4:182
Volume 4: Base IA-32 Instruction Reference
FSUB/FSUBP/FISUB—Subtract
Description
Subtracts the source operand from the destination operand and stores the difference in
the destination location. The destination operand is always an FPU data register; the
source operand can be a register or a memory location. Source operands in memory
can be in single-real, double-real, word-integer, or short-integer formats.
The no-operand version of the instruction subtracts the contents of the ST(0) register
from the ST(1) register and stores the result in ST(1). The one-operand version
subtracts the contents of a memory location (either a real or an integer value) from the
contents of the ST(0) register and stores the result in ST(0). The two-operand version,
subtracts the contents of the ST(0) register from the ST(
i
) register or vice versa.
The FSUBP instructions perform the additional operation of popping the FPU register
stack following the subtraction. To pop the register stack, the processor marks the
ST(0) register as empty and increments the stack pointer (TOP) by 1. The no-operand
version of the floating-point subtract instructions always results in the register stack
being popped. In some assemblers, the mnemonic for this instruction is FSUB rather
than FSUBP.
The FISUB instructions convert an integer source operand to extended-real format
before performing the subtraction.
The following table shows the results obtained when subtracting various classes of
numbers from one another, assuming that neither overflow nor underflow occurs. Here,
the SRC value is subtracted from the DEST value (DEST
SRC = result).
When the difference between two operands of like sign is 0, the result is +0, except for
the round toward
mode, in which case the result is
0. This instruction also
guarantees that +0
(
0) = +0, and that
0
(+0) =
0. When the source operand is
an integer 0, it is treated as a +0.
When one operand is
, the result is
of the expected sign. If both operands are
of
the same sign, an invalid-operation exception is generated.
Opcode
Instruction
Description
D8 /4
FSUB
m32real
Subtract
m32real
from ST(0) and store result in ST(0)
DC /4
FSUB
m64real
Subtract
m64real
from ST(0) and store result in ST(0)
D8 E0+i
FSUB ST(0), ST(
i
)
Subtract ST(
i
) from ST(0) and store result in ST(0)
DC E8+i
FSUB ST(
i
), ST(0)
Subtract ST(0) from ST(
i
) and store result in ST(
i
)
DE E8+i
FSUBP ST(
i
), ST(0)
Subtract ST(0) from ST(
i
), store result in ST(
i
), and pop register
stack
DE E9
FSUBP
Subtract ST(0) from ST(1), store result in ST(1), and pop
register stack
DA /4
FISUB
m32int
Subtract
m32int
from ST(0) and store result in ST(0)
DE /4
FISUB
m16int
Subtract
m16int
from ST(0) and store result in ST(0)
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Страница 1494: ...4 192 Volume 4 Base IA 32 Instruction Reference FWAIT Wait See entry for WAIT ...
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