Editing UNIVAC 1101 (section)

==Description==
[[Image:UNIVAC_1101_"Atlas"_cryptanalytic_installation.jpg|thumb|right|ATLAS]]
This computer was {{convert|38|ft|m|abbr=on}} long, {{convert|20|ft|m|abbr=on}} wide, weighed about {{Convert|8.4|ST|MT}}<ref name=":0"/><ref>{{Cite web|url=http://ed-thelen.org/comp-hist/BRL-t-z.html#UNIVAC-SCI%20%28ERA-1101%29|title=UNIVAC-SCI (ERA-1101)|last=Weik|first=Martin H.|date=December 1955|website=ed-thelen.org|series=A Survey of Domestic Electronic Digital Computing Systems}}</ref><ref>(16,000 lbs + 17,400 lbs ) / 2 =  '''{{Convert|{{#expr:(16000+17400)/2}}|lb|ST}}'''<br />
{{Convert|16,000|lb|ST MT}}<br />
{{Convert|17,400|lb|ST MT}}</ref> and used 2700 [[vacuum tube]]s for its logic circuits. Its drum memory was {{convert|8.5|in|cm|abbr=on}} in diameter, rotated at 3500 rpm, had 200 read-write heads, and held 16,384 [[24-bit]] words (a memory size equivalent to 48 [[kilobyte|kB]]) with access time between 32 microseconds and 17 milliseconds.

[[Instruction set|Instructions]] were 24 bits long, with six bits for the [[opcode]], four bits for the "skip" value (telling how many memory locations to skip to get to the next instruction in program sequence), and 14 bits for the memory address. Numbers were binary with negative values in [[ones' complement]]. The addition time was 96 microseconds and the multiplication time was 352 microseconds.

The single 48-bit [[Accumulator (computing)|accumulator]] was fundamentally subtractive, addition being carried out by subtracting the ones' complement of the number to be added. This may appear rather strange, but the ''[[subtractive adder]]'' reduces the chance of getting negative zero in normal operations.

The machine had 38 instructions.

===Instruction set===
<ref>{{Cite journal|date=1952|title=Automatic Computing Machinery: Discussions - Minimum Access Programming|journal=Mathematics of Computation|language=en-US|volume=6|issue=39|pages=172–182|doi=10.1090/S0025-5718-52-99392-7|issn=0025-5718|doi-access=free}}</ref>

;Conventions
* y is memory box at address y
* X = X-Register (24 bits)

* ( ) is interpreted as the contents of
* Q = Q-Register (24 bits)

* A = Accumulator (48 bits)

;Arithmetic

* Insert (y) in A
* Insert complement of (y) in A

* Insert (y) in A [multiple precision]
* Insert complement of (y) in A [multiple precision]

* Insert absolute value (y) in A
* Insert complement of absolute value (y) in A

* Add (y) to (A)
* Subtract (y) from (A)

* Add (y) to (A) [multiple precision]
* Subtract (y) from (A) [multiple precision]

* Add absolute value of (y) to (A)
* Subtract absolute value of (y) from (A)

* Insert (Q) in A
* Clear right half of A

* Add (Q) to (A)
* Transmit (A) to Q

* Insert [(y) + 1] in A

;Multiply and divide

* Form product (Q) * (y) in A 
* Add logical product (Q) * (y) to (A)

* Form logical product (Q) * (y) in A 
* Divide (A) by (y), (quotient forms in Q, non-negative remainder left in A)

* Add product (Q) * (y) to (A) 

;Logical and control flow

* Store right half of (A) at y
* Shift (A) left

* Store (Q) at y
* Shift (Q) left

* Replace (y) with (A) using (Q) as operator
* Take (y) as next order

* Replace (y) with (A) [address portion only]
* Take (y) as next order if (A) is not zero

* Insert (y) in Q
* Take (y) as next order if (A) is negative

* Take (y) as next order if (Q) is negative

;Input Output and control

* Print right-hand 6 digits of (y)
* Optional Stop

* Print and punch right-hand 6 digits of (y)
* Intermediate Stop

* Final Stop