Intel 80286, 80287 manual Processor Control Instructions

PROGRAMMING NUMERIC APPLICATIONS

FLDLG2

FLDLG2 (load log base 10 of 2) loads (pushes) the value LOG lO 2 onto the stack.

FLDLN2

FLDLN2 (load log base e of 2) loads (pushes) the value LOGe 2 onto the stack.

Processor Control Instructions

The processor control instructions shown in table 2-11 are not typically used in calculations; they provide control over the 80287 NPX for system-level activities. These activities include initialization, exception handling, and task switching.

As shown in table 2-11, many of the NPX processor control instructions have two forms of assembler mnemonic:

•A wait form, where the mnemonic is prefixed only with an F, such as FSTSW. This form checks for unmasked numeric errors.

•A no-waitform, where the mnemonic is prefixed with an FN, such as FNSTSW. This form ignores unmasked numeric errors.

When the control instruction is coded using the no-waitform of the mnemonic, the ASM286 assembler does not precede the ESC instruction with a wait instruction, and the CPU does not test the ERROR status line from the NPX before executing the processor control instruction.

Only the processor control class of instructions have this alternate no-wait form. All numeric instruc- tions are automatically synchronized by the 80286, with the CPU testing the BUSY status line and only executing the numeric instruction when this line is inactive. Because of this automatic synchroni- zation by the 80286, numeric instructions for the 80287 need not be preceded by a CPU wait instruc- tion in order to execute correctly.

Table 2-11. Processor Control Instructions

FINITjFNINIT	Initialize processor
FSETPM	Set Protected Mode
FLDCW	Load control word
FSTCWjFNSTCW	Store control word
FSTSWjFNSTSW	Store status word
FSTSW AXjFNSTSW AX	Store status word to AX
FCLEXjFNCLEX	Clear exceptions
FSTENVjFNSTENV	Store Environment
FLDENV	Load environment
FSAVEjFNSAVE	Save state
FRSTOR	Restore state
FINCSTP	Increment stack pointer
FDECSTP	Decrement stack pointer
FFREE	Free register
FNOP	No operation
FWAIT	CPU Wait

2-15