path: root/Documentation/arm/nwfpe/TODO
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authorLinus Torvalds <>2005-04-16 15:20:36 -0700
committerLinus Torvalds <>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/arm/nwfpe/TODO
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
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+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.
+There are a couple of ways to approach the implementation of these. One
+method would be to use accurate table methods for these routines. I have
+a couple of papers by S. Gal from IBM's research labs in Haifa, Israel that
+seem to promise extreme accuracy (in the order of 99.8%) and reasonable speed.
+These methods are used in GLIBC for some of the transcendental functions.
+Another approach, which I know little about is CORDIC. This stands for
+Coordinate Rotation Digital Computer, and is a method of computing
+transcendental functions using mostly shifts and adds and a few
+multiplications and divisions. The ARM excels at shifts and adds,
+so such a method could be promising, but requires more research to
+determine if it is feasible.
+Rounding Methods
+The IEEE standard defines 4 rounding modes. Round to nearest is the
+default, but rounding to + or - infinity or round to zero are also allowed.
+Many architectures allow the rounding mode to be specified by modifying bits
+in a control register. Not so with the ARM FPA11 architecture. To change
+the rounding mode one must specify it with each instruction.
+This has made porting some benchmarks difficult. It is possible to
+introduce such a capability into the emulator. The FPCR contains
+bits describing the rounding mode. The emulator could be altered to
+examine a flag, which if set forced it to ignore the rounding mode in
+the instruction, and use the mode specified in the bits in the FPCR.
+This would require a method of getting/setting the flag, and the bits
+in the FPCR. This requires a kernel call in ArmLinux, as WFC/RFC are
+supervisor only instructions. If anyone has any ideas or comments I
+would like to hear them.
+[NOTE: pulled out from some docs on ARM floating point, specifically
+ for the Acorn FPE, but not limited to it:
+ The floating point control register (FPCR) may only be present in some
+ implementations: it is there to control the hardware in an implementation-
+ specific manner, for example to disable the floating point system. The user
+ mode of the ARM is not permitted to use this register (since the right is
+ reserved to alter it between implementations) and the WFC and RFC
+ instructions will trap if tried in user mode.
+ Hence, the answer is yes, you could do this, but then you will run a high
+ risk of becoming isolated if and when hardware FP emulation comes out
+ -- Russell].