Page "Accumulator (computing)" Paragraph 7

This instruction would add the value read from the memory location at memaddress to the value from the accumulator, placing the result in the accumulator.

The accumulator is not identified in the instruction by a register number ; it is implicit in the instruction and no other register can be specified in the instruction.

Some architectures use a particular register as an accumulator in some instructions, but other instructions use register numbers for explicit operand specification.