User's Manual

OVERVIEW OF NUMERIC PROCESSING
80287 STACK:
TAG FIELD
79
78
64
63
0
1
0
Rl
SIGN
EXPONENT SIGNIFICAND
R2
R3
R4
r--
-
R5
R6
R7
R8
15 0
CONTROL REGISTER
STATUS REGISTER
TAG WORD
t- INSTRUCTION POINTER
DATA POINTER
G3010B
Figure 1-3_ 80287 Register Set
Many numeric instructions have several addressing modes that permit the programmer to implicitly
operate
on the top of the stack, or to explicitly operate on specific registers relative to the ST. The
ASM286 Assembler supports these register addressing modes, using the expression ST(O), or simply
ST, to represent the current Stack Top and STeil to specify the ith register from ST in the stack (0 ::s
i ::s 7). For example, if ST contains OllB (register 3 is the top of the stack), the following statement
would add the contents of the top two registers on the stack (registers 3 and 5):
FADD
ST,ST(2)
The stack organization and top-relative addressing of the numeric registers simplify subroutine
programming by allowing routines
to pass parameters on the register stack. By using the stack to pass
parameters rather than using
"dedicated" registers, calling routines gain more flexibility in how they
use the stack.
As long as the stack is not full, each routine simply loads the parameters onto the stack
before calling a particular subroutine
to perform a numeric calculation. The subroutine then addresses
its parameters
as ST, ST(l), etc., even though ST may, for example, refer to physical register 3 in one
invocation and physical register 5
in another.
The NPX Status Word
The l6-bit status word shown in figure 1-4 reflects the overall state of the 80287. This status word may
be stored into memory using the FSTSW/FNSTSW,'FSTENV/FNSTENV, and FSAVEjFNSAVE
instructions, and can be transferred into the 80286 AX register with the FSTSW AX/FNSTSW AX
instructions, allowing the
NPX status to be inspected by the CPU.
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