Routines
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C59B: Draw Wall Strips
Used by the routine at DrawCellWithWalls.
Draws two wall tiles, one above the other, for a step along the edge of a cell. A bit in ColumnFlags is set to record that the column has a wall in it, and if it already has one the step is skipped. The horizontal position in ScreenHorizontalPosition chooses the bit, and is written into the BIT instruction at DrawWallStrips_Test and the SET instruction at C5BF. The lower tile is drawn at ScreenVerticalPosition by DrawWallStrip, and the upper tile 40 above it. The step is skipped if either position is off the screen. It always moves on to the next two tiles of the list.
Input
BC The count of steps in B
IY The address of the first tile of the step in its list
Return unless the column is on the screen.
DrawWallStrips C59B LD HL,($A24E) Fetch ScreenHorizontalPosition.
C59E LD A,H Jump to DrawWallStrips_Skip if it is off the screen.
C59F AND A
C5A0 JR NZ,DrawWallStrips_Skip
Keep the position and change the BIT and SET instructions.
C5A2 LD A,L Keep the horizontal position in E.
C5A3 LD E,A
C5A4 RRCA Move the bits which say which sixteen pixels it is in...
C5A5 AND %00111000 ...to make the number of a bit...
C5A7 OR %01000110 ...in the second byte of a BIT instruction.
C5A9 LD ($C5BC),A Store it as the second byte of the BIT instruction at DrawWallStrips_Test.
C5AC OR %10000000 Change it to the second byte of a SET instruction.
C5AE LD ($C5C0),A Store it as the second byte of the SET instruction at C5C0.
Choose the byte of ColumnFlags.
C5B1 LD A,L Fetch the horizontal position again.
C5B2 RLCA Move the top bit of the horizontal position...
C5B3 AND %00000001 ...into a single bit.
C5B5 LD HL,$A248 Point HL at ColumnFlags.
C5B8 JR Z,DrawWallStrips_Test Jump to DrawWallStrips_Test if the position is in the first half of the screen.
C5BA INC HL Point HL at the second byte of the flags.
Test and set the flag for this column.
DrawWallStrips_Test C5BB BIT 0,(HL) Jump to DrawWallStrips_Skip if there is already a wall there.
C5BD JR NZ,DrawWallStrips_Skip
C5BF SET 0,(HL) Set the bit.
Draw the lower tile.
C5C1 LD HL,($A250) Fetch ScreenVerticalPosition.
C5C4 LD A,H Jump to DrawWallStrips_Skip if it is off the screen.
C5C5 AND A
C5C6 JR NZ,DrawWallStrips_Skip
C5C8 LD D,L Keep the low byte of the vertical position in D.
C5C9 PUSH BC Stash the count of steps and the position on the stack.
C5CA PUSH DE
C5CB CALL DrawWallStrip Call DrawWallStrip.
C5CE POP DE Restore the position.
C5CF INC IY Move on to the next tile of the list.
Draw the upper tile.
C5D1 LD HL,($A250) Fetch ScreenVerticalPosition.
C5D4 LD BC,$0040 Set BC to 40.
C5D7 ADD HL,BC Move up by that much.
C5D8 LD A,H Jump to DrawWallStrips_RestoreCount if it is off the screen.
C5D9 AND A
C5DA JR NZ,DrawWallStrips_RestoreCount
C5DC LD D,L Keep the low byte of the vertical position in D.
C5DD CALL DrawWallStrip Call DrawWallStrip.
Restore the count and move on.
DrawWallStrips_RestoreCount C5E0 POP BC Restore the count of steps.
C5E1 INC IY Move on to the next tile of the list.
C5E3 RET Return.
There is nothing to draw, so skip both of the tiles.
DrawWallStrips_Skip C5E4 INC IY Move on past the two tiles of the list.
C5E6 INC IY
C5E8 RET Return.
View the equivalent code in;
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