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FE1F: Build up a hash value from the screen memory
Used by the routine at FF70.
Input
A On exit, holds the computed hash value
FE1F LD HL,$4000 Start of block
FE22 LD BC,$1B00 Length
FE25 XOR A Initialise A to 0
FE26 XOR (HL) Flip bits against the current byte
FE27 LD D,A Remember new value
FE28 INC HL Point to next byte
FE29 DEC BC Decrease length
FE2A LD A,B Is length 0?
FE2B OR C
FE2C LD A,D Restore A
FE2D JP NZ,$FE26 Loop back if more to do
FE30 RET All done
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