Architecture and Instruction Set of the C6x Processor - Digital Signal Processing and Applications with the C6713 and C6416 DSK - page 96

31 30 23 22 0

s e f

( a )

31 30 20 19 0 31 0

s e f f

( b )
FIGURE 3.5. Data format: ( a ) single precision; ( b ) double precision.

2. int or signed int : of size 32 bits represented as 2's complement with a
range from 2 31 to (2 31

1)
3. float : of size 32 bits represented as IEEE 32-bit with a range from 2 -126

- -

=

10 38
4. double : of size 64 bits represented as IEEE 64-bit with a range from 2 -1022

10 -38 to 2 +128

1.175494 ¥ = 3.40282346 ¥

=

10 -308 to 2 +1024 10 +308

2.22507385 ¥ = 1.79769313 ¥

Data types such as short for fixed-point multiplication can be more efficient (fewer
cycles) than using int . Use of const can also increase code performance. Nota-
tions such as Uint16 and Uint32 are supported for casting 16- and 32-bit unsigned
integers, respectively.

3.18.2 Floating-Point Format

With a much wider dynamic range in a floating-point processor, scaling is not an
issue. A floating-point number can be represented using single precision with 32 bits
or double precision with 64 bits, as shown in Figure 3.5. In single-precision format,
bit 31 represents the sign bit, bits 23 through 30 represent the exponent bits, and
bits 0 through 22 represent the fractional bits, as shown in Figure 3.5 a . Numbers as
small as 10 -38 and as large as 10 +38 can be represented. In double-precision format,
more exponent and fractional bits are available, as shown in Figure 3.5 b . Since 64
bits are represented, a pair of registers is used. Bits 0 through 31 of the first regis-
ter pair represent the fractional bits. Bits 0 through 19 of the second register pair
also represent the fractional bits, with bits 20 through 30 representing the exponent
bits and bit 31 the sign bit. As a result, numbers as small as 10 -308 and as large as

10 +308 can be represented.
Instructions ending in either SP or DP represent single and double precision,
respectively. Some of the floating-point instructions (available on the C67x floating-
point processor) have more latencies than do fixed-point instructions. For example,
the fixed-point multiplication MPY requires one delay or NOP , whereas the single-
precision MPYSP requires three delays and the double-precision instruction MPYDP
requires nine delays.
The single-precision floating-point instructions ADDSP and MPYSP have three
delay slots and take four cycles to complete execution. The double-precision instruc-