The P101 Registers
As already seen, the registers of the P101 are the following: A, R, M, B, C, D, E, F.
B, C, D, E, F are called "storage registers" and are used to store numbers. Each of them can contain a signed 22-digits number or can be split in two parts each containing a signed 11-digits number. The "splitting" operator is the slash "/", and for example one might refer to B as a whole registers or to B/ as the first half of the same register. Confusingly enough, the other half of the split B register is called B, just like the whole register.
In print-outs, split registers are printed in lower case (e.g. the two parts of B are b and B), but when keying programs the B/ form must be used.
A, R and M are called the "operating registers" and have special functions.
A is the accumulator: the first operand of math operations must be placed in it and the result of operations is found in it.
M (I don't know what M stands for) holds the second operator for math operation and also holds data entered from the keyboard.
R has different functions for each operation. For add, subtract and multipliy it holds a copy of the result of the operation prior to rounding off. After a division, instead, it holds the remainder of the division.
Transfer between registers is limited, not all registers can be transferred into any register: for futher details, see the section about transfer functions.