FPU Accumulator

This design for a better FPU block for 8/16 bit access, which also integrates 32-bit integer processing. While it's 32 bits, copying around 32-bit values with the CPU is mostly avoided.

There are 256 32-bit registers (TBD- RAM or FPGA registers. Former allows swapping in new sets, but more complicated bus mastering, steal cpu cycles) and one 32-bit accumulator. Any of the registers can hold useful constants (including 0, 1, etc) for reference. It's similar to the 6502 accumulator and zeropage values.

Commands are issued by writing a byte to an I/O location. The location identifies the command to trigger, and the byte written is often a selector for which register number to use, sometimes a literal integer value, or ignored.

Status bits (TBD) are available to read, with various errors, and integer as well as fp Z/N/V/C flags just like the 6502. V and C are only for integer ops.

• Overflow/underflow (fp/int range, as well as f2i)
• Division by zero
• Int conversion lost fractional portion

Set the integer fixed point location (TBD) which affects multiply, divide, and conversion with floats.

TODO - should there be separate signed & unsigned variants of integer operations (including F2I), or a mode bit for signedness?

Shared Operations

Cmd #	Name	Parameter	Description
	LOAD	Reg	A = Reg
	STORE	Reg	Reg = A
	SWAP	Reg	(A, Reg) = (Reg, A)

Floating Point Operations

Cmd #	Name	Parameter	Description
	FADD	Reg	A = A + Reg
	FSUB	Reg	A = A - Reg
	FRSUB	Reg	A = Reg - A
	FMUL	Reg	A = A * Reg
	FDIV	Reg	A = A / Reg
	FRDIV	Reg	A = Reg / A
	F2I	Signed flag	A = Int(A) or UInt(A)
	FMIN	Reg	A = Min(A, Reg)
	FMAX	Reg	A = Max(A, Reg)
	FABS	Ignored	A = Abs(A)
Maybe
	FADDB	Byte	A = A + Val
	FSUBB	Byte	A = A - Val
	FMULB	Byte	A = A * Val
	FDIVB	Byte	A = A / Val
	FCLR	Ignored	A = 0.0 (LOAD instead)
	FNEG	Ignored	A = -A (FRSUB instead)
	FRECIP	Ignored	A = 1.0/A (FRDIV instead)
	FPOW	Reg	A = A ^ Reg
	FSQRT	Ignored	A = Sqrt(A)
	FTRIG	0	A = Sin(A)
		1	A = Cos(A)
		2	A = Tan(A)
	FATAN	Reg	A = Atan(A, Reg)
			A = Sec(A)
			A = Csc(A)
			A = Cot(A)
	FACOT	Reg	A = Arccot(A, Reg)
			A = Arcsin(A)
			A = Arccos(A)
	FFLOOR	Reg	A = Floor(A), Reg = Frac(A) (options?)
	FCEIL	Reg	A = Ceil(A), Reg = Frac(A) (options?)

Integer Operations

Cmd #	Name	Parameter	Description
	IADD	Reg	A = A + Reg
	IADDC	Reg	A = A + Reg + C
	ISUB	Reg	A = A - Reg
	ISUBC	Reg	A = A - Reg - !C
	IRSUB	Reg	A = Reg - A
	ISETC	0 or 1	C = Value
	IMUL	Reg	A = A * Reg (TBD - 64-bit result? Could auto-write Reg 0?)
	IDIV	Reg	A = A / Reg (TBD - combined DIVMOD? 64/32 DIV?)
	IMOD	Reg	A = A % Reg
	ICMP	Reg	Status = A - Reg
	INEG	0	A = -A
	IAND	Reg	A = A & Reg
	IOR	Reg	A = A | Reg
	IXOR	Reg	A = A ^ Reg
	I2F	Signed Flag	A = Float(Int(A)) or Float(UInt(A))
	ISHL	Count	A = A << Value
	ISHR	Count	A = A >> Value
	ISSHR	Count	A = A >>(signed) Value
	IROLL	Count	A = (A << Count) | (A >> (32 - Count))
	IUMIN	Reg	A = Min(A, Reg)
	ISMIN	Reg	A = Min(A, Reg)
	IUMAX	Reg	A = Max(A, Reg)
	ISMAX	Reg	A = Max(A, Reg)
	IABS	Ignored	A = Abs(A)