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WINDOW2.DOC
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1987-05-04
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---WINDOW2.DOC---
The Extra Display Window
When you invoke D86, you get a sign-on message in the upper right
quadrant of the screen, with a version number, legal terms, and
my name, address, and phone number. The contents of this
quadrant may be toggled by two keys:
One key, Alt-F10, toggles you between help and non-help modes.
(On the Wang-PC the key is HELP; on the TI-PC, the key is F11;
on the Sanyo, the key is CTRL-PF5.)
Another key, F10, toggles you between the windows of whatever
mode (help or non-help) you are in. In other words, if you are
in help mode, and there is more than one window-full of help
available, F10 will toggle you through the help-windows. If you
are in non-help mode, F10 will toggle you through the non-help
windows.
Non-Help Windows Now Available
In this preliminary version of D86, there are only 3 non-help
windows available. One is the sign-on window. The second is a
blank window. The third, available only if you have a floating-
point chip (8087 or 287) in your computer, displays the floating-
point state.
The Floating-Point Display Window
NOTE: The following description does not attempt to teach you the
meanings of the various fields of the 87-- if you don't know them
all, you'll be confused by some of the descriptions. If you wish
to learn all the gory details, a good book is "The 80286
Architecture", by Morse and Albert, Chapter 4.
The floating-point display contains the following components:
1. The top 8 lines contain the contents of each floating-point
register. The registers are displayed as a stack, with ST(0)
on top, numbered with "0:". The possible displays for
register contents are:
---- denoting a stack slot tagged empty.
a decimal number, displayed in scientific notation if
necessary.
Infinity for an overflowed result.
NaN for Not a Number, followed by the hexadecimal codes of
the significand field of the NaN
Unn for unnormal, followed by the number of bits of precision
lost. I'll add a display of the value in a later D86.
Den for denormal, followed by the number of bits of precision
lost. I'll add a display of the value in a later D86.
Pseudo 0 for a pseudo zero, followed by the contents of the
exponent field.
2. English-language displays for the infinity, precision, and
rounding mode settings for the chip.
3. The value of the instruction pointer for the instruction that
caused the last floating-point exception. If all exceptions
are masked this doesn't change.
4. The address of the memory operand for the instruction that
caused the last floating-point exception. If all exceptions
are masked this doesn't change either.
5. The value of the Tag register, for registers 7 through 0.
These register numbers are NOT the same as the stack element
numbers-- they are rotated by the value of ST, given on the
next line. The possible values for the Tag register fields
are:
f for finite non-zero number
z for zero
i for infinite number
- for empty slot
6. The value of the stack pointer ST. The tag-register number
for ST(i) is ST plus i.
7. A display of which 87 exceptions are masked, and thus will not
cause interrupts. If the exception is masked, its letter is
displayed. If it is not masked (interrupt will occur), a
blank is displayed. The letters are:
p for precision exception
u for underflow exception
o for overflow exception
z for zero-divide exception
d for denormal exception
i for invalid operation exception
8. A display of which masked exceptions have occurred since the
last time the status bits were cleared. The exceptions have
the same letters as the masked-display.
9. A display of the 87 Condition Flags C3 through C0. A
displaying character is shown if the flag is set; a blank is
shown if the flag is cleared. The characters are:
z for C3 (corresponds to the Zero flag)
u for C2 (set if a result is "unordered")
. for C1
c for C0 (corresponds to the Carry flag)
10. Another display of C3 through C0, this time showing the
results of an FXAM instruction. This display is always
present, even when FXAM is not the last flag-setting
instruction executed (in which case the display is
meaningless).