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+module decoder (
+);
+
+// instructions
+//  add (reg0 <- reg1 + reg2)
+//  sub (reg0 <- reg1 - reg2)
+//  mul (reg0 <- reg1 * reg2)
+//  div (reg0 <- reg1 / reg2)
+//  shl (reg0 <- reg1 >> reg2, sign extend)
+//  shr (reg0 <- reg1 << reg2, zero extend)  <-- could maybe be one instruction with a direction and extend flags
+//  and (reg0 <- reg1 & reg2)
+//  or  (reg0 <- reg1 | reg2)
+//  xor (reg0 <- reg1 ^ reg2)
+//  not (reg0 <- !reg1)
+//  neg (reg0 <- -reg1)  <-- could maybe be another mode of not instruction where it also adds 1.
+//  bts (reg1th bit of reg0 moved to "zero" status bit, reg1th bit of reg0 is set or reset)
+//  mov (reg0 <- reg1, *reg0 <- reg1, reg0 <- *reg1, imm8 -> reg0H, imm8 -> reg0L)
+//  b** (relative branch: ne, eq, gt, lt, ge, le, always)
+//  jmp (absolute branch)
+
+// 8 registers, 3 bits per register argument
+// 4 bits for instruction
+// instr with 3 reg params is 13 bits
+
+// add/sub/mul/div/and/or/xor
+//  0000 + [reg0] + [op-msb] + [reg1] + [reg2] + [op-2lsb]
+//    op:  0 00 add
+//         0 01 sub
+//         0 10 mul
+//         0 11 div
+//         1 00 and
+//         1 01 or
+//         1 10 xor
+//         1 11 ???
+
+// shl/slr
+//  0001 + [reg0] + [dir] + [reg1] + [reg2] + [extend]
+//    dir:     0 = left, 1 = right
+//    extend:  00 zero
+//             01 one
+//             10 sign/last bit  (copy bit on the end)
+//             11 barrel shift
+
+// not/neg
+//  0010 + [reg0] + [which] + [reg1] + 000 + 00
+//    which:   0 = not, 1 = neg
+
+// bts
+//  0011 + [reg0] + [set or reset] + [reg1] + 00000
+//    set or reset: 0 = reset, 1 = set
+
+// mov
+//  reg0 <- reg1:   0100 + [reg0] + 0 + [reg1] + [dest byte] + [src byte] + [byte or word] + 00
+//  *reg0 <- reg1:  0100 + [reg0] + 1 + [reg1] + 0 + [src byte] + [byte or word] + 0 + 0
+//  reg0 <- *reg1:  0100 + [reg0] + 1 + [reg1] + [dest byte] + 0 + [byte or word] + 0 + 1
+//  reg0L <- imm8:  0101 + [reg0] + 0 + [immediate value]
+//  reg0H <- imm8:  0101 + [reg0] + 1 + [immediate value]
+//    byte or word: 0 = word, 1 = byte
+//                     for mem operations, word ops must be word-aligned (lsb must be 0)
+//    src/dest byte: 0 = low byte, 1 = high byte (ignored for word operations)
+
+// b**
+//  0110 + [which] + 0 + [immediate offset]
+//    which:  000 eq
+//            001 ne
+//            010 gt
+//            011 ge
+//            100 lt
+//            101 le
+//            110 ??
+//            111 always
+
+// jmp
+//  0111 + [reg0] + 0 + 00000000
+//    basically moves reg0 to pc
+
+endmodule