User's Manual

OVERVIEW OF NUMERIC PROCESSING
15
I B I Co I ST I c" I c, I eoJEsl X I PE IUEIOEIZEIDEIIE I
I
EXCE
PTION
FLAGS (1 ~ EXCEPTION HAS OCCURRED)
INVALID OPERATION'
DENORMALIZED OPERAND'
ZERO DIVIDE'
OVERFLOW'
UNDERFLOW'
PRECISION'
(RESE
RVED)
ERRO
R SUMMARY STATUS(1)
ITION
CODE(2)
COND
STAC
K TOP POINTER(3)
BUSY NEU
(1) ES IS SET IF ANY UNMASKED EXCEPTION BIT IS SET, CLEARED OTHERWISE.
(2) SEE TABLE 1-4 FOR CONDITION CODE INTERPRETATION.
(3) ST VALUES
000 ~ REGISTER 0 IS TOP OF STACK
00 1 ~ REGISTER 1 IS TOP OF STACK
111 ~ REGISTER 7 IS TOP OF STACK
'FOR DEFINITIONS,
SEE THE SECTION ON EXCEPTION HANDLING
Figure 1-4. 80287 Status Word
G30108
The Busy bit (bit 15) and the BUSY pin indicate whether the 80287's execution unit is idle (B=O) or
is executing a numeric instruction or signalling an exception (B= 1). (The instructions FNSTSW,
FNSTSW AX, FNSTENV, and FNSAVE do not set the Busy bit themselves, nor do they require the
Busy bit to be clear in order to execute.)
The four
NPX condition code bits (C
O
-C
3
)
are similar to the flags in a CPU: the 80287 updates these
bits to reflect the outcome of arithmetic operations. The effect of these instructions
on the condition
code bits
is summarized in table 1-4. These condition code bits are used principally for conditional
branching. The FSTW AX instruction stores the
NPX status word directly into the CPU AX register,
allowing these condition codes to be inspected efficiently by
80286 code.
Bits 12-14 of the status word point to the
80287 register that is the currentStack Top (ST). The
significance of the stack top has been described in the section
on the Register Stack.
Figure
1-4 shows the six error flags in bits 0-5 of the status word. Bit 7 is the error summary status
(ES) bit.
ES is set if any unmasked exception bits are set, and is cleared otherwise. If this bit is set,
the
ERROR signal is asserted. Bits 0-5 indicate whether the NPX has detected one of six possible
:::i:::::pti~!), c~!),d!t!a!!~ ~im:o:' tho:'5o:' 5t~hl~ hit~ were last cleared or reset.
Control Word
The NPX provides the programmer with several processing options, which are selected by loading a
word from memory into the control word. Figure
1-5 shows the format and encoding of the fields in
the control word.
1-10