Routines
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9F2B: Seven shifts, entered part-way down
Not a loop: seven copies of the same two instructions, one after another, so that jumping in n pairs from the top performs 7 - n shifts with no counter and no branch. The whole cost of positioning a sprite to the pixel is one patched jump.
SHIFT_CHAIN 9F2B ADD HL,HL Seven ADD HL,HL / ADC A,A pairs. Entering at the top shifts seven times, two bytes in six, and so on.
9F2C ADC A,A
9F2D ADD HL,HL
9F2E ADC A,A
9F2F ADD HL,HL
9F30 ADC A,A
9F31 ADD HL,HL
9F32 ADC A,A
9F33 ADD HL,HL
9F34 ADC A,A
9F35 ADD HL,HL
9F36 ADC A,A
9F37 ADD HL,HL
9F38 ADC A,A
9F39 EX DE,HL Whatever the shift, the row ends up here to be XORed onto the screen.
9F3A EX (SP),HL
9F3B XOR (HL)
9F3C LD (HL),A
9F3D INC L
9F3E JR XOR_BYTE
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