xref: /kernel/linux/linux-6.6/Documentation/arch/arm/nwfpe/netwinder-fpe.rst (revision 62306a36)

=============
Current State
=============

The following describes the current state of the NetWinder's floating point
emulator.

In the following nomenclature is used to describe the floating point
instructions.  It follows the conventions in the ARM manual.

::

  <S|D|E> = <single|double|extended>, no default
  {P|M|Z} = {round to +infinity,round to -infinity,round to zero},
            default = round to nearest

Note: items enclosed in {} are optional.

Floating Point Coprocessor Data Transfer Instructions (CPDT)
------------------------------------------------------------

LDF/STF - load and store floating

<LDF|STF>{cond}<S|D|E> Fd, Rn
<LDF|STF>{cond}<S|D|E> Fd, [Rn, #<expression>]{!}
<LDF|STF>{cond}<S|D|E> Fd, [Rn], #<expression>

These instructions are fully implemented.

LFM/SFM - load and store multiple floating

Form 1 syntax:
<LFM|SFM>{cond}<S|D|E> Fd, <count>, [Rn]
<LFM|SFM>{cond}<S|D|E> Fd, <count>, [Rn, #<expression>]{!}
<LFM|SFM>{cond}<S|D|E> Fd, <count>, [Rn], #<expression>

Form 2 syntax:
<LFM|SFM>{cond}<FD,EA> Fd, <count>, [Rn]{!}

These instructions are fully implemented.  They store/load three words
for each floating point register into the memory location given in the
instruction.  The format in memory is unlikely to be compatible with
other implementations, in particular the actual hardware.  Specific
mention of this is made in the ARM manuals.

Floating Point Coprocessor Register Transfer Instructions (CPRT)
----------------------------------------------------------------

Conversions, read/write status/control register instructions

FLT{cond}<S,D,E>{P,M,Z} Fn, Rd          Convert integer to floating point
FIX{cond}{P,M,Z} Rd, Fn                 Convert floating point to integer
WFS{cond} Rd                            Write floating point status register
RFS{cond} Rd                            Read floating point status register
WFC{cond} Rd                            Write floating point control register
RFC{cond} Rd                            Read floating point control register

FLT/FIX are fully implemented.

RFS/WFS are fully implemented.

RFC/WFC are fully implemented.  RFC/WFC are supervisor only instructions, and
presently check the CPU mode, and do an invalid instruction trap if not called
from supervisor mode.

Compare instructions

CMF{cond} Fn, Fm        Compare floating
CMFE{cond} Fn, Fm       Compare floating with exception
CNF{cond} Fn, Fm        Compare negated floating
CNFE{cond} Fn, Fm       Compare negated floating with exception

These are fully implemented.

Floating Point Coprocessor Data Instructions (CPDT)
---------------------------------------------------

Dyadic operations:

ADF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - add
SUF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - subtract
RSF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - reverse subtract
MUF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - multiply
DVF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - divide
RDV{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - reverse divide

These are fully implemented.

FML{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - fast multiply
FDV{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - fast divide
FRD{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - fast reverse divide

These are fully implemented as well.  They use the same algorithm as the
non-fast versions.  Hence, in this implementation their performance is
equivalent to the MUF/DVF/RDV instructions.  This is acceptable according
to the ARM manual.  The manual notes these are defined only for single
operands, on the actual FPA11 hardware they do not work for double or
extended precision operands.  The emulator currently does not check
the requested permissions conditions, and performs the requested operation.

RMF{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - IEEE remainder

This is fully implemented.

Monadic operations:

MVF{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - move
MNF{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - move negated

These are fully implemented.

ABS{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - absolute value
SQT{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - square root
RND{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - round

These are fully implemented.

URD{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - unnormalized round
NRM{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - normalize

These are implemented.  URD is implemented using the same code as the RND
instruction.  Since URD cannot return a unnormalized number, NRM becomes
a NOP.

Library calls:

POW{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - power
RPW{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - reverse power
POL{cond}<S|D|E>{P,M,Z} Fd, Fn, <Fm,#value> - polar angle (arctan2)

LOG{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - logarithm to base 10
LGN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - logarithm to base e
EXP{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - exponent
SIN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - sine
COS{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - cosine
TAN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - tangent
ASN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arcsine
ACS{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arccosine
ATN{cond}<S|D|E>{P,M,Z} Fd, <Fm,#value> - arctangent

These are not implemented.  They are not currently issued by the compiler,
and are handled by routines in libc.  These are not implemented by the FPA11
hardware, but are handled by the floating point support code.  They should
be implemented in future versions.

Signalling:

Signals are implemented.  However current ELF kernels produced by Rebel.com
have a bug in them that prevents the module from generating a SIGFPE.  This
is caused by a failure to alias fp_current to the kernel variable
current_set[0] correctly.

The kernel provided with this distribution (vmlinux-nwfpe-0.93) contains
a fix for this problem and also incorporates the current version of the
emulator directly.  It is possible to run with no floating point module
loaded with this kernel.  It is provided as a demonstration of the
technology and for those who want to do floating point work that depends
on signals.  It is not strictly necessary to use the module.

A module (either the one provided by Russell King, or the one in this
distribution) can be loaded to replace the functionality of the emulator
built into the kernel.