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3-398 Vol. 2A FSUB/FSUBP/FISUB—Subtract
INSTRUCTION SET REFERENCE, A-M
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-precision or double-precision floating-point format or in
word or doubleword integer format.
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 floating-point 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.
Opcode Instruction 64-Bit
Mode
Compat/
Leg Mode
Description
D8 /4 FSUB m32fp Valid Valid Subtract m32fp from ST(0)
and store result in ST(0).
DC /4 FSUB m64fp Valid Valid Subtract m64fp from ST(0)
and store result in ST(0).
D8 E0+i FSUB ST(0), ST(i) Valid Valid Subtract ST(i) from ST(0) and
store result in ST(0).
DC E8+i FSUB ST(i), ST(0) Valid Valid Subtract ST(0) from ST(i) and
store result in ST(i).
DE E8+i FSUBP ST(i), ST(0) Valid Valid Subtract ST(0) from ST(i),
store result in ST(i), and pop
register stack.
DE E9 FSUBP Valid Valid Subtract ST(0) from ST(1),
store result in ST(1), and pop
register stack.
DA /4 FISUB m32int Valid Valid Subtract m32int from ST(0)
and store result in ST(0).
DE /4 FISUB m16int Valid Valid Subtract m16int from ST(0)
and store result in ST(0).