Routines
Prev: 63171 Up: Map Next: 63470
63343: XOR Blit Sprite
Used by the routines at AnimateIntroSprites and DrawIntroSprites.
Takes the two-byte position entry at HL (byte 0 = Y pixel row, byte 1 = X byte column) and the attribute byte in E (shadow; bit 7 set = erase mode, bit 7 clear = draw mode). Computes each screen pixel address by reading the row base from the screen row LUT at 35840 (indexed by Y) and adding the X column byte offset stored in shadow D. XORs 4 bytes of the fixed bitmap at 27900 into consecutive screen pixel memory for each of 32 pixel rows (4 attribute-cell rows of 8 pixel rows each). After the first pixel row of each attribute-cell row, derives the attribute memory high byte from the pixel address high byte and writes the attribute byte to 4 consecutive attribute cells — suppressed in erase mode.
Input
HL Pointer to the two-byte position entry (Y pixel row at +0, X byte column at +1)
Initialise the bitmap source (HL=27900 via EX DE,HL) and compute IX into the screen row LUT at 35840 indexed by the Y pixel row. The row is shifted left into the LUT index directly (IX=35840 + Y × 2). Store the X column byte offset in shadow D and set the outer loop count (C=4 shadow, one iteration per attribute-cell row of 8 pixel rows).
XorBlitSprite 63343 LD DE,27900 DE=27900 (bitmap source base address; becomes HL via EX DE,HL).
63346 LD B,70 B=70 (initial high byte; becomes 140 or 141 after RL B).
63348 LD C,(HL) C=Y pixel row (byte 0 of the position entry).
63349 INC HL Advance HL to byte 1 (X byte column).
63350 SLA C SLA C; carry = bit 7 of Y (selects high byte 140 or 141).
63352 RL B RL B; B=140 (or 141 if carry set) — high byte of LUT pointer.
63354 PUSH BC Stash BC on the stack.
63355 LD A,(HL) A=X byte column (byte 1 of the position entry).
63356 EXX Switch to the shadow registers.
63357 LD D,A D=X byte column offset (held in shadow across all loop iterations).
63358 EXX Switch back to the normal registers.
63359 POP IX POP IX; IX=35840 + Y × 2 (pointer into the screen row LUT).
63361 EX DE,HL HL=27900 (bitmap source pointer); DE=old HL (discarded).
63362 EXX Switch to the shadow registers.
63363 LD C,4 C=4 (shadow, outer loop count: 4 attribute-cell rows).
63365 EXX Switch back to the normal registers.
Outer loop (4 iterations, one per attribute-cell row of 8 pixel rows). Compute the screen pixel address for the first row of this attribute cell: read shadow D (X column), add the LUT low byte from *(IX) to give E, then read the LUT high byte from *(IX+1) into D. Advance IX by two to the next LUT entry.
XorBlitSprite_OuterLoop 63366 EXX Switch to the shadow registers.
63367 LD A,D A=D (X byte column offset).
63368 EXX Switch back to the normal registers.
63369 ADD A,(IX+0) A += *(IX) (LUT low byte); low byte of screen pixel address.
63372 INC IX Advance IX to the LUT high byte.
63374 LD E,A E=A (low byte of screen pixel address).
63375 LD D,(IX+0) D=*(IX) (high byte of screen pixel address from LUT).
63378 INC IX Advance IX to the next LUT entry.
XOR 4 consecutive bytes of the bitmap at (HL)–(HL+3) into screen pixel memory at (DE)–(DE+3), advancing both pointers.
63380 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63381 XOR (HL)
63382 LD (DE),A
63383 INC DE
63384 INC HL
63385 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63386 XOR (HL)
63387 LD (DE),A
63388 INC DE
63389 INC HL
63390 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63391 XOR (HL)
63392 LD (DE),A
63393 INC HL
63394 INC DE
63395 LD A,(DE) *(DE) ^= *(HL); advance HL.
63396 XOR (HL)
63397 LD (DE),A
63398 INC HL
Derive the attribute memory address high byte from the current pixel address high byte in D. Three right-rotations expose the screen-third index in bits 0–1; OR with 88 gives the attribute area high byte (88–91).
63399 LD A,D D = (RRCA × 3 of D) AND 3 OR 88 (attribute address high byte).
63400 RRCA
63401 RRCA
63402 RRCA
63403 AND 3
63405 OR 88
63407 LD D,A
Test the erase-mode flag (bit 7 of shadow E). If set, skip writing the attribute byte so that the background colour is preserved during erase.
63408 EXX Switch to shadow; A=E (attribute byte); switch back.
63409 LD A,E
63410 EXX
63411 BIT 7,A Test bit 7 of A (erase-mode flag).
63413 JR NZ,XorBlitSprite_InnerSetup If in erase mode, jump to XorBlitSprite_InnerSetup (skip attribute write).
63415 LD (DE),A Write attribute byte to (DE), (DE−1), (DE−2) and (DE−3).
63416 DEC DE
63417 LD (DE),A
63418 DEC DE
63419 LD (DE),A
63420 DEC DE
63421 LD (DE),A
XorBlitSprite_InnerSetup 63422 EXX Switch to shadow; B=7 (remaining pixel rows in this attribute-cell row); switch back.
63423 LD B,7
63425 EXX
Inner loop (7 iterations, one per remaining pixel row in this attribute-cell row). Compute the screen address from the LUT and XOR 4 bitmap bytes into pixel memory. No attribute write.
XorBlitSprite_InnerLoop 63426 EXX Switch to the shadow registers.
63427 LD A,D A=D (X byte column offset).
63428 EXX Switch back to the normal registers.
63429 ADD A,(IX+0) A += *(IX) (LUT low byte for this pixel row).
63432 INC IX Advance IX to the LUT high byte.
63434 LD E,A E=A (low byte of screen pixel address).
63435 LD D,(IX+0) D=*(IX) (high byte of screen pixel address from LUT).
63438 INC IX Advance IX to the next LUT entry.
63440 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63441 XOR (HL)
63442 LD (DE),A
63443 INC DE
63444 INC HL
63445 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63446 XOR (HL)
63447 LD (DE),A
63448 INC DE
63449 INC HL
63450 LD A,(DE) *(DE) ^= *(HL); advance DE and HL.
63451 XOR (HL)
63452 LD (DE),A
63453 INC HL
63454 INC DE
63455 LD A,(DE) *(DE) ^= *(HL); advance HL.
63456 XOR (HL)
63457 LD (DE),A
63458 INC HL
63459 EXX Switch to shadow; decrement B (inner pixel row counter); switch back.
63460 DEC B
63461 EXX
63462 JR NZ,XorBlitSprite_InnerLoop Loop back to XorBlitSprite_InnerLoop until all 7 pixel rows are blitted.
63464 EXX Switch to shadow; decrement C (outer attribute-row counter); switch back.
63465 DEC C
63466 EXX
63467 JR NZ,XorBlitSprite_OuterLoop Loop back to XorBlitSprite_OuterLoop until all 4 attribute-cell rows are blitted.
63469 RET Return.
Prev: 63171 Up: Map Next: 63470