INTEL387TMDX データシートの表示（PDF） - Intel

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INTEL387TMDX

Intel387™ DX MATH COPROCESSOR

Intel 

Intel387TM DX MATH COPROCESSOR

ES is set if any unmasked exception bit is set cleared otherwise

See Table 2 2 for interpretation of condition code

TOP values

000 e Register 0 is Top of Stack

001 e Register 1 is Top of Stack







111 e Register 7 is Top of Stack

For definitions of exceptions refer to the section entitled

‘‘Exception Handling’’

240448 – 3

Figure 2 2 MCP Status Word

2 3 3 STATUS WORD

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

in Figure 2 2 reflects the overall state of the MCP It

may be read and inspected by CPU code

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

patibility only It reflects the contents of the ES bit

(bit 7 of the status word) not the status of the

BUSY output of the Intel387 DX MCP

Bits 13–11 (TOP) point to the Intel387 DX MCP reg-

ister that is the current top-of-stack

The four numeric condition code bits (C3 – C0) are

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 2

through 2 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

asserted

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

SF is set bit 9 (C1) distinguishes between stack

overflow (C1 e 1) and underflow (C1 e 0)

Figure 2 2 shows the six exception flags in bits 5 – 0

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

the MCP 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

MCP is activated immediately

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