Processor Description Languages

202 CHAPTER 8 TIE

operation ADD {out AR res, in AR in0, in AR in1} {

assign res = in0 + in1;

}

operation SUB {out AR res, in AR in0, in AR in1} {

assign res = in0 – in1;

}

semantic addsub {ADD, SUB} {

wire carry = SUB;

wire [31:0] tmp = ADD ? in1 : ∼ in1;

assign res = TIEadd(in0, tmp, carry);

}

The semantic addsub implements the instructions ADD and SUB. It uses the simple

principle of 2’s complement arithmetic that (

a ⫺ b) is equal to (a ⫹ ∼b ⫹ 1). For

every instr uction implemented in aTIE semantic,a 1-hot signal with the same name

as the instruction is available in the semantic. This signal is true when the corre-

sponding instruction is being executed, ...