Intel 80287, 80286 manual FAD D destination, source, Data Transfer Instructions

PROGRAMMING NUMERIC APPLICATIONS

Whether supplied by the programmer or utilized automatically, the two basic types of operands are sources and destinations. A source operand simply supplies one of the inputs to an instruction; it is not altered by the instruction. Even when an instruction converts the source operand from one format to another (e.g., real to integer), the conversion is actually performed in an internal work area to avoid altering the source operand. A destination operand may also provide an input to an instruction. It is distinguished from a source operand, however, because its content may be altered when it receives the result produced by the operation; that is, the destination is replaced by the result.

Many instructions allow their operands to be coded in more than one way. For example, FADD (add real) may be written without operands, with only a source or with a destination and a source. The instruction descriptions in this section employ the simple convention of separating alternative operand forms with slashes; the slashes, however, are not coded. Consecutive slashes indicate an option of no explicit operands. The operands for FADD are thus described as

//source/destination, source

This means that FADD may be written in any of three ways:

FADD

FAD D source

FAD D destination, source

When reading this section, it is important to bear in mind that memory operands may be coded with any of the CPU's memory addressing modes. To review these modes-direct, register indirect, based, indexed, based indexed-refer to the 80286 Programmer's Reference Manual. Table 2-17later in this chapter also provides several addressing mode examples.

Data Transfer Instructions

These instructions (summarized in table 2-1)move operands among elements of the register stack, and between the stack top and memory. Any of the seven data types can be converted to temporary real and loaded (pushed) onto the stack in a single operation; they can be stored to memory in the same manner. The data transfer instructions automatically update the 80287 tag word to reflect the register contents following the instruction.

FLO source

FLD (load real) loads (pushes) the source operand onto the top of the register stack. This is done by decrementing the stack pointer by one and then copying the content of the source to the new stack top. The source may be a register on the stack (ST(i)) or any of the real data types in memory. Short and long real source operancis are conveneu lu lempunuy fed: dutvllla.tica.lly. CGdir:g FLD ST(O) d!!p!!c!!.te~ the stack top.

FST destination

FST (store real) transfers the stack top to the destination, which may be another register on the stack or a short or long real memory operand. If the destination is short or long real, the significand is rounded to the width of the destination according to the RC field of the control word, and the exponent is converted to the width and bias of the destination format.

2-2