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Some Atlas Word Formats
Atlas Instruction 09 F 1016 Ba 1723 Bm 2447 023 N Atlas Address 0 111 1220 21 Block number Word number in block 22-23 Character address Floating Point 0 17 EXPONENT 8 947 MANTISSA 6-bit Characters 05 0 611 1 1217 2 1823 3 2429 4 3035 5 3641 6 4247 7

Atlas had 127 index registers B1 to B127.
B127 contains the instruction address in normal execution
B126 the instruction address within an extracode