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ozpropdev
Posts: **2,696**

in Propeller 2

Hi All

Can someone confirm the operation of the SCL/SCLU instruction.

It's my understanding the SCLU is basically a MUL instruction followed by a SAR 16 bits.

I assume the same goes for SCL, a MULS followed by a SAR 16 bits.

Is that correct?

If so, here is what I am seeing.

SCL seems to give a result tat is out by 2 bits.

* Test1 SCLU with unsigned values = Ok.

* Test2 SCL with unsigned values = Needs extra SAR 2 to be correct.

* Test3 SCLU with 1 signed value = Fails as expected.

* Test4 SCL with 1 signed value = Needs extra SAR 2 to be correct.

Can someone confirm the operation of the SCL/SCLU instruction.

It's my understanding the SCLU is basically a MUL instruction followed by a SAR 16 bits.

I assume the same goes for SCL, a MULS followed by a SAR 16 bits.

Is that correct?

If so, here is what I am seeing.

SCL seems to give a result tat is out by 2 bits.

* Test1 SCLU with unsigned values = Ok.

* Test2 SCL with unsigned values = Needs extra SAR 2 to be correct.

* Test3 SCLU with 1 signed value = Fails as expected.

* Test4 SCL with 1 signed value = Needs extra SAR 2 to be correct.

Test #1 R0 = $000004D2 0000001234 R1 = $000004D2 0000001234 MUL R0,R1 = $00173C44 0001522756 SAR 16 = $00000017 0000000023 SCLU R0,R1 = $00000017 0000000023 * SAR 2 = $00000005 0000000005 ========================================== Test #2 R0 = $000004D2 0000001234 R1 = $000004D2 0000001234 MULS R0,R1 = $00173C44 0001522756 SAR 16 = $00000017 0000000023 SCL R0,R1 = $0000005C 0000000092 * SAR 2 = $00000017 0000000023 ========================================== Test #3 R0 = $FFFFFB2E -0000001234 R1 = $000004D2 0000001234 MUL R0,R1 = $04BAC3BC 0079348668 SAR 16 = $000004BA 0000001210 SCLU R0,R1 = $000004BA 0000001210 * SAR 2 = $0000012E 0000000302 ========================================== Test #4 R0 = $FFFFFB2E -0000001234 R1 = $000004D2 0000001234 MULS R0,R1 = $FFE8C3BC -0001522756 SAR 16 = $FFFFFFE8 -0000000024 SCL R0,R1 = $FFFFFFA3 -0000000093 * SAR 2 = $FFFFFFE8 -0000000024

## Comments

12,809SCLU multiplies D[15:0] by S[15:0] and substitutes product >> 16 into the next instruction's S value. For example:

SCLU A,B 'Add (A[15:0] * B[15:0]) >> 16 into C

ADD C,0

SCL multiplies D[15:0] by S[15:0] and substitutes product ~> 14 into the next instruction's S value (~> is like SAR). For example:

SCL A,B 'Add (A[15:0] * B[15:0]) ~> 14 into C

ADD C,0

2,340This allows fractional multiply with one factor as a signed 16bit signal and the other factor as a coefficient in the range -2.0 .. +1.99994.

So the SAR by 2 you do in the Tests, is normally done by the correct scaling of the coefficient. $4000 correspond to 1.0.

Fractional multiply is important for DSP application where you work mostly in the range -1.0 to +1.0, Chip wanted to have the scaling so that 1.0 is possible. And for FIR filters the coefficients can go bigger than 1.0, so +-2 is a good decision.

The downside is that the resolution of the fractional part is only 14 bits now. For more resolution we would need a better multiplier with 18x18 bits or more.

Andy

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