Routines
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BD5E: Draw 3-Wide Sprite Column
Used by the routine at Draw_Sprites.
Draws a three-column-wide sprite to the screen buffer. Looks up the sprite graphic data from the frame table at Sprite_01, then shifts and ORs the pixel data across two adjacent screen columns, repeated for two column pairs (covering three columns total). The shift amount is self-modified at BD82.
Input
A Shift amount (self-modified into the code)
HL Screen buffer address
IX Pointer to sprite data (IX+03 = one-indexed frame number)
Look up the sprite graphic data address. Each frame is 20 bytes.
Draw_3Wide_Sprite_Column BD5E LD ($BD82),A Self-modify the shift jump offset at BD82.
BD61 PUSH HL Stash the screen buffer address on the stack.
BD62 LD DE,$AB3B Point DE at Sprite_01 (3-wide sprite frame table base).
BD65 LD L,(IX+$03) Load the frame ID from *IX+03 and decrement to make it zero-indexed.
BD68 DEC L
BD69 LD H,$00 Multiply the frame index by 20 and add to the table base to get the graphic data address.
BD6B ADD HL,HL
BD6C ADD HL,HL
BD6D ADD HL,HL
BD6E ADD HL,HL
BD6F ADD HL,HL
BD70 ADD HL,DE
BD71 EX DE,HL Exchange so DE points to the sprite graphic data.
BD72 POP HL Restore the screen buffer address from the stack.
Draw two column pairs, each 10 pixels tall, covering three screen columns. C holds a mask of 07 used to detect character cell boundaries for screen address adjustment.
BD73 LD B,$02 Set the column pair counter to 02 in B.
Draw_3Wide_Column_Loop BD75 LD C,$07 Set the row boundary mask to 07 in C.
Draw_3Wide_Sprite_Column_0 BD77 PUSH BC Stash the column pair counter and screen buffer address on the stack.
BD78 PUSH HL
BD79 LD B,$10 Set the row counter to 10 in B.
For each row, read a byte of sprite data, shift it left by the self-modified amount and OR the resulting two bytes across two adjacent screen buffer columns.
Draw_3Wide_Row_Loop BD7B PUSH DE Stash the graphic data pointer and screen buffer address on the stack.
BD7C PUSH HL
Read the sprite data byte and shift it into a 16-bit value in HL.
The shift amount at BD82 was self-modified on entry to control the horizontal pixel alignment.
BD7D LD A,(DE) Load the graphic byte from *DE.
BD7E LD L,A Transfer sprite byte to HL (clearing the high byte).
BD7F LD H,$00
BD81 JR Shift_3Wide_Fine Jump into the shift sequence (self-modified offset controls the number of shifts applied).
Shift_3Wide_Extra BD83 ADD HL,HL Shift HL left one position (extra shift).
Shift_3Wide_Fine BD84 ADD HL,HL Shift HL left seven positions; the total shift spreads the graphic byte across H and L.
BD85 ADD HL,HL
BD86 ADD HL,HL
BD87 ADD HL,HL
BD88 ADD HL,HL
BD89 ADD HL,HL
BD8A ADD HL,HL
OR the shifted sprite data onto the screen buffer across two adjacent columns.
BD8B POP DE Restore the screen buffer address into DE, keeping a copy on the stack.
BD8C PUSH DE
BD8D EX DE,HL Exchange so HL=screen buffer address, DE=shifted sprite data.
BD8E LD A,D OR the high byte of the shifted data onto the first screen column at *HL.
BD8F OR (HL)
BD90 LD (HL),A
BD91 INC L Advance to the next column and OR the low byte onto the second screen column at *HL.
BD92 LD A,E
BD93 OR (HL)
BD94 LD (HL),A
BD95 POP HL Restore the screen buffer address and graphic data pointer from the stack.
BD96 POP DE
Move down one pixel row in the screen buffer and check for a character cell boundary crossing.
BD97 INC H Move down one pixel row in the screen buffer.
BD98 LD A,H Test if we've crossed a character cell boundary (every 08 rows).
BD99 AND C
BD9A CALL Z,Adjust_Screen_Address Call Adjust_Screen_Address to adjust the screen buffer address if a character cell boundary was crossed.
Move to the next sprite data byte and loop.
BD9D INC DE Advance the sprite data pointer.
BD9E DJNZ Draw_3Wide_Row_Loop Decrease the row counter and loop back to Draw_3Wide_Row_Loop until all 10 rows are drawn.
Move to the next column in the screen buffer and loop.
BDA0 POP HL Restore the screen buffer address and column pair counter from the stack.
BDA1 POP BC
BDA2 INC L Move one byte to the right in the screen buffer.
BDA3 DJNZ Draw_3Wide_Sprite_Column_0 Decrease the column pair counter and loop back to Draw_3Wide_Column_Loop until both column pairs are drawn.
BDA5 RET Return.
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