Status Register (ST0)
2-32
OVM
Bit 1
Overflow mode bit. When ACC accepts the result of an addition or subtraction and the
result causes an overflow, OVM determines how the CPU handles the overflow as fol-
lows:.
0 Results overflow normally in ACC. The OVC reflects the overflow (see the de-
scription for the OVC on page 2-16)
1 ACC is filled with either its most positive or most negative value as follows:
If ACC overflows in the positive direction (from 7FFFFFFF
16
to 80000000
16
),
ACC is then filled with 7FFFFFFF
16
.
If ACC overflows in the negative direction (from 80000000
16
to 7FFFFFFF
16
),
ACC is then filled with 80000000
16
.
This bit can be individually set and cleared by the SETC OVM instruction and
CLRC OVM instruction, respectively. At reset, OVM is cleared.
SXM
Bit 0
Sign-extension mode bit. SXM affects the MOV, ADD, and SUB instructions that use a
16-bit value in an operation on the 32-bit accumulator. When the 16-bit value is loaded
into (MOV), added to (ADD), or subtracted from (SUB) the accumulator, SXM deter-
mines whether the value is sign extended during the operation as follows:
0 Sign extension is suppressed. (The value is treated as unsigned.)i
1 Sign extension is enabled. (The value is treated as signed.)
SXM also determines whether the accumulator is sign extended when it is shifted right
by the SFR instruction. SXM does not affect instructions that shift the product register
value; all right shifts of the product register value use sign extension.
This bit can be individually set and cleared by the SETC SXM instruction and
CLRC SXM instruction, respectively. At reset, SXM is cleared. Table 2−10 lists the in-
structions that are affected by SXM. See Chapter 6 for more details on instructions.
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