Texas Instruments TMS320C67X/C67X+ DSP - Manual (Page 171)

Extract and Sign-Extend a Bit Field EXT
3-111 Instruction Set SPRU733
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.
csta
x
cstb − csta
src2
dst
x x x x x x x x 1 1 0 1 x x x x x x x x x x x x x 0 1 0 x 1 0
1 1 0 1 0 0 1 1 0 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
x x x 0 0 0 0 0 1 1 1 1 1 0 0 0 x x x x x x x x 0 0 0 0 0 0 0 0
Shifts left by 12 to produce:
Then shifts right by 23 to produce:
1)
2)
3)
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
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
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 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 src1
9..5
, src1
4..0
→ dst
else nop
Pipeline
Stage
E1
Read src1, src2
Written dst
Unit in use .S
Pipeline