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Routines |
| Prev: E967 | Up: Map | Next: E9EF |
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Used by the routine at GameStart.
Compare the primary and secondary display buffers and blit changed tiles to the Spectrum screen. Tiles with attribute bit 07 set are horizontally mirrored via BitmapMirrorLookup (see Blit_DisplayBufferRowLoop).
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Choose which display buffer is "live" versus "previous" from DisplayBufferSelectFlag.
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| Blit_DisplayBuffer | E991 | LD IX,$5800 | Point IX at the attribute file (5800 (attribute buffer)). | |
| E995 | LD HL,$63A0 | Point HL at Primary Display Buffer (PrimaryDisplayBuffer). | ||
| E998 | LD DE,$6B20 | Point DE at Secondary Display Buffer (SecondaryDisplayBuffer). | ||
| E99B | LD A,($72EA) | Load A with the display-buffer select flag (DisplayBufferSelectFlag). | ||
| E99E | OR A | Is the display-buffer select flag (DisplayBufferSelectFlag) zero? If so, jump to Blit_DisplayBuffer_AfterSelect. | ||
| E99F | JR Z,Blit_DisplayBuffer_AfterSelect | |||
| E9A1 | EX DE,HL | Exchange DE and HL. | ||
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After selecting primary/secondary, walk 14 rows × 20 columns for dirty tiles.
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| Blit_DisplayBuffer_AfterSelect | E9A2 | PUSH DE | Copy DE into IY (using the stack). | |
| E9A3 | POP IY | |||
| E9A5 | LD B,$14 | Load B with 14. | ||
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Begin one display-buffer row of 20 tile comparisons.
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| Blit_DisplayBufferLoop | E9A7 | PUSH BC | Stash BC on the stack. | |
| E9A8 | LD B,$20 | Load B with loop count 20. | ||
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Compare one tile's graphic-set / type / attribute triple against the secondary buffer.
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| Blit_DisplayBufferLoopB | E9AA | PUSH BC | Stash BC on the stack. | |
| E9AB | LD A,(HL) | Fetch the next primary-buffer graphic-set byte. | ||
| E9AC | INC HL | Advance the display-buffer pointer in HL by one byte. | ||
| E9AD | OR A | Is A zero? If so, jump to Blit_DisplayBufferLoopB6. | ||
| E9AE | JP Z,Blit_DisplayBufferLoopB6 | |||
| E9B1 | LD C,A | Keep the remaining count in C. | ||
| E9B2 | LD E,$00 | Load E with 00. | ||
| E9B4 | CP (IY+$00) | Does A match the referenced entity's type? If not, jump to Blit_DisplayBufferLoopB2. | ||
| E9B7 | JR NZ,Blit_DisplayBufferLoopB2 | |||
| E9B9 | INC E | Advance the low byte of the pointer/offset in DE. | ||
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Match the tile's entity-type byte; count matches in E.
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| Blit_DisplayBufferLoopB2 | E9BA | LD A,(HL) | Fetch the next script byte. | |
| E9BB | OR A | Is the next script byte zero? If so, jump to Blit_DisplayBufferLoopB6. | ||
| E9BC | JR Z,Blit_DisplayBufferLoopB6 | |||
| E9BE | LD B,A | Keep a copy of A in B. | ||
| E9BF | CP (IY+$01) | Does A match the referenced entity's flags byte 1? If not, jump to Blit_DisplayBufferLoopB3. | ||
| E9C2 | JR NZ,Blit_DisplayBufferLoopB3 | |||
| E9C4 | INC E | Advance the low byte of the pointer/offset in DE. | ||
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Match the tile's attribute byte; blit via Blit_DisplayBufferRow unless both prior bytes already matched.
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| Blit_DisplayBufferLoopB3 | E9C5 | INC HL | Advance the display-buffer pointer in HL by one byte. | |
| E9C6 | LD A,(HL) | Fetch the next script byte. | ||
| E9C7 | CP (IY+$02) | Does A match the referenced entity's flags byte 2? If not, jump to Blit_DisplayBufferLoopB4. | ||
| E9CA | JR NZ,Blit_DisplayBufferLoopB4 | |||
| E9CC | BIT 1,E | Is bit 01 of E set? If so, jump to Blit_DisplayBufferLoopB5. | ||
| E9CE | JR NZ,Blit_DisplayBufferLoopB5 | |||
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Blit this changed tile to the Spectrum screen through Blit_DisplayBufferRow.
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| Blit_DisplayBufferLoopB4 | E9D0 | PUSH HL | Stash HL on the stack. | |
| E9D1 | CALL Blit_DisplayBufferRow | Call Blit_DisplayBufferRow to blit Display Buffer Row. | ||
| E9D4 | POP HL | Restore HL from the stack. | ||
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Advance past this tile's triple and move to the next attribute column.
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| Blit_DisplayBufferLoopB5 | E9D5 | INC HL | Advance the display-buffer pointer in HL by one byte. | |
| E9D6 | INC IX | Advance IX to the next attribute-file column. | ||
| E9D8 | LD DE,$0003 | Load DE with 0003. | ||
| E9DB | ADD IY,DE | IY+=DE. | ||
| E9DD | POP BC | Restore BC from the stack. | ||
| E9DE | DJNZ Blit_DisplayBufferLoopB | Decrease the loop counter in B and jump to Blit_DisplayBufferLoopB if more iterations remain. | ||
| E9E0 | POP BC | Restore BC from the stack. | ||
| E9E1 | DEC B | Decrease B by one. | ||
| E9E2 | JP NZ,Blit_DisplayBufferLoop | Jump to Blit_DisplayBufferLoop to continue if the comparison did not match. | ||
| E9E5 | RET | Return. | ||
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Treat a zero graphic-set as "blank": force bit 06 and skip the blit.
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| Blit_DisplayBufferLoopB6 | E9E6 | INC HL | Advance the display-buffer pointer in HL by one byte. | |
| E9E7 | LD A,(HL) | Fetch the next script byte. | ||
| E9E8 | SET 6,A | Set bit 06 of A. | ||
| E9EA | LD (IX+$00),A | Set the current entity's entity type. | ||
| E9ED | JR Blit_DisplayBufferLoopB5 | Jump to Blit_DisplayBufferLoopB5 to continue the loop. | ||
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