Routines
Prev: B278 Up: Map Next: B2C5
B29A: Build Lookup Tables
Used by the routine at RunGame.
Builds the tables that the sprite drawing and flipping code uses. ReverseTable gets the reversed bits of each byte value. The shift tables fill the pages from ShiftTables to FFFF: for each of the seven shifts, one page holds the low byte and the page below it holds the high byte of every byte value shifted left by that number of bits, with all the bits inverted.
Build the shift tables, one pair of pages for each shift of one to seven bits.
BuildLookupTables B29A LD L,$00 Start with the byte value 00 in L.
BuildLookupTables_NextValue B29C LD D,$00 Clear D, the high byte of the shifted value.
B29E LD E,L Copy the byte value to E, the low byte of the shifted value.
B29F LD H,$FF Point H at the top page, FF.
B2A1 LD B,$07 Set a counter in B of seven shifts.
BuildLookupTables_NextShift B2A3 SLA E Shift the value left by one bit more.
B2A5 RL D
B2A7 LD A,E Fetch the low byte of the value.
B2A8 CPL Invert the low byte of the shifted value.
B2A9 LD (HL),A Write the inverted low byte to the shift table.
B2AA DEC H Move down to the next page.
B2AB LD A,D Fetch the high byte of the value.
B2AC CPL Invert the high byte of the shifted value.
B2AD LD (HL),A Write the inverted high byte to the shift table.
B2AE DEC H Move down to the next page.
B2AF DJNZ BuildLookupTables_NextShift Decrease the shift counter and loop until all seven shifts are done.
B2B1 INC L Move on to the next byte value.
B2B2 JR NZ,BuildLookupTables_NextValue Jump to BuildLookupTables_NextValue until all 0100 byte values are done.
Build the table of bytes with the order of their bits reversed.
B2B4 LD HL,$F100 Start the table of reversed bytes at ReverseTable.
BuildLookupTables_Reverse B2B7 LD D,L Fetch the byte value to be reversed in D.
B2B8 LD B,$08 Set a counter in B of eight bits.
BuildLookupTables_ReverseBit B2BA SRL D Shift the lowest bit of D out into the carry flag.
B2BC RL E Shift it into E, so the bits arrive in the opposite order.
B2BE DJNZ BuildLookupTables_ReverseBit Decrease the bit counter and loop until all eight bits are done.
B2C0 LD (HL),E Write the reversed value to the table.
B2C1 INC L Move on to the next byte value.
B2C2 JR NZ,BuildLookupTables_Reverse Jump to BuildLookupTables_Reverse until all 0100 values are done.
B2C4 RET Return once both tables are built.
View the equivalent code in;
Prev: B278 Up: Map Next: B2C5