2026-04-22 18:46:54 +00:00
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# Practical Example: Disassembling a Boot Sequence
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This walkthrough uses verified opcodes from the open21xx assembler
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to demonstrate analysis of a typical ADSP-2191 initialization.
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## Hex Dump (from examples/isa_test.bin)
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0x000 000000 NOP
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0x003 412340 AX0 = 0x1234
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0x006 456781 AX1 = 0x5678
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0x009 4AAAA4 AY0 = 0xAAAA
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0x00C 440002 MX0 = 0x4000
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0x00F 420006 MY0 = 0x2000
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0x012 501000 I0 = 0x0100
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0x015 502001 I1 = 0x0200
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0x018 303000 I4 = 0x0300
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## Step-by-Step Analysis
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### 1. NOP at Reset Vector (0x000000)
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The processor starts at PM address 0x0000. A NOP here means the
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real entry point follows immediately, or there is a JUMP to the
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main code further ahead.
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### 2. Register Initialization (Type 6: Dreg = Imm16)
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0x412340 -> AX0 = 0x1234
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Bit layout: 01.00.0001.00100.011.0100.0000
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Bits 23-20 = 0100 (Type 6), bits 3-0 = 0000 (AX0)
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Immediate value in bits 19-4 = 0x1234
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### 3. DAG Setup (Type 7: Reg1/Reg2 = Imm16)
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0x501000 -> I0 = 0x0100
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Bits 23-20 = 0101 (Type 7, REG1), bits 3-0 = 0000 (I0)
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0x303000 -> I4 = 0x0300
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Bits 23-20 = 0011 (Type 7, REG2), bits 3-0 = 0000 (I4)
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This is the classic DAG initialization pattern: set index registers
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to buffer start addresses, then set modifier and length registers.
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### 4. Compute Instructions (Type 9: ALU/MAC)
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0x22600F -> AR = AX0 + AY0
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Bits 23-21 = 001, 20-19 = 00 (Type 9)
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AMF = bits 17-13 = 10011 = X+Y
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XOP = bits 10-8 = 000 (AX0), YOP = bits 12-11 = 00 (AY0)
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COND = bits 3-0 = 1111 (TRUE = unconditional)
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0x20800F -> MR = MX0 * MY0 (SS)
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AMF = bits 17-13 = 00100 = X*Y (SS)
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### 5. Jumps (Type 10 vs 10a)
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Type 10 (conditional, 13-bit address):
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0x180010 -> IF EQ JUMP 0x0001
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Bits 23-20 = 0001, 19 = 1, 18 = 0 (Type 10)
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B = bit 17 = 0 (no delay slot)
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Address = bits 16-4, COND = bits 3-0 = 0000 (EQ)
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Type 10a (unconditional, 16-bit address):
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0x1C0010 -> JUMP 0x0001
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Bits 23-18 = 000111 (Type 10a)
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Address = bits 17-4 + bits 1-0 as MSBs
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S = bit 2 (0=JUMP, 1=CALL), B = bit 3 (delayed branch)
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## Patterns to Look For
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- **FIR/IIR kernels**: Tight DO...UNTIL CE loops containing
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Type 1 multifunction instructions (MAC + dual memory read).
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- **Initialization**: Sequences of Type 6/7 loads setting up
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DAG registers (I/M/L) before entering a processing loop.
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- **Data tables in PM**: Regions that disassemble as nonsense
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are likely coefficient tables (24-bit PM data words).
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