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INSTRUCTION SET REFERENCE
FADD/FADDP/FIADD—Add
Description
This instruction adds the destination and source operands and stores the sum in the destination
location. The destination operand is always an FPU 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 adds the contents of the ST(0) register to the ST(1)
register. The one-operand version adds the contents of a memory location (either a real or an
integer value) to the contents of the ST(0) register. The two-operand version, adds the contents
of the ST(0) register to the ST(i) register or vice versa. The value in ST(0) can be doubled by
coding:
FADD ST(0), ST(0);
The FADDP instructions perform the additional operation of popping the FPU register stack
after storing the result. 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 add
instructions always results in the register stack being popped. In some assemblers, the
mnemonic for this instruction is FADD rather than FADDP.)
The FIADD instructions convert an integer source operand to extended-real format before
performing the addition.
The table on the following page shows the results obtained when adding various classes of
numbers, assuming that neither overflow nor underflow occurs.
When the sum of two operands with opposite signs is 0, the result is +0, except for the round
toward
??
mode, in which case the result is
?
0. When the source operand is an integer 0, it is
treated as a +0.
When both operand are infinities of the same sign, the result is
?
of the expected sign. If both
operands are infinities of opposite signs, an invalid operation exception is generated.
Opcode
Instruction
Description
D8 /0
FADD m32 real
Add m32real to ST(0) and store result in ST(0)
DC /0
FADD m64real
Add m64real to ST(0) and store result in ST(0)
D8 C0+i
FADD ST(0), ST(i)Add ST(0) to ST(i) and store result in ST(0)
DC C0+i
FADD ST(i), ST(0)Add ST(i) to ST(0) and store result in ST(i)
DE C0+i
FADDP ST(i), ST(0)Add ST(0) to ST(i), store result in ST(i), and pop the
register stack
DE C1
FADDP
Add ST(0) to ST(1), store result in ST(1), and pop the
register stack
DA /0
FIADD m32int
Add m32int to ST(0) and store result in ST(0)
DE /0
FIADD m16int
Add m16int to ST(0) and store result in ST(0)
