Stage3: Execution Stage

Figure 4.29 Execute stage

Arithmetic Logic Unit (ALU) performs the four operations.

ADD, SUB, ADN, XOR

Inputs

• ALU_fun:

OP (0,1,2,3) <- ALU_fun(icode, ifun)

• ALU_A:

op1 <- (valC, valB, +8, -8, Z)

• ALU_B:

op2 <- (valB, 0, Z)

Outputs

• valE: op1 OP op2

• ZF/SF/OF: set by OPq instructions

for t = a + b

CF	(unsigned) t < (unsigned) a	Unsigned overflow
ZF	(t == 0)	Zero
SF	(t < 0)	Negative
OF	(a < 0 == b < 0) && ( t < 0 != a < 0)	Signed overflow

Condition Code (CC) module

CC stores the last condition code (ZF/SF/OF)

SetCC module

Generate a signal to store CC. It is set if the icode is 6 (OPq).

Cond module

The signal of Cond is used by jXX in NewPC, cmovXX in Write Back stage.

Cnd <- Cond(CC, ifun)

instruction	Synonym	Jump condition	Description
jmp Label		1	Direct jump
je Label	jz	ZF	Equal / zero
jne Label	jnz	~ZF	Not equal / not zero
jg Label	jnle	~(SF ^ OF) & ~ZF	Greater (signed >)
jge Label	jnl	~(SF ^ OF)	Greater or euqal (signed >=)
jl Label	jnge	SF ^ OF	Less (signed <)
jle Label	jng	(SF ^ OF) | ZF	Less or equal (signed <=)

Control logic for Execution stage.

instruction	ALU_A (valA/valC/-8/8/X)	ALU_B (valB/0/X)	alufun (0/1/2/3)	set_cc (0/1)	valE (op1 OP op2)	CC (ZF/SF/OF)	Cnd (0/1)
halt
nop
rrmovq rA, rB
irmovq V, rB
rmmovq rA, D(rB)
mrmovq D(rB), rA
OPq rA, rB
jXX Dest
cmovXX rA, rB
call Dest
ret
pushq rA
popq rA