PRB: 8087 mnemonics without Operators

• Microsoft Macro Assembler for MS-DOS, versions 1.25, 5.0, 5.1, and 6.0
• Microsoft Macro Assembler for OS/2, versions 5.1 and 6.0

SYMPTOMS

When assembled with no operands, the 8087 instructions FADD, FDIV, FDIVR, FMUL, FSUB, and FSUBR assemble as if they were the instructions "FADDP ST(1),ST", "FDIVP ST(1),ST", and so forth.

CAUSE

The assembler is correct. When used with no operands, these instructions perform "classical stack" operations.

MORE INFORMATION

The following is from Pages 6-23 of the "Intel iAPX 86/88, 186/188 User's Manual Programmer's Reference":

The classical stack form may be used to make the 8087 operate like a classical stack machine. No operands are coded in this form, only the instruction mnemonic. The NDP (8087) picks the source operand from the stack top and the destination from the next stack element. It then pops the stack, performs the operation, and returns the result to the new stack top, effectively replacing the operands by the result.

Note: FADD, FSUB, FMUL, and FDIV also pop if no operands are specified, in spite of the fact that the mnemonics do not have P as a last character.

A similar passage can be found on Pages 2-5 of the Numeric Supplement to the "iAPX 286 Programmer's Reference" manual.