Add missing CPU components todo
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todo.md
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# Missing CPU Components
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## Core CPU Components
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| Component | Description | Status |
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|-------------------------|-----------------------------|--------------------------------|
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| SUB | Subtraction circuit | Missing - need NOT(B)+1+A path |
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| DIV | Division circuit | Missing |
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| NEG | Two's complement negate | Missing |
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| Program Counter | PC register + increment | Missing |
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| PC Load | Load PC from jump target | Missing |
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| Register File MUX | Select 1-of-4 GPRs | Missing |
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| Register Write Enable | Write back to register | Missing |
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| Memory Address Register | MAR latch | Missing |
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| Memory Data Register | MDR latch | Missing |
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| Memory Read/Write | R/W enable signals | Missing |
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| Instruction Register | IR latch | Missing |
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| Instruction Decoder | Opcode → microops | Missing |
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| Sequencer FSM | Fetch/Decode/Execute states | Missing |
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## Extended Operations
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| Component | Description | Status |
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|-----------|--------------------------------------|-----------------------------------------------------|
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| ROL | Rotate left through carry | Missing |
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| ROR | Rotate right through carry | Missing |
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| ASR | Arithmetic shift right (sign-extend) | Missing |
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| ADC | Add with carry input | Missing |
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| SBC | Subtract with borrow | Missing |
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| CMP | Compare (SUB without writeback) | Missing (comparators exist but not via subtraction) |
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## System
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| Component | Description | Status |
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|------------------------|------------------|---------|
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| Interrupt Controller | IRQ/NMI handling | Missing |
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| Interrupt Vector Table | Jump addresses | Missing |
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| Interrupt Enable Flag | Global IE bit | Missing |
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| I/O Ports | IN/OUT data path | Missing |
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| HALT | Stop execution | Missing |
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| NOP | No operation | Missing |
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| Watchdog Timer | Reset on timeout | Missing |
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