Extract and Sign-Extend a Bit Field EXT

 

Description

The field in src2, specified by csta and cstb, is extracted and sign-extended

 

to 32 bits. The extract is performed by a shift left followed by a signed shift right.

 

csta and cstb are the shift left amount and shift right amount, respectively. This

 

can be thought of in terms of the LSB and MSB of the field to be extracted. Then

 

csta = 31 − MSB of the field and cstb = csta + LSB of the field. The shift left and

 

shift right amounts may also be specified as the ten LSBs of the src1 register

 

with cstb being bits 0−4 and csta bits 5−9. In the example below, csta is 12 and

 

cstb is 11 + 12 = 23. Only the ten LSBs are valid for the register version of the

 

instruction. If any of the 22 MSBs are non-zero, the result is invalid.

src2 1)

csta

cstb − csta

x x x x x x x x x x x x 1 0 1 0 0 1 1 0 1 x x x x x x x x x x x

31

30 29

28 27 26

25 24 23

22

21

20 19

18

17

16

15

14

13

12

11 10

9

8

7

6

5

4

3

2

1

0

Shifts left by 12 to produce:

2) 1 0 1 0 0 1 1 0 1 x x x x x x x x x x x 0 0 0 0 0 0 0 0 0 0 0 0

31

30 29

28 27 26

25 24 23

22

21

20 19

18

17

16

15

14

13

12

11 10

9

8

7

6

5

4

3

2

1

0

dst 3)

Then shifts right by 23 to produce:

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

 

1

0

1

0

0

1

1

0

1

 

31

30 29

28 27 26

25 24 23

22

21

20 19

18

17

16

15

14

13

12

11 10

9

8

7

6

5

4

3

2

1

0

 

Execution

Pipeline

If the constant form is used:

if (cond) src2 ext csta, cstb dst else nop

If the register form is used:

if (cond) src2 ext src19..5, src14..0 dst else nop

Pipeline

E1

Stage

 

 

Read

src1, src2

Written

dst

Unit in use

.S

 

 

SPRU733

Instruction Set

3-111

Page 171
Image 171
Texas Instruments TMS320C67X/C67X+ DSP manual If cond src2 ext csta, cstb → dst else nop