Rcpdp (Double-Precision Floating-Point Reciprocal Approximation) - Texas Instruments TMS320C67X Reference Manual

RCPDP
Syntax
Compatibility
Opcode
31 29 28 27
creg z dst
3 1 5
Description
Execution
SPRU733
Double-Precision Floating-Point Reciprocal Approximation
Double-Precision Floating-Point Reciprocal Approximation
RCPDP (.unit) src2, dst
.unit = .S1 or .S2
C67x and C67x+ CPU
23 22 18
src2
5
Opcode map field used...
src2
dst
The 64-bit double-precision floating-point reciprocal approximation value of
src2 is placed in dst. The operand is read in one cycle by using the src1 port
for the 32 LSBs and the src2 port for the 32 MSBs.
The RCPDP instruction provides the correct exponent, and the mantissa is
accurate to the eighth binary position (therefore, mantissa error is less
−8
than 2 ). 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 RCPDP. For each iteration,
the accuracy doubles. Thus, with one iteration, accuracy is 16 bits in the
mantissa; with the second iteration, the accuracy is 32 bits; with the third itera-
tion, the accuracy is the full 52 bits.
if (cond) rcp(src2) → dst
else nop
17 13 12 11
reserved x 1
5 1
For operand type...
dp
dp
RCPDP
6 5 4 3
0 1 1 0 1 1 0 0 0 s p
Unit
.S1, .S2
Instruction Set
2 1 0
1 1
3-197