Freescale Semiconductor SC140 specifications Limiting, Calculating the Ln Bit, Scaling Example

DALU

2.2.1.5 Scaling

The data shifters in the shifter/limiter unit can perform the following data shift operations:

•Scale up—Shift data one bit to the left

•Scale down—Shift data one bit to the right

•No scaling—Pass the data unshifted

The eight shifters permit direct dynamic scaling of fixed-point data without additional program steps. For example, this permits straightforward block floating-point implementation of Fast Fourier Transforms (FFTs).

Scaling occurs if programmed in the scaling mode bits S0 and S1 (bits 4 and 5 in the SR). Scaling of operands only occurs with the MOVES.F, MOVES.2F, MOVES.4F, and MOVES.L instructions, moving data from a DALU register (or registers) to memory. The data in the register is not changed, only the data that is transferred. The scaling mode also affects the Ln bit calculation and the rounding function for a set of DALU instructions. Scaling is disabled when the arithmetic saturation mode is set. See Section 3.1.1, “Status Register (SR),” and below for further details. An example of scaling is provided in Table 2-7.

Table 2-7. Scaling Example

Instruction	Memory/	New Value	Comments
	Register
move.w #$0030,r0	r0	$0000 0030	R0 initialized for first memory write
moveu.w #$0200,d0.h	d0	$0200 0000	D0 written
bmset #$10,sr.l	sr	$0000 0010	Scale down set in SR
moves.f d0,(r0)+	$0030	$0100	Memory written with scaled down value
move.l #$00e40020,sr	sr	$00e4 0020	Scale up set in SR
moves.f d0,(r0)	$0032	$0400	Memory written with scaled up value

2.2.1.6 Limiting

The limiting capability is enabled only for the MOVES.F, MOVES.2F, MOVES.4F, and MOVES.L instructions, and not for any other fractional moves such as MOVE.F. These instructions move data from DALU register(s) to memory. The limiting operation takes place in two steps: first, calculating the Ln bit when a previous ALU instruction wrote to a register, and second, transferring the data from that register with a MOVES instruction. The transferred data is limited if the Ln bit is set.

2.2.1.6.1 Calculating the Ln Bit

The Ln bit can be affected by ALU instructions which are capable of using the extension portion of a data register. The only use of the Ln bit is to set up or prepare for a subsequent MOVES instruction. The Ln bit is calculated based on the effective extension bits shown in Table 2-8. These are the bits to the left of the implied decimal point after scaling. If the bits are not all zeros or all ones, the extension is effectively in use and the Ln bit will be set. The Ln bit is cleared as data is written to a DALU register if the defining bits below are all zeros or all ones.

2-14 SC140 DSP Core Reference Manual