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10. 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.

10.1. 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.

10.2. 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

10.3. 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

10.4. 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.

10.5. 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.

10.6. Convert between integer and float

  • vcvt_f32_s32(Sd, Sm) Sd = float(Sm)

  • vcvt_s32_f32(Sd, Sm) Sd = int(Sm)