The PMBASE write-only register sets the base of memory occupied by Player-Missile Graphics. According to bit 4 of DMACTL:

• when it is 0, the PMG memory area occupies 1024 ($400) bytes of memory and only upper 6 bits of PMBASE matter, so the PM memory area can be set to begin of 1KB boundary.
• when it is 1, the PMG memory area occupies 2048 ($800) bytes of memory and only upper 5 bits of PMBASE matter, so the PM memory area can be set to begin of 2KB boundary.

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