I've "de-optimized" Arlet's opcode state machine in order to make sure I have every single bit perfect as it needs to be to obey the opcode table above, or the Sim will crash as it has been doing all day, at least on my fast machine, with ISE13.2, it is crashing. On my slow laptop, with ISE13.4, the Sim appears to be correct... I'm not blaming ISE at this point. I find errors in my opcode decoding frequently. It's not pretty, but progress will be made!
Code: /*
* Microcode state machine */...
casex ( IR[15:0] )
16'b0000_0000_0000_0000: state <= BRK0;
16'b0000_0000_0010_0000: state <= JSR0;
16'b0000_0000_0100_0000: state <= RTI0; //
16'b0000_0000_0100_1100: state <= JMP0;
16'b0000_0000_0110_0000: state <= RTS0;
16'b0000_0000_0110_1100: state <= JMPI0;
16'bxxxx_xxxx_xxx0_01xx: state <= ZP0; // even rows, columns 4,5,6,7
16'b0000_0000_xxx1_0000: state <= BRA0; // odd rows, column 0
16'bxxxx_xxxx_xxx0_0001: state <= INDX0; // even rows, column 1 --(zp,x)
16'bxxxx_xxxx_xxx1_0001: state <= INDY0; // odd rows, column 1 --(zp),y
16'bxxxx_xxxx_xxx1_0010: state <= INDY0; // odd rows, column 2 --(zp),w
16'bxxxx_xxxx_0xx1_0101: state <= ZPX0; // row 1,3,5,7, column 5
16'bxxxx_xxxx_0xx1_0110: state <= ZPX0; // row 1,3,5,7, column 6
16'bxx00_xx00_10x1_010x: state <= ZPX0; // row 9,B, column 4,5
16'b0000_0000_0111_0100: state <= ZPX0; // STW zpx
16'bxxxx_xxxx_11x1_01xx: state <= ZPX0; // row D,F, column 4,5,6,7
16'bxxxx_xxxx_0010_1100: state <= ABS0; // BIT abs
16'bxxxx_xxxx_0xx0_1101: state <= ABS0; // row 0,2,4,6 even D column
16'bxxxx_xxxx_0xx0_1110: state <= ABS0; // row 0,2,4,6 even E column
16'b0000_0000_1xx0_11xx: state <= ABS0; // row 8,A,C,E, column C,D,E,F --X/Y/W abs
16'bxxxx_xxxx_xxx1_1xx1: state <= ABSX0; // odd rows, column 9,B,D,F
16'bxxxx_xxxx_xxx1_1110: state <= ABSX0; // odds rows, column E
16'bxx00_xx00_0x00_1000: state <= PUSH0; // PH[A..Q], PHP
16'b0000_0000_x101_1010: state <= PUSH0; // PHX, PHY
16'b0000_0000_0100_1011: state <= PUSH0; // PHW
16'b00xx_00xx_0x10_1000: state <= PULL0; // PL[A..Q], PLP
16'b0000_0000_x111_1010: state <= PULL0; // PLY, PLX
16'b0000_0000_0110_1011: state <= PULL0; // PLW
16'b0000_0000_1xx0_00x0: state <= FETCH; // IMM, row 8,A,C,E, column 0,2
16'bxxxx_xxxx_xxx0_1001: state <= FETCH; // IMM, even rows, column 9
16'b0000_0000_0xx1_1000: state <= REG; // CLC, SEC, CLI, SEI
16'bxxxx_xxxx_1xxx_1000: state <= REG; // DEY, TY[A..Q], T[A..Q]Y, INY, INX, INW, DEW
16'bxxxx_xxxx_xxx0_1010: state <= REG; // <shift/rotate> [A..Q], TX[A..Q], NOP
16'bxxxx_xxxx_00x1_1010: state <= REG; // INC/DEC [A..Q]
16'bxxxx_xxxx_10x1_1010: state <= REG; // TSX, TXS
16'bxxxx_xxxx_1xx0_1011: state <= REG; // T[A..Q][A..Q],TYX,TXY
16'bxxxx_xxxx_1111_00x0: state <= REG; // TW[A..Q], T[A..Q]W
endcase
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But the way I see the code is: Verilog will look at index_w first to see if it's a 1 or 0. Then it will look at index_y and regsel will be SEL_Y if index_y is 1, or SEL_W or SEL_X depending on the value of index_w.