Analog Devices adsp-2100 Manual page 60

2 Computational Units
There is one additional normalization situation, requiring the HI-extended
(HIX) state. This is specifically when normalizing ALU results (AR) that
may have overflowed. This operation reads the arithmetic status word
(ASTAT) overflow bit (AV) and the carry bit (AC) in conjunction with the
value in AR. AV is set (1) if an overflow has occurred. AC contains the true
sign of the twos-complement value.
For example, given these conditions:
AR =
AV =
AC =
1. Detect Exponent, Modifier = HIX
SE gets set to
2. Normalize, Modifier = HI, SE = 1
AR = 11111010 00110010
SR = 01111101 00011001
The AC bit is supplied as the sign bit, shown in bold above.
The HIX operation executes properly whether or not there has actually been
an overflow. Consider this example:
AR =
AV =
AC =
1. Detect Exponent, Modifier = HIX
SE set to –2
2. Normalize, Modifier = HI, SE = –2
AR = 11100011 01011011
SR = 10001101 01101 000 00000000 00000000
The AC bit is not used as the sign bit. A brief examination of Table 2.4
shows that the HIX mode is identical to the HI mode when AV is not set.
When the NORM, LO operation is done, the extension bit is zero; when the
NORM, HI operation is done, the extension bit is AC.
2 – 36
11111010 00110010
1, indicating overflow
0, the true sign bit of this value
+1
11100011 01011011
0, indicating no overflow
0, not meaningful if AV = 0