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- Floating Point instructions
- ==============================
- These instructions support the use of the ARM floating point coprocessor
- (on platforms such as the Pyboard which are equipped with one). The FPU
- has 32 registers known as ``s0-s31`` each of which can hold a single
- precision float. Data can be passed between the FPU registers and the
- ARM core registers with the ``vmov`` instruction.
- Note that MicroPython doesn't support passing floats to
- assembler functions, nor can you put a float into ``r0`` and expect a
- reasonable result. There are two ways to overcome this. The first is to
- use arrays, and the second is to pass and/or return integers and convert
- to and from floats in code.
- Document conventions
- --------------------
- Notation: ``Sd, Sm, Sn`` denote FPU registers, ``Rd, Rm, Rn`` denote ARM core
- registers. The latter can be any ARM core register although registers
- ``R13-R15`` are unlikely to be appropriate in this context.
- Arithmetic
- ----------
- * vadd(Sd, Sn, Sm) ``Sd = Sn + Sm``
- * vsub(Sd, Sn, Sm) ``Sd = Sn - Sm``
- * vneg(Sd, Sm) ``Sd = -Sm``
- * vmul(Sd, Sn, Sm) ``Sd = Sn * Sm``
- * vdiv(Sd, Sn, Sm) ``Sd = Sn / Sm``
- * vsqrt(Sd, Sm) ``Sd = sqrt(Sm)``
- Registers may be identical: ``vmul(S0, S0, S0)`` will execute ``S0 = S0*S0``
-
- Move between ARM core and FPU registers
- ---------------------------------------
- * vmov(Sd, Rm) ``Sd = Rm``
- * vmov(Rd, Sm) ``Rd = Sm``
- The FPU has a register known as FPSCR, similar to the ARM core's APSR, which stores condition
- codes plus other data. The following instructions provide access to this.
-
- * vmrs(APSR\_nzcv, FPSCR)
- Move the floating-point N, Z, C, and V flags to the APSR N, Z, C, and V flags.
- This is done after an instruction such as an FPU
- comparison to enable the condition codes to be tested by the assembler
- code. The following is a more general form of the instruction.
- * vmrs(Rd, FPSCR) ``Rd = FPSCR``
- Move between FPU register and memory
- ------------------------------------
- * vldr(Sd, [Rn, offset]) ``Sd = [Rn + offset]``
- * vstr(Sd, [Rn, offset]) ``[Rn + offset] = Sd``
- Where ``[Rn + offset]`` denotes the memory address obtained by adding Rn to the offset. This
- is specified in bytes. Since each float value occupies a 32 bit word, when accessing arrays of
- floats the offset must always be a multiple of four bytes.
- Data Comparison
- ---------------
- * vcmp(Sd, Sm)
- Compare the values in Sd and Sm and set the FPU N, Z,
- C, and V flags. This would normally be followed by ``vmrs(APSR_nzcv, FPSCR)``
- to enable the results to be tested.
- Convert between integer and float
- ---------------------------------
- * vcvt\_f32\_s32(Sd, Sm) ``Sd = float(Sm)``
- * vcvt\_s32\_f32(Sd, Sm) ``Sd = int(Sm)``
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