= = = DUO ENTERPRISE = = =

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MACHINE DESCRIPTION

The DUO Enterprise has an 8-bit data bus and 24-bit address bus. The CPU contains the following registers:

• 24-bit PC register – Program counter for locating the current instruction.
• 24-bit A register – Memory address to use when executing instructions.
• 24-bit T register – Temporary register for indirect addresses.
• 16-bit D register – Data inputs for ALU operations.
• 1-bit carry flag – Used for ALU operations and branching.

Note that the A, T, and D registers have the following 8-bit divisions:

• A register → A0, A1, A2
• T register → T0, T1, T2
• D register → D0, D1

Every instruction has the following format:

(1-bit condition) (3-bit source) (4-bit destination) (8-bit constant)

Conditions:
0 = Always execute
1 = Execute if carry flag is 1

Sources:
000 = Memory at A
001 = Constant
010 = T (destination may only be PC or A)
011 = D0
100 = D0 NOR D1
101 = D0 + D1 with carry
110 = D0 << 1 with carry
111 = D0 >> 1 with carry

Destinations:
0000 = Memory at A
0001 = PC (source may only be T)
0010 = A (source may only be T)
0011 = A0
0100 = A1
0101 = A2
0110 = T0
0111 = T1
1000 = T2
1001 = D0
1010 = D1

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