Freescale Semiconductor SC140 specifications Equation Mode, S1-S0 Scaling Mode Bits Specify

Core Control Registers

Table 3-1. Status Register Description (Continued)

Name	Description	Settings
S	Scaling Bit — Set when moving a result
Bit 6	from a data register (D0–D15) to
						Scaling
	memory using a MOVES (saturated		S1	S0					S Equation
						Mode
	move) instruction. The scaling bit is set
	when the absolute value of the data that
			0	0		No scaling			S = (D30 XOR D29)
	is moved to memory (after scaling and
									OR S (previous)
	limiting) is greater than or equal to 0.25
	and less than 0.75.		0	1		Scale down			S = (D31 XOR D30)
	The logical equations of this bit, if viewed								OR S (previous)
	as functions of the data in the register,
			1	0		Scale up			S = (D29 XOR D28)
	are dependent on the scaling mode.
	If limiting occurs during a data register								OR S (previous)
	transfer to memory, the scaling bit is not
			1	1		Reserved			S = Undefined
	affected. This bit is a sticky bit and it
	remains set until explicitly cleared.
	This bit is cleared at core reset.
S1–S0	Scaling Mode Bits — Specify the
Bits 5–4	scaling to be performed in the DALU
					Rounding
	shifter/limiter as well as the rounding		S1	S0					Scaling Mode
						Bit
	position in the DALU MAC unit.
	The shifter/limiter scaling mode affects
		0		0	15				No scaling
	data read from the D0–D15 registers out
	to the data memory bus using a MOVES	0		1	16				Scale down
	instruction. The scaling mode also							(1-bit Arithmetic Right
	affects the calculation of the Ln bit for a								Shift)
	class of DALU instructions. See
		1		0	14				Scale up
	Section 2.2.1.5, “Scaling,” and
	Section 2.2.1.6, “Limiting,” for more						(1-bit Arithmetic Left Shift)
	information.
		1		1		—			Reserved
	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	Rounding Mode Bit — Selects the type	0 = Convergent rounding selected
Bit 3	of rounding performed by the DALU	1 = Two’s complement rounding selected
	during arithmetic operations that involve
	rounding. SeeSection 2.2.2.6, “Rounding
	Modes.”
	This bit is cleared at core reset.

SC140 DSP Core Reference Manual 3-5