D17133CS datasheet(108/289 Pages) NEC

108 / 289 page

CHAPTER 11

ARITHMETIC AND LOGIC UNIT

11.2.6 Performing Operations in the ALU Block

When arithmetic operations, logical operations, bit evaluations, comparison evaluations or rotations in a program

are executed, the first data operand is stored in temporary register A and the second data operand is stored in

temporary register B.

The first data operand is four bits of data used to specify the contents of an address in the general register or

data memory. The second data operand is four bits of data used to either specify the contents of an address in data

memory or to be used as an immediate value. For example, in the instruction

ADD r, m

Second data operand

First data operand

the first data operand, r, is used to specify the contents of an address in the general register. The second data operand,

m, is used to specify the contents of an address in data memory. In the instruction

ADD m, #n4

the first data operand, m, is used to specify an address in data memory. The second operand, #n4, is immediate

data. In the rotation instruction

RORC r

only the first data operand, r (used to specify the contents of an address in the general register) is used.

Next, using the data stored in temporary registers A and B, the ALU executes the operation specified by the

instruction (arithmetic operation, logical operation, bit evaluation, comparison evaluation, or rotation). When the

instruction being executed is an arithmetic operation, logical operation, or rotation, the data processed by the ALU

is stored in the location specified by the first data operand (general register address or data memory address) and

the operation terminates. When the instruction being executed is a bit evaluation or comparison evaluation, the result

processed by the ALU is used to determine whether or not to skip the next instruction (whether to treat next

instruction as a no operation instruction: NOP) and the operation terminates.

D17133CS Datasheet(PDF) 108 Page - NEC