MC68040RC25A Datasheet, PDF (248/442 Pages) Motorola, Inc – Freescale Semiconductor, Inc.

Freescale Semiconductor, Inc.
EXCEPTION ENABLE
MODE CONTROL
15
14 13 12 11 10
9
8
7
6
5
4
3
2
1
0
BSUN SNAN OPERR OVFL UNFL DZ INEX2 INEX1 PREC
RND
0
ROUNDING MODE
ROUNDING PRECISION
INEXACT DECIMAL INPUT
INEXACT OPERATION
DIVIDE BY ZERO
UNDERFLOW
OVERFLOW
OPERAND ERROR
SIGNALING NOT-A-NUMBER
BRANCH/SET ON UNORDERED
Figure 9-2. Floating-Point Control Register
9.2.3 Floating-Point Status Register (FPSR)
The FPSR (see Figure 9-1) contains a floating-point condition code (FPCC) byte, a
quotient byte, a floating-point exception status byte (EXC), and a floating-point accrued
exception byte (AEXC). The user can read or write to all bits in the FPSR. Execution of
most floating-point instructions modifies this register. The reset function or a restore
operation of the null state clears the FPSR. Floating-point conditional operations are not
guaranteed if the FPSR is written directly, because the FPSR is only valid as a result of a
floating-point instruction.
9.2.3.1 FLOATING-POINT CONDITION CODE BYTE. The FPCC byte (see Figure 9-3)
contains four condition code bits that are set at the end of all arithmetic instructions
involving the floating-point data registers. These bits are sign of mantissa (N), zero (Z),
infinity (I), and NAN. The FMOVE FPm,< ea>, FMOVEM FPm, and FMOVE FPCR
instructions do not affect the FPCC.
31
30
29
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27
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25
24
0
N
Z
I
NAN
NOT-A-NUMBER OR UNORDERED
INFINITY
ZERO
NEGATIVE
Figure 9-3. FPSR Condition Code Byte
To aid programmers of floating-point subroutine libraries, the MC68040 implements the
four FPCC bits in hardware instead of only implementing the four IEEE conditions. An
instruction derives the IEEE conditions when needed. For example, the programmers of a
complex arithmetic multiply subroutine usually prefer to handle special data types such as
9-4
M68040 USER’S MANUAL
MOTOROLA
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