80C187

The 80C187 register set can be accessed either as

a stack with instructions operating on the top one or

two stack elements or as individually addressable

registers The TOP field in the status word identifies

the current top-of-stack register A ‘‘push’’ operation

decrements TOP by one and loads a value into the

new top register A ‘‘pop’’ operation stores the value

from the current top register and then increments

TOP by one The 80C187 register stack grows

‘‘down’’ toward lower-addressed registers

Instructions may address the data registers either

implicitly or explicitly Many instructions operate on

the register at the TOP of the stack These instruc-

tions implicitly address the register at which TOP

points Other instructions allow the programmer to

explicitly specify which register to use This explicit

addressing is also relative to TOP

TAG WORD

The tag word marks the content of each numeric

data register as Figure 3 shows Each two-bit tag

represents one of the eight data registers The prin-

cipal function of the tag word is to optimize the

NPX’s performance and stack handling by making it

possible to distinguish between empty and nonemp-

ty register locations It also enables exception han-

dlers to identify special values (eg NaNs or denor-

mals) in the contents of a stack location without the

need to perform complex decoding of the actual

data

STATUS WORD

The 16-bit status word (in the status register) shown

in Figure 4 reflects the overall state of the 80C187 It

may be read and inspected by programs

Bit 15 the B-bit (busy bit) is included for 8087 com-

patibility only It always has the same value as the

ES bit (bit 7 of the status word) it does not indicate

the status of the BUSY output of 80C187

Bits 13 – 11 (TOP) point to the 80C187 register that

is the current top-of-stack

similar to the flags in a CPU instructions that per-

form arithmetic operations update these bits to re-

flect the outcome The effects of these instructions

on the condition code are summarized in Tables 2

through 5

Bit 7 is the error summary (ES) status bit This bit is

set if any unmasked exception bit is set it is clear

otherwise If this bit is set the ERROR signal is as-

serted

Bit 6 is the stack flag (SF) This bit is used to distin-

guish invalid operations due to stack overflow or un-

derflow from other kinds of invalid operations When

Figure 4 shows the six exception flags in bits 5 – 0 of

the status word Bits 5 – 0 are set to indicate that the

80C187 has detected an exception while executing

an instruction A later section entitled ‘‘Exception

Handling’’ explains how they are set and used

Note that when a new value is loaded into the status

word by the FLDENV or FRSTOR instruction the

value of ES (bit 7) and its reflection in the B-bit (bit

15) are not derived from the values loaded from

memory but rather are dependent upon the values of

the exception flags (bits 5 – 0) in the status word and

their corresponding masks in the control word If ES

is set in such a case the ERROR output of the

80C187 is activated immediately

15

0

TAG (7)

TAG (6)

TAG (5)

TAG (4)

TAG (3)

TAG (2)

TAG (1)

TAG (0)

NOTE

The index i of tag(i) is not top-relative A program typically uses the ‘‘top’’ field of Status Word to determine

which tag(i) field refers to logical top of stack

TAG VALUES

Figure 3 Tag Word

5