preservaction-prologue-epilogue.md

Project Preservaction

Table Of Contents

Prologue/Epilogue Variants

It's interesting to see that the prologue and epilogue sections belong together as pairs: the prologue sets up the environment for the cheat data blocks, and the epilogue restores the original state and jumps back into the game so that execution can continue uninterrupted.

Taken in context together with the cheat data blocks, they form tiny self-contained opcode programs that temporarily take control of the CPU to modify memory before returning to normal gameplay.

Variant 1 (Cybernator UK)

Prologue

8F004200

Byte	Opcode	Meaning
8F	STA (absolute long)	Store A to address $004200
00 42 00	Operand	The NMI/timer control register in I/O space

This prologue doesn't perform any CPU setup - instead, it preserves the original game logic. These four bytes correspond to the instruction that was overwritten by the trainer hook. The trainer replays this instruction first to maintain the game's intended behavior before applying any cheat modifications.

Epilogue

5C7C8280

Byte	Opcode	Meaning
5C	JML	Long jump to absolute 24-bit address.
7C 82 80	Operand	Jump target $80:827C (start address + 4 bytes).

The epilogue performs a long jump back to the instruction immediately following the hook point at $80:827C. This returns control to the game's normal execution flow once all cheat operations have completed, ensuring the gameplay continues seamlessly without disruption.

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Variant 2 (Mystical Ninja 2 JP)

Prologue

E230EAEA

Byte	Opcode	Effect
E2 30	SEP #$30	Set processor status bits for 8-bit accumulator and 8-bit index registers (sets M and X flags).
EA EA	NOP; NOP	No operation (padding / alignment).

This prologue prepares the CPU for the cheat operations by switching both the accumulator and index registers to 8-bit mode. This guarantees that all immediate load instructions (A9 vv) in the cheat data blocks are interpreted as 8-bit values. (The two NOP bytes serve only as padding and alignment to maintain consistent block size and timing.)

Epilogue

C2105CB8
87800000

Byte	Opcode	Meaning
C2 10	REP #$10	Clear X flag <- set 16-bit index registers again.
5C B8 87 80	JML $80:87B8	Return to address $80:87B8 (entry + 4).
00 00	-	Padding.

The epilogue restores the index registers to 16-bit mode, reversing the changes made by the prologue. After all cheat data blocks have executed, the routine performs a long jump back to $80:87B8, the instruction immediately following the hook point. This ensures the trainer exits cleanly and the game resumes normal execution without any disruption.

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Variant 3 (Super Taz Mania US)

Prologue

08E220EA

Byte	Opcode	Meaning
08	PHP	Push Processor Status Register onto the stack (saves current flags).
E2 20	SEP #$20	Set only the M flag <- forces 8-bit accumulator, leaves index registers untouched.
EA	NOP	Padding byte.

This prologue safeguards the current processor state by saving the status flags to the stack, then switches the CPU into 8-bit accumulator mode to ensure the cheat data blocks execute consistently. (The single NOP instruction acts as harmless filler for alignment and timing consistency within the trainer code.)

Epilogue

2868AF11
42005C15
FDC00000

Byte	Opcode	Meaning
28	PLP	Pull Processor Status (restore flags).
68	PLA	Restore Accumulator.
5C 15 FD C0	JML $C0:FD15	Jump to same offset as start, different bank (ROM mapping).
00 00	-	Padding.

The epilogue cleanly restores both the processor flags and the accumulator, reversing the changes made by the prologue. It then performs a long jump to $C0:FD15, which corresponds to the same offset as the hook entry ($00:FD15) but within the active ROM execution bank. This ensures that once all cheat operations are complete, the game continues seamlessly from its original instruction flow.

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Variant 4 (Wolfenstein 3D)

Prologue

4808E230

Byte	Opcode	Meaning
48	PHA	Push Accumulator onto the stack (save A).
08	PHP	Push Processor Status Register onto the stack (save flags).
E2 30	SEP #$30	Set bits M and X → force 8-bit accumulator and 8-bit index registers.

This prologue saves the accumulator and CPU status flags, then switches the processor into 8-bit mode for both the accumulator and index registers. This ensures that the following cheat data blocks - which use 8-bit immediate loads (A9 vv) - operate correctly regardless of the game's current CPU mode.

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Epilogue

2868AF11
42005C15
FDC00000

Byte	Opcode	Meaning
28	PLP	Pull Processor Status from stack (restore flags).
68	PLA	Pull Accumulator from stack (restore A).
AF 11 42 00	LDA $004211	Read hw register $004211 (NMI flag / joypad latch). Commonly a safe timing read before returning.
5C 15 FD C0	JML $C0:FD15	Long jump back to $C0:FD15 - the same offset as the hook address, but in the executable ROM bank.
00 00	-	Padding / alignment bytes.

The epilogue restores both the accumulator and the status register, performs a harmless read from a hardware register (a timing-safe filler operation), and then jumps back to $C0:FD15. This address corresponds to the same offset as the hook entry ($00:FD15) but in the active ROM bank. Together with the prologue, this forms a complete, reversible routine: the prologue establishes a controlled CPU environment, the cheat data blocks perform memory writes, and the epilogue restores the state and returns execution to the game.