Routines
Prev: B453 Up: Map Next: B4B1
B48E: Get Buffer Address
Used by the routine at DrawSprite.
Finds the address in the play window buffer of the byte that holds a given pixel position. The address is F400 plus 18 times (AF minus the y position) plus the x position divided by 08.
Input
B y position
C x position
Output
BC Address in the buffer
GetBufferAddress B48E PUSH HL Save HL.
B48F PUSH BC Save the x position.
B490 LD A,B Take the y position, subtract 20 and flip the bits (which is the same as taking it away from 011F), then add 90 to give AF minus the y position.
B491 SUB $20
B493 CPL
B494 ADD A,$90
B496 LD L,A Put the result in HL.
B497 XOR A
B498 LD H,A
B499 LD B,H Copy it into BC.
B49A LD C,L
B49B ADD HL,HL Multiply by 18: double it, add the original, then double three times.
B49C ADD HL,BC
B49D ADD HL,HL
B49E ADD HL,HL
B49F ADD HL,HL
B4A0 POP BC Restore the x position into C.
B4A1 LD B,A Clear the high byte of the x position.
B4A2 SRL C Divide the x position by 08.
B4A4 SRL C
B4A6 SRL C
B4A8 ADD HL,BC Add it to the offset.
B4A9 LD BC,$F400 Add the base of the buffer.
B4AC ADD HL,BC
B4AD LD C,L Put the address into BC.
B4AE LD B,H
B4AF POP HL Restore HL.
B4B0 RET Return.
Prev: B453 Up: Map Next: B4B1