Editing IBM 704 (section)

==Registers==
{| class="infobox" style="font-size:88%;width:34em;"
|+ IBM 704 registers
|-
|
{| style="font-size:88%;width:100%;"
|-
| style="width:10px; text-align:left"  | <sup>3</sup><sub>7</sub>
| style="width:10px; text-align:left"  | <sup>3</sup><sub>6</sub>
| style="width:10px; text-align:left"  | <sup>3</sup><sub>5</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:left"  | <sup>3</sup><sub>1</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:right" | <sup>2</sup><sub>3</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>1</sup><sub>5</sub>
| style="width:10px; text-align:center"| <sup>1</sup><sub>4</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>0</sup><sub>7</sub>
| style="width:60px; text-align:center"| ...
| style="width:10px; text-align:center"| <sup>0</sup><sub>0</sub>
| style="width:auto; background:white; color:black" | ''(bit position)''
|-
|colspan="15" | '''Data registers'''
|- style="background:silver;color:black"
| style="text-align:center;" colspan="14"| AC
| style="background:white; color:black" | Accumulator
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="2" | &nbsp;
| style="text-align:center;" colspan="12"| MQ
| style="background:white; color:black" | Multiplier/Quotient
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="2" | &nbsp;
| style="text-align:center;" colspan="12"| SI
| style="background:white; color:black" | Sense Indicator
|-
|colspan="15" | '''Index registers'''
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="9" | &nbsp;
| style="text-align:center;" colspan="5"| XR1
| style="background:white; color:black" | Index 1
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="9" | &nbsp;
| style="text-align:center;" colspan="5"| XR2
| style="background:white; color:black" | Index 2
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="9" | &nbsp;
| style="text-align:center;" colspan="5"| XR3
| style="background:white; color:black" | Index 3
|-
|colspan="15" | '''Program counter'''
|- style="background:silver;color:black"
| style="text-align:center;background:white;" colspan="9"| &nbsp;
| style="text-align:center;" colspan="5"| PC
| style="background:white; color:black;"| Program&nbsp;Counter
|}
|}

[[File: Circuito di calcolo per il sistema IBM 704 - Museo scienza tecnologia Milano D1207.jpg|thumb|IBM 704 vacuum-tube circuit module]]
The IBM 704 had a 38-bit [[Accumulator (computing)|accumulator]], a 36-bit multiplier/quotient register, and three 15-bit [[index register]]s. The contents of the index registers are subtracted from the base address, so the index registers are also called "decrement registers". All three index registers can participate in an instruction: the 3-bit ''tag'' field in the instruction is a bit map specifying which of the registers participate in the operation. However, when more than one index register is selected, then their contents are bit-wise [[OR gate|ORed]] &ndash; not added &ndash; together before the decrement takes place. This behavior persisted in later scientific-architecture machines (such as the [[IBM&nbsp;709]] and [[IBM&nbsp;7090]]) until the [[IBM&nbsp;7094]]. The IBM&nbsp;7094, introduced in 1962, increased the number of index registers to seven and only selected one at a time; the "or" behavior remained available in a compatibility mode of the IBM&nbsp;7094.<ref>{{Citation |url=http://bitsavers.trailing-edge.com/pdf/ibm/7094/A22-6703-4_7094_PoO_Oct66.pdf |title=IBM 7094 Principles of Operation |edition=fifth |year=1962 |page=8 |id=A22-6703-4 |publisher=IBM |series=IBM Systems Reference Library }}.</ref>