2 data formats, Data formats -18, Dalu data representations -18 – Freescale Semiconductor StarCore SC140 User Manual
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2-18
SC140 DSP Core Reference Manual
DALU
Figure 2-3. DALU Data Representations
2.2.2.2 Data Formats
Three types of two’s complement data formats are supported by the SC140 core:
•
Signed fractional (SF)
•
Signed integer (SI)
•
Unsigned integer (UI)
The ranges for each of these formats, described below, apply to all data stored in memory as well as data
stored in the data registers. The extension associated with each register allows word growth so that the
most positive fractional number that can be represented in a register is almost 256.0 with the most negative
fractional number being exactly -256.0. When the register extension is in use, the data contained in the
register cannot be stored exactly in memory or in other registers in a single move. In these cases, the
storage error can be minimized by limiting the data to the most positive or most negative number
consistent with the size of the destination, the sign of the register and the MSB of the extension.
2.2.2.2.1 Signed Fractional
In this format, without extension bits 39-32, the N-bit operand is represented using the 1.[N-1] bit format
(1 sign bit, N-1 fractional bits). Signed fractional numbers lie in the following range:
-1.0
≤
SF
≤
+1.0 - 2
-[N-1]
For words and long-word signed fractions, the most negative number that can be represented is exactly
–1.0, of which the internal representation is $8000 and $8000 0000, respectively. The most positive word
is $7FFF or 1.0–2
-15
, and the most positive long word is $7FFF FFFF or 1.0–2
-31
.
If the extension bits are in use, the most positive number is 256 – 2
–31
represented by $7F FFFF FFFF, and
the most negative number is –256, represented by $80 0000 0000.
16-bit word operand
D0.h—D15.h,
16-bit memory
40-bit registers
D0—D15
16-bit word operand
D0.l—D15.l,
16-bit memory
40-bit registers
D0—D15
Signed Fractional Two’s Complement Representations
Signed Integer Two’s Complement Representations
.
–
2
0
2
–15
2
0
2
–15
2
–16
2
–31
–
2
8
–
2
15
2
0
2
14
2
31
2
16
2
15
2
0
–
2
39
.