The following instructions perform arithmetic operations between rA and rX register and memory contents.
Add and set OV if overflow. OPCODE = 1, MOD = fspec.
rA <- rA +V.
Sub and set OV if overflow. OPCODE = 2, MOD = fspec.
rA <- rA - V.
Multiply V times rA and store the 10-bytes product in rAX.
OPCODE = 3, MOD = fspec.
rAX <- rA x V.
rAX is considered a 10-bytes number, and it is divided by V.
OPCODE = 4, MOD = fspec.
rA <- rAX / V,
rX <- reminder.
In all the above instructions, ‘[rA]’ is one of the operands of the binary arithmetic operation, the other being ‘V’ (that is, the specified subfield of the memory cell with address ‘M’), padded with zero bytes on its left-side to complete a word. In multiplication and division, the register ‘X’ comes into play as a right-extension of the register ‘A’, so that we are able to handle 10-byte numbers whose more significant bytes are those of ‘rA’ (the sign of this 10-byte number is that of ‘rA’: ‘rX’’s sign is ignored).
Addition and subtraction of MIX words can give rise to overflows, since the result is stored in a register with room to only 5 bytes (plus sign). When this occurs, the operation result modulo 1,073,741,823 (the maximum value storable in a MIX word) is stored in ‘rA’, and the overflow toggle is set to TRUE.