Intel Itanium 9150M, Manual

Page: 140 / 420

Volume 3: Instruction Reference 3:131
frsqrta
frsqrta — Floating-point Reciprocal Square Root Approximation
Format: (
qp
) frsqrta.
sf f
1
,
p
2
=
f
3
F7
Description:
If PR
qp
is 0, PR
p
2
is cleared and FR
f
1
remains unchanged.
If PR
qp
is 1, the following will occur:
• FR
f
1
is either set to an approximation (with a relative error < 2
-8.831
) of the
reciprocal square root of FR
f
3
, or set to the IEEE-754 mandated square root of FR
f
3
— if FR
f
3
is in the set {-Infinity, -Finite, -0, Pseudo-zero, +0, +Infinity, NaN,
Unsupported}.
• If FR
f
1
is set to an approximation of the reciprocal square root of FR
f
3
, then PR
p
2
is
set to 1; otherwise, it is set to 0.
• If FR
f
3
is such the approximation of its reciprocal square root may cause the
Newton-Raphson iterations to fail to produce the correct IEEE-754 square root
result, then a Floating-point Exception fault for Software Assist occurs.
System software is expected to compute the IEEE-754 square root, return the
result in FR
f
1
, and set PR
p
2
to 0.
• If FR
f
3
is a NaTVal, FR
f
1
is set to NaTVal instead of the computed result, and PR
p
2
is cleared.
The mnemonic values for
sf
are given in Table 2-23 on page 3:56 .
Operation: if (PR[ qp ]) {
fp_check_target_register( f
1
);
if (tmp_isrcode = fp_reg_disabled( f
1
, f
3
, 0, 0))
disabled_fp_register_fault(tmp_isrcode, 0);
if (fp_is_natval(FR[ f
3
])) {
FR[ f
1
] = NATVAL;
PR[ p
2
] = 0;
} else {
tmp_default_result = frsqrta_exception_fault_check( f
3
, sf ,
&tmp_fp_env);
if (fp_raise_fault(tmp_fp_env))
fp_exception_fault(fp_decode_fault(tmp_fp_env));
if (fp_is_nan(tmp_default_result)) {
FR[ f
1
] = tmp_default_result;
PR[ p
2
] = 0;
} else {
tmp_fr3 = fp_normalize(fp_reg_read(FR[ f
3
]));
if (fp_is_zero(tmp_fr3)) {
FR[ f
1
] = tmp_fr3;
PR[ p
2
] = 0;
} else if (fp_is_pos_inf(tmp_fr3)) {
FR[ f
1
] = tmp_fr3;
PR[ p
2
] = 0;
} else {
FR[ f
1
] = fp_ieee_recip_sqrt(tmp_fr3);
PR[ p
2
] = 1;
}
}
fp_update_fpsr(
sf , tmp_fp_env);
}