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IBM 709
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==Registers== {| class="infobox" style="font-size:88%;width:34em;" |+ IBM 709 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" | Accumlator |- style="background:silver;color:black" | style="text-align:center;background:white;" colspan="2" | | 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" | | 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" | | 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" | | 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" | | 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"| | style="text-align:center;" colspan="5"| PC | style="background:white; color:black;"| Program Counter |} |} The IBM 709 has a 38-bit accumulator, a 36-bit multiplier/quotient register, and three 15-bit [[index register]]s whose contents are subtracted from the base address instead of being added to it. All three index registers can participate in an instruction: the three-bit ''tag'' field in the instruction is a bit map specifying which of the registers participate in the operation, however if more than one index register is specified, their contents are combined by a [[logical or]] operation, not addition.<ref name=709refman>IBM 709 Reference Manual, Form A22-6501-0, 1958</ref><sup>p. 12</sup>
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