Olivetti Programma 101 Reference Manual page 8

The following formats show the separation and capacities of the ten registers.
Format 1 - This represents the storage registers used as complete registers: M, A, R: always
complete; B, C, D, E, F: complete in this case.
Format 2 - This represents the storage registers when split: b, B; c, C; d, D; e, E;
and f, F.
Format 3 - This represents the split registers with numeric and instruction storage: e.g. d, D;
e, E: and f, F.
Format 4 - This represents the complete instruction registers. Registers 1 and 2; registers F,
E and D when used only for instructions.
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