Routines
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41396: Build Flag Mask
Used by the routines at TestFlag, SetFlag, ClearFlag and Handler_RoomExits.
Turns a bit index into the flag address and the bit mask used by TestFlag, SetFlag and ClearFlag.
Input
A Bit index
HL Flag base address
Output
HL Address of the flag byte
A Bit mask
BuildFlagMask 41396 PUSH DE Stash DE and BC on the stack.
41397 PUSH BC
41398 EX DE,HL Switch the flag table base into DE so HL can hold the bit index.
41399 LD L,A Put the bit index into HL.
41400 LD H,0
41402 CALL CalculateBitAddress Call CalculateBitAddress to split the bit index into a byte offset (HL) and a bit number (C).
41405 ADD HL,DE Point HL at the flag byte holding this bit.
41406 LD B,C Use the bit number as the rotate count.
41407 INC B Count one extra rotate so the mask lands on this bit.
41408 XOR A Start the mask with no bits set.
41409 SCF Seed a single set bit for the mask.
BuildFlagMask_0 41410 RLA Build the mask for this flag bit.
41411 DJNZ BuildFlagMask_0 Keep rotating until the set bit reaches this flag's position.
41413 POP BC Restore BC and DE from the stack.
41414 POP DE
41415 RET Return.
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