ed-thelen.org|series=A Third Survey of Domestic Electronic Digital Computing Systems}}</ref> and could perform about 1,905 operations per second running on a 2.25 [[megahertz|MHz]] clock. The Central Complex alone (i.e. the processor and memory unit) was 4.3 m by 2.4 m by 2.6 m high. The complete system occupied more than 35.5 m<sup>2</sup> (382&nbsp;ft<sup>2</sup>) of floor space.{{citation needed|date=July 2022}}

===Main memory details===
[[Image:Mercury memory.jpg|300px|right|thumb|Mercury delay-line memory of UNIVAC&nbsp;I]]
The main memory consisted of 1000&nbsp;[[Word (data type)|words]] of 12&nbsp;characters each. When representing numbers, they were written as 11 [[decimal]] digits plus [[signed number representations|sign]]. The 1000&nbsp;words of memory consisted of 100&nbsp;channels of 10-word [[delay-line memory|mercury delay-line]] [[register (computing)|registers]]. The [[input/output]] buffers were 60&nbsp;words each, consisting of 12&nbsp;channels of 10-word mercury delay-line registers. There are six channels of 10-word mercury delay-line registers as spares. With modified circuitry, seven more channels control the temperature of the seven mercury tanks, and one more channel is used for the 10-word "Y" register. The total of 126 mercury channels is contained in the seven mercury tanks mounted on the backs of sections MT, MV, MX, NT, NV, NX, and GV. Each mercury tank is divided into 18 mercury channels.{{citation needed|date=July 2022}}

Each 10-word mercury delay-line channel is made up of three sections:
# A channel in a column of mercury, with receiving and transmitting [[quartz]] piezo-electric [[crystal]]s mounted at opposite ends.
# An intermediate frequency chassis, connected to the receiving crystal, containing amplifiers, detector, and compensating delay, mounted on the shell of the mercury tank.
# A recirculation chassis, containing cathode follower, pulse former and retimer, modulator, which drives the transmitting crystal, and input, clear, and memory-switch gates, mounted in the sections adjacent to the mercury tanks.{{citation needed|date=July 2022}}

[[File:UNIVAC 1 Recirculation Chassis Board (1951).jpg|thumb|UNIVAC 1 recirculation chassis board]]

===Instructions and data===
[[Instruction set|Instructions]] were six [[alphanumeric]] characters, packed two instructions per word. The addition time was 525 [[microseconds]] and the multiplication time was 2150 microseconds. A non-standard modification called "Overdrive" did exist, that allowed for three four-character instructions per word under some circumstances. (Ingerman's simulator for the UNIVAC, referenced below, also makes this modification available.){{citation needed|date=July 2022}}

[[File:UNIVAC I Interior.jpg|thumb|Internal view of UNIVAC I]]
Digits were represented internally using [[excess-3]] ("XS3") [[binary-coded decimal]] (BCD) arithmetic with six bits per digit using the same value as the digits of the alphanumeric character set (and one [[parity bit]] per digit for [[redundancy check|error checking]]), allowing 11-digit [[signed number representations#Sign–magnitude|signed magnitude]] numbers. But with the exception of one or two machine instructions, UNIVAC was considered by programmers to be a decimal machine, not a binary machine, and the binary representation of the characters was irrelevant. If a non-digit character was encountered in a position during an arithmetic operation the machine passed it unchanged to the output, and any carry into the non-digit was lost. (Note, however, that a peculiarity of UNIVAC I's addition/subtraction circuitry was that the "ignore", space, and minus characters were occasionally treated as numeric, with values of –3, –2, and –1, respectively, and the apostrophe, ampersand, and left parenthesis were occasionally treated as numeric, with values 10, 11, and 12.){{citation needed|date=July 2022}}

===Input/output===
Besides the operator's console, the only [[input/output|I/O]] devices connected to the UNIVAC I were up to 10 [[UNISERVO]] tape drives, a [[Remington Standard]] [[electric typewriter]] <!-- Maint. Man. page 1-29. I maintained Univac I's for 13 years. --> and a [[Tektronix]] [[oscilloscope]]. The UNISERVO was the first commercial computer tape drive commercially sold. It used data density 128 bits per inch (with real transfer rate 7,200 characters per second) on magnetically plated phosphor bronze tapes. The UNISERVO could also read and write UNITYPER created tapes at 20 bits per inch. The [[UNITYPER]] was an offline typewriter to tape device, used by programmers and for minor data editing. Backward and forward tape read and write operations were possible on the UNIVAC and were fully overlapped with instruction execution, permitting high system throughput in typical sort/merge data processing applications. Large volumes of data could be submitted as input via magnetic tapes created on offline card to tape system and made as output via a separate offline tape to printer system. The operators console had three columns of decimal coded switches that allowed any of the 1000 memory locations to be displayed on the oscilloscope. Since the mercury delay-line memory stored bits in a serial format, a programmer or operator could monitor any memory location continuously and with sufficient patience, decode its contents as displayed on the scope. The on-line typewriter was typically used for announcing program breakpoints, checkpoints, and for memory dumps.{{citation needed|date=July 2022}}

===Operations===
A typical UNIVAC I installation had several ancillary devices. There were:
* The UNIPRINTER read metal UNIVAC magnetic tape using a tape reader and typed the data at 10 characters per second using a modified Remington typewriter.
* The UNIVAC Card to Tape converter read punched cards at 240 cards per minute and wrote their data on metal UNIVAC magnetic tape using a UNISERVO tape drive.
* A tape-to-card converter, that read a magnetic tape and produced punched cards.
UNIVAC did not provide an operating system. Operators loaded on a UNISERVO a program tape which could be loaded automatically by processor logic. The appropriate source and output data tapes would be mounted and the program started. Results tapes then went to the offline printer or typically for data processing into short-term storage to be updated with the next set of data produced on the offline card to tape unit. The mercury delay-line memory tank temperature was very closely controlled as the speed of sound in mercury varies with temperature. In the event of a power failure, many hours could elapse before the temperature stabilized.{{citation needed|date=July 2022}}

===Reliability===
Eckert and Mauchly were uncertain about the reliability of digital logic circuits—little was known about them at the time.  The UNIVAC had been designed with parallel computation circuits and a [[statistics|statistical]] comparison of the results.  In practice, however, only failing ''components,'' i.e., the vacuum tubes, yielded comparison faults, as the circuit designs as such proved very reliable. A regimen was established to ensure the reliability of the fragile vacuum tubes, the [[choke point]] of the entire operation. Prior to use large lots of the predominant tube type [[25L6]] were burned in and thoroughly tested. (Often half of any given production lot would be thrown away.) Technicians would then install a tested and burned-in tube in an easily diagnosed location such as the memory recirculate amplifiers. Then, when further proven aged and proven reliable, this "golden" tube was sent to stock to be pulled out for difficult-to-diagnose logic positions.

Furthermore, it took approximately 30 minutes to turn on the computer—all [[cathode heater]] power was stepped up gradually in order to reduce the in-rush current the concominant thermal stress on the tubes. As a result of these measures, uptimes ([[Mean time between failures|MTBF]]) of many days to weeks were eventually obtained on the processor. (The UNISERVO did not have vacuum columns but rather springs and strings to buffer the tape from the reels to the [[Capstan (tape recorder)|capstan]]. These mechanical components then became the most frequent source of failures.){{citation needed|date=July 2022}}