Freescale Semiconductor StarCore SC140 User Manual

Page 105

Core Control Registers

SC140 DSP Core Reference Manual

3-5

S
Bit 6

Scaling Bit — Set when moving a result
from a data register (D0–D15) to
memory using a MOVES (saturated
move) instruction. The scaling bit is set
when the absolute value of the data that
is moved to memory (after scaling and
limiting) is greater than or equal to 0.25
and less than 0.75.
The logical equations of this bit, if viewed
as functions of the data in the register,
are dependent on the scaling mode.
If limiting occurs during a data register
transfer to memory, the scaling bit is not
affected. This bit is a sticky bit and it
remains set until explicitly cleared.
This bit is cleared at core reset.

S1–S0
Bits 5–4

Scaling Mode Bits — Specify the
scaling to be performed in the DALU
shifter/limiter as well as the rounding
position in the DALU MAC unit.
The shifter/limiter scaling mode affects
data read from the D0–D15 registers out
to the data memory bus using a MOVES
instruction. The scaling mode also
affects the calculation of the Ln bit for a
class of DALU instructions. See

Section 2.2.1.5, “Scaling,”

and

Section 2.2.1.6, “Limiting,”

for more

information.
The scaling mode also affects the MAC
rounding bit position. Correct rounding is
maintained when different portions of the
registers are read out to the data
memory buses. For more information,
see

Section 2.2.2.6, “Rounding Modes.”

During arithmetic saturation mode, the
scaling bits are ignored for most DALU
instructions. See

Section 2.2.2.7,

“Arithmetic Saturation Mode.”

These bits are cleared at the start of an
exception service routine as well as at
core reset.

RM
Bit 3

Rounding Mode Bit — Selects the type
of rounding performed by the DALU
during arithmetic operations that involve
rounding. See

Section 2.2.2.6, “Rounding

Modes.”

This bit is cleared at core reset.

0 = Convergent rounding selected
1 = Two’s complement rounding selected

Table 3-1. Status Register Description (Continued)

Name

Description

Settings

Scaling

Mode

S Equation

No scaling

S = (D30 XOR D29)

OR S (previous)

Scale down

S = (D31 XOR D30)

OR S (previous)

Scale up

S = (D29 XOR D28)

OR S (previous)

Reserved

S = Undefined

Rounding

Bit

Scaling Mode

No scaling

Scale down

(1-bit Arithmetic Right

Shift)

Scale up

(1-bit Arithmetic Left Shift)

—

Reserved