Operands assigned in this order, masked against the global OP_MASK (0 is enabled):
	MULT1	OP_MASK.b4
	MULT2	OP_MASK.b3
	ALU1	OP_MASK.b2
	ALU2	OP_MASK.b1
	BS	OP_MASK.b0
If no extra operands exist, the writeback EA is always WritebackEA.
If extra operands _do_ exist, and WritebackEA is not assigned (or assigned to zero), LastEA is used as writeback EA.

In dsp_insns.txt, these are assigned to numbers in order; that is, Oper0 is the first Op0.

-------------------------------------------------------------------------------
OP & 0x8000 == 0x0000:
 15  14       13 10 9        8 5 4        3 0
|...|Op0Indir|Op0  |Op1Indir|Op1|Op2Indir|Op2|

LastEA = Op2EA

Op0:
	Op0EA = Op0Indir ? IMEM[REGADDR[Op0]] : REGADDR[Op0]
	Op0Value = IMEM[Op0EA]
Op1:
	Op1EA = Op1Indir ? IMEM[REGADDR[Op1]] : REGADDR[Op1]
	Op1Value = IMEM[Op1EA]
Op2:
	Op2EA = Op2Indir ? IMEM[REGADDR[Op2]] : REGADDR[Op2]
	Op2Value = IMEM[Op2EA]

-------------------------------------------------------------------------------
OP & 0xE000 == 0x8000:
 15 13 12 11        10       9     0
|...  |--|Writeback|Indirect|Address|

LastEA = Op0EA
if(Writeback) WritebackEA = Op0EA

Op0:
	Op0EA = Indirect ? IMEM[Address] : Address
	Op0Value = IMEM[Op0EA]

-------------------------------------------------------------------------------
OP & 0xE000 == 0xA000:
 15 13 12         11         10     9        8 5 4        3 0
|...  |Writeback0|Writeback1|UseOp0|Op0Indir|Op0|Op1Indir|Op1|

if(UseOp0) {
	if(Writeback0) WritebackEA = Op0EA
	Op0:
		Op0EA = Op0Indir ? IMEM[REGADDR[Op0]] : REGADDR[Op0]
		Op0Value = IMEM[Op0EA]
}else{
	Op1 is actually Op0, and there is no Op0
}

LastEA = Op1EA
if(Writeback1) WritebackEA = Op1EA

Op1:
	Op1EA = Op1Indir ? IMEM[REGADDR[Op1]] : REGADDR[Op1]
	Op1Value = IMEM[Op1EA]

-------------------------------------------------------------------------------
OP & 0xC000 == 0xC000:
 15 14 13    12  0
|...  |Shift|Value|

LastEA = Op0Value

Op0:
	Op0Value = Value << (Shift ? 3 : 0)

===============================================================================
Public domain 3DO chip documentation, by trap15
Last update 2020-05-09T20:10+09

Knowledge derived from FreeDO source.

FreeDO Project authors:
Alexander Troosh
Maxim Grishin
Allen Wright
John Sammons
Felix Lazarev