Rcpsp – Texas Instruments TMS320C67X/C67X+ DSP User Manual
Page 259
Single-Precision Floating-Point Reciprocal Approximation
RCPSP
3-199
Instruction Set
SPRU733
Single-Precision Floating-Point Reciprocal Approximation
RCPSP
Syntax
RCPSP (.unit) src2, dst
.unit = .S1 or .S2
Compatibility
C67x and C67x+ CPU
Opcode
31
29
28
27
23
22
18
17
13
12
11
6
5
4
3
2
1
0
creg
z
dst
src2
0 0 0 0 0 x 1 1 1 1 0 1 1 0 0 0 s p
3
1
5
5
1
1
1
Opcode map field used...
For operand type...
Unit
src2
dst
xsp
sp
.S1, .S2
Description
The single-precision floating-point reciprocal approximation value of src2 is
placed in dst.
The RCPSP instruction provides the correct exponent, and the mantissa is
accurate to the eighth binary position (therefore, mantissa error is less
than 2
−8
). This estimate can be used as a seed value for an algorithm to
compute the reciprocal to greater accuracy.
The Newton-Rhapson algorithm can further extend the mantissa’s precision:
x[n + 1] = x[n](2 − v × x[n])
where v = the number whose reciprocal is to be found.
x[0], the seed value for the algorithm, is given by RCPSP. For each iteration,
the accuracy doubles. Thus, with one iteration, accuracy is 16 bits in the
mantissa; with the second iteration, the accuracy is the full 23 bits.
Execution
if (cond)
rcp(src2) → dst
else
nop