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Glossary
The maximum positive value in a 16-bit register is 0x7FFF. The maxi-
mum negative value is 0x8000. For a signed two's-complement 1.15
fractional notation, the allowable range is –1 through (1–2–15).
The maximum positive value in a 32-bit register is 0x7FFF FFFF. The
maximum negative value is 0x8000 0000. For a signed two's-complement
fractional data in 1.31 format, the range of values that the register can
hold are –1 through (1–2–31).
The maximum positive value in a 40-bit register is 0x7F FFFF FFFF. The
maximum negative value is 0x80 0000 0000. For a signed two's-comple-
ment 9.31 fractional notation, the range of values that can be represented
is –256 through (256–2–31).
For example, if a 16-bit register containing 0x1000 (decimal integer
+4096) was shifted left 3 places without saturation, it would overflow to
0x8000 (decimal –32,768). With saturation, however, a left shift of 3 or
more places would always produce the largest positive 16-bit number,
0x7FFF (decimal +32,767).
Another common example is copying the lower half of a 32-bit register
into a 16-bit register. If the 32-bit register contains 0xFEED 0ACE and
the lower half of this negative number is copied into a 16-bit register with-
out saturation, the result is 0x0ACE, a positive number. But if saturation
is enforced, the 16-bit result maintains its negative sign and becomes
0x8000.
The MSA implements 40-bit saturation for all arithmetic operations that
write an Accumulator destination except as noted in the individual
instruction descriptions when an optional 32-bit saturation mode can
constrain a 40-bit Accumulator to the 32-bit register range. The MSA per-
forms 32-bit saturation for 32-bit register destinations only as noted in the
instruction descriptions.
Overflow is the alternative to saturation. The number is allowed to simply
exceed its bounds and lose its most significant bit(s); only the lowest
(least-significant) portion of the number can be retained. Overflow can
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ADSP-BF53x/BF56x Blackfin Processor Programming Reference
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