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3-5
ADVANCED MATH FEATURES
3.4 ACCUMULATOR CONTROL AND STATUS REGISTER (ACC_STAT)
The ACC_STAT register controls the operating mode and reflects the status of the accumulator.
The mode bits (FME and SME) are effective only for signed multiplication. Table 3-2 describes
the 80C196NU’s operation with each of the four possible configurations of these bits.
ACC_STAT
(80C196NU)
Address:
Reset State:
0BH
00H
The accumulator control and status (ACC_STAT) register enables and disables fractional and
saturation modes and contains three status flags that indicate the status of the accumulator’s
contents.
7 0
80C196NU
FME SME — — — STOVF OVF STSAT
Bit
Number
Bit
Mnemonic
Function
7 FME Fractional Mode Enable
Set this bit to enable fractional mode. (See Table 3-2.) In this mode, the
result of a signed multiplication instruction is shifted left by one bit before it
is added to the contents of the accumulator.
For unsigned multiplication, this bit is ignored.
6 SME Saturation Mode Enable
Set this bit to enable saturation mode. (See Table 3-2.) In this mode, the
result of a signed multiplication operation is not allowed to overflow or
underflow.
For unsigned multiplication, this bit is ignored.
5:3 — Reserved; for compatibility with future devices, write zeros to these bits.
2 STOVF Sticky Overflow Flag
For unsigned multiplication, this bit is set if a carry out of bit 31 occurs.
Unless saturation mode is enabled, this bit is set for signed multiplication to
indicate that the sign bit of the accumulator and the sign bit of the addend
are equal, but the sign bit of the result is the opposite. (See Table 3-2.)
Software can clear this flag; hardware does not clear it.
1 OVF Overflow Flag
This bit indicates that an overflow occurred during the preceding accumu-
lation. (See Table 3-2.)
This flag is dynamic; it can change after each accumulation.
0 STSAT Sticky Saturation Flag
This bit indicates that a saturation has occurred during accumulation with
saturation mode enabled. (See Table 3-2.)
Software can clear this flag; hardware does not clear it.
Figure 3-2. Accumulator Control and Status (ACC_STAT) Register