Cirrus Logic CS485 Manual de usuario descargar pdf (Pagina 17)

CS485G Spring 2015 17

11. Result of compilation

pushl %ebp # save base pointer

movl %esp,%ebp # new base pointer

movl 12(%ebp),%eax # $a = y

xorl 8(%ebp),%eax # $a = y ˆ x [t1]

sarl $17,%eax # $a = (y ˆ x) >> 17 [t2]

andl $8185,%eax # $a = t2 & mask [rval]

popl %ebp # retore base pointer

ret # return

20 Control based on condition codes

1. The four condition codes, each Boolean

(a) CF: Carry ﬂag

(b) ZF: Zero ﬂag

(d) OF: Overﬂow ﬂag (for signed integers)

2. Arithmetic operations implicitly set these ﬂags, but the lea instruc-

tion does not set them.

3. Example: addition t = a + b

(a) sets CF if there is a carry from most signiﬁcant bit

(b) sets ZF if the t is zero

(d) sets OF if there is a two’s complement overﬂow:

(a > 0 ∧ b > 0 ∧ t < 0) ∨ (a < 0 ∧ b < 0 ∧ t > 0)

4. The compare instruction (cmpl b, a) also sets the ﬂags; it’s like

computing a − b without modifying the destination.

(a) sets CF if there is a carry from most signiﬁcant bit

(b) sets ZF if a = b

(d) sets OF if there is a two’s complement overﬂow:

(a > 0 ∧ b < 0 ∧ (a − b) < 0) ∨ (a < 0 ∧ b > 0 ∧ (a − b) > 0)

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Cirrus Logic CS485 Manual de usuario Pagina 17