FPU instruction error?

Discussion in 'Computer Information' started by Robert Baer, Sep 7, 2004.

  1. Robert Baer

    Robert Baer Guest

    It looks like i have found a serious error in the documentation if the
    FSUBP ST(i),ST instruction.
    Both the Intel i486 Microprocessor Programmers Reference Manual (1990
    McGraw-Hill), and the 80486 Programming by Brumm, Brumm & Scanlon (1991
    McGraw-Hill) have this error (i had understood the authors of the second
    book were intimately involved with the 486 at Intel).
    The documentation sez "Replaces ST(i) with ST-ST(i); pop ST".
    However, i get the additive inverse of that, or ST(i)-ST--->ST(i); pop
    ST.
     
    Robert Baer, Sep 7, 2004
    #1
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  2. Robert Baer

    Wizard Guest

    So who cares that processor is at least 6 years obsolete.

    Robert Baer wrote:
    >
    > It looks like i have found a serious error in the documentation if the
    > FSUBP ST(i),ST instruction.
    > Both the Intel i486 Microprocessor Programmers Reference Manual (1990
    > McGraw-Hill), and the 80486 Programming by Brumm, Brumm & Scanlon (1991
    > McGraw-Hill) have this error (i had understood the authors of the second
    > book were intimately involved with the 486 at Intel).
    > The documentation sez "Replaces ST(i) with ST-ST(i); pop ST".
    > However, i get the additive inverse of that, or ST(i)-ST--->ST(i); pop
    > ST.
     
    Wizard, Sep 7, 2004
    #2
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  3. Robert Baer

    Robert Baer Guest

    Wizard wrote:
    >
    > So who cares that processor is at least 6 years obsolete.
    >
    > Robert Baer wrote:
    > >
    > > It looks like i have found a serious error in the documentation if the
    > > FSUBP ST(i),ST instruction.
    > > Both the Intel i486 Microprocessor Programmers Reference Manual (1990
    > > McGraw-Hill), and the 80486 Programming by Brumm, Brumm & Scanlon (1991
    > > McGraw-Hill) have this error (i had understood the authors of the second
    > > book were intimately involved with the 486 at Intel).
    > > The documentation sez "Replaces ST(i) with ST-ST(i); pop ST".
    > > However, i get the additive inverse of that, or ST(i)-ST--->ST(i); pop
    > > ST.


    Once an instruction is carved in stone (embedded in silicon, in this
    case), it tends to remain unaltered.
    If you had looked, the same FPU instructions that existed way back in
    the 8088 daze (when the FPU was a seperate chip), are still with us;
    instructions added for the 286 then for the 386 and the 486, and MMX
    instructions are also still with us.
    So.
    What are the FSUB ST(i),ST and FSUBP ST(i),ST instructions supposed to
    do in the FPU stack on the Pentium?
     
    Robert Baer, Sep 8, 2004
    #3
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