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OAM DMA hijacking: Difference between revisions
→Setup (additional arbitrary code execution)
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2. (Option 2 - Keeps OAM sprites) A more elaborate code may be written to keep OAM sprite functionality. Note the ld a,C3 ld (ff00+46),a works with a delay to keep the program counter in HRAM until FF89's ret (jumping there if the z flag is unset, from the code ld a,28 dec a jr nz,ff86). If the program counter remains in HRAM for the wrong amount of time, the game will freeze after leaving it (for example, if the 28 was changed to a lower value). Hence, any new delay should address this before leaving HRAM, yet if properly accounted for it is possible to run code outside of HRAM in addition to OAM sprites still working.
2. (Option 3 - Keeps OAM sprites but at the cost of potentially freezing the game if the HRAM addresses the player chose to change are overwritten again); Another option is to overwrite other HRAM address that don't often change beginning from a relative jump at FF89 (such as to the money coins amount at FF9F) to run additional code (and with a potential series of other relative jumps across HRAM) which can be ended with a ret, without having to ever leave HRAM as part of the routines the player writes.
Here is an example for Red and Blue which keeps overworld sprites but disables moving the character, as documented by luckytyphlosion:
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