Software Techniques
8.2.5 Arithmetic Shifts by a Fixed Amount
Arithmetic shifts (left or right) by a fixed amount can be emulated with the ASRxx operations.
8.2.5.1 Right Shifts (ASR12–ASR20)
For arithmetic right shifts there is a faster way to shift an accumulator for large shift counts. The following code shows how to perform arithmetic right shifts of 12 through 20 bits on an accumulator. This emulation works without destroying any registers on the chip. If desired, it is possible to use this technique for bit shifts greater than 20, but it is not possible to use this technique for shifts of 11 or fewer bits without losing information.
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DSP56800 Family Manual |
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16and 32-Bit Shift Operations
;ASR12 Operation
;Emulated in 8 Icyc, 8 Instruction Words
ASL |
A |
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ASL |
A |
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ASL |
A |
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ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A2,A |
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POP |
A0 |
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;ASR13 Operation
;Emulated in 7 Icyc, 7 Instruction Words
ASL |
A |
|
ASL |
A |
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ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A2,A |
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POP |
A0 |
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;ASR14 Operation
;Emulated in 6 Icyc, 6 Instruction Words
ASL |
A |
|
ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A2,A |
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POP |
A0 |
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;ASR15 Operation
;Emulated in 5 Icyc, 5 Instruction Words
ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A2,A |
|
POP |
A0 |
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;ASR16 Operation
;Emulated in 2 Icyc, 2 Instruction Words
MOVE |
A1,A0 |
; (Assumes EXT contains sign extension) |
MOVE |
A2,A1 |
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;ASR17 Operation
;Emulated in 3 Icyc, 3 Instruction Words
ASR |
A |
|
MOVE |
A1,A0 |
; (Assumes EXT contains sign extension) |
MOVE |
A2,A1 |
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;ASR18 Operation
;Emulated in 4 Icyc, 4 Instruction Words
ASR |
A |
|
ASR |
A |
|
MOVE |
A1,A0 |
; (Assumes EXT contains sign extension) |
MOVE |
A2,A1 |
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;ASR19 Operation
;Emulated in 5 Icyc, 5 Instruction Words
ASR |
A |
|
ASR |
A |
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ASR |
A |
|
MOVE |
A1,A0 |
; (Assumes EXT contains sign extension) |
MOVE |
A2,A1 |
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;ASR20 Operation
;Emulated in 6 Icyc, 6 Instruction Words
ASR |
A |
ASR |
A |
ASR |
A |
ASR |
A |
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Software Techniques |
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Software Techniques |
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MOVE |
A1,A0 |
; (Assumes EXT contains sign extension) |
MOVE |
A2,A1 |
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8.2.5.2 Left Shifts (ASL16–ASL19)
For arithmetic left shifts there is a faster way to shift an accumulator for large shift counts. The following code shows how to perform arithmetic left shifts of 16 through 19 bits on an accumulator. This emulation works without destroying any registers on the chip. If desired, it is possible to use this technique for bit shifts greater than 19, but it is not possible for shifts of 15 or fewer bits without losing information.
;ASL16 Operation
;Emulated in 4 Icyc, 4 Instruction Words
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A0,A |
|
POP |
A2 |
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;ASL17 Operation
;Emulated in 5 Icyc, 5 Instruction Words
ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A0,A |
|
POP |
A2 |
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;ASL18 Operation
;Emulated in 6 Icyc, 6 Instruction Words
ASL |
A |
|
ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A0,A |
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POP |
A2 |
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;ASL19 Operation
;Emulated in 7 Icyc, 7 Instruction Words
ASL |
A |
|
ASL |
A |
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ASL |
A |
|
PUSH |
A1 |
; (PUSH is a 2-word, 2 Icyc macro) |
MOVE |
A0,A |
|
POP |
A2 |
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DSP56800 Family Manual |
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