HIGH  LOW
0000  iiii MOV A, imm4
0001  iiii MOV B, imm4
0010  dddd ALU operation -> ALU  ; ALU operations on registers
0011  0000 MOV A, ALU  ; Register operations?
0011  0001 MOV B, ALU
      0010 MOV IX, AB
      0011 MOV IY, AB
      0100 MOV B, A
      0101 MOV OUT, AB
      0110 MOV IN, A
      0111 MOV IN, B
      1000 MOV SP, AB
      1001 MOV A, A (NOP)
      1010-1111: Unused

0100 ; IX operations
0100 dddd ; ALU operations on memory using IX
     0x0: ADD  [IX]
     0x1: ADDC [IX]
     0x2: SUB  [IX]
     0x3: SUBB [IX]
     0x4: AND  [IX]
     0x5: OR   [IX]
     0x6: XOR  [IX]
     0x7: NOT  [IX]
     0x8: INCA  [IX]
     0x9: DECA  [IX]
     0xA: INCB  [IX]
     0xB: DECB  [IX]
     0xC: PASSA [IX]
     0xD: PASSB [IX]
0100 1110 inc IX
0100 1111 dec IX

0101 dddd ; ALU operations on memory using IY
0110
0111
1000

1001 ; Memory operations
1001 0000 MOV AB, [IX]
1001 0001 MOV [IX], AB
     0010 MOV [IX], ALU0
  0011-0111: unused
     1000 PUSH AB
     1001 PUSH PC
  1010-1101: unused
     1110 POP IX
     1111 POP AB


# Absolute jumps: Set PC to IX/IY
1010 0000 JZ  IX
1010 0001 JC  IX
1010 0010 JNC IX
1010 0011 JMP IX
1010 0100 JNZ IX
1010 0101 JZ  IY
1010 0110 JC  IY
1010 0111 JNC IY
1010 1000 JMP IY
1010 1001 JNZ IY
# Far jumps: Set CS to A, and PC to IX
1010 1010 JZ Far  A:IX
1010 1011 JC Far  A:IX
1010 1100 JNC Far A:IX
1010 1101 JMP Far A:IX
1010 1110 JMP Far A:IX


# Immediate jumps:
# Set PC to hhiiii00
# (top two PC bits unchanged, middle nibble supplied by imm4, bottom two zeroed)
1011 dddd JZ  imm4
1100 dddd JC  imm4
1101 dddd JNC imm4
1110 dddd JNZ imm4
1111 dddd JR  imm4


# Decoder ROM

Inputs:
0:7 Instruction
8   Carry flag
9   Zero flag
10  Reserved?

Outputs:
0:3 SRC
  - 0000 AB
  - 0001 imm4
  - 0010 ALUou
  - 0011 PC
  - 0100 Unused
  - 0101 Unused  
  - 0110 Input
  - 0111 RAM
  - 1000-1111
4:7 DST
  - 0000 A
  - 0001 IX
  - 0010 B
  - 0011 ALU
  - 0100 AB
  - 0101 OUTPUT
  - 0110 SP
  - 0111 RAM
  - 1000-1111

8:9  PC_LOAD
  - 00 increment
  - 01 Index_register
  - 10 no advance
  - 11 imm4
10:11 INDEX_SELECT
     - 00 none
     - 01 IX
     - 10 IY
     - 11 SP
12:13 INDEX_INCDEC
     - 00 none
     - 01 IX
     - 10 IY
     - 11 SP
14: Direction
     - 0 increment
     - 1 decrement

16:19 ALUs0:3        
20  ALUm     
21:22 ALUclear
 - 00 ALU_NO_CLEAR
 - 01 ALUclearA
 - 10 ALUclearB
 - 11 ALU_NO_STORE
23  ALUcarry        

24  Store_flags

25:26 RAM_Segment_select
     - 00 DS
     - 01 ES
     - 10 SS
     - 11 GS


26:32 unused