Routines
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C381: Draw Block Shape
Used by the routine at DrawBlock.
Copies the four step bytes of the shape into this routine and sign-extends each one, then draws the block. A shape below 09 and shape 11 is drawn here a row at a time. Shapes 09 to 10 are drawn by DrawBlockShapeThree and shapes 12 to 15 by DrawBlockShapeTwo. DrawBlock patches in the shape number and the row count.
Input
IY The shape
Each step byte of the shape is sign-extended into a word that the row loops add to the screen address and the sprite address.
DrawBlockShape C381 LD A,(IY+$01) Read the shape's first step.
C384 LD ($C3DC),A Keep it as the low byte of a row step.
C387 RLA Sign-extend it when its top bit is set.
C388 LD A,$00
C38A JR NC,DrawBlockShape_High
C38C DEC A The high byte is FF.
DrawBlockShape_High C38D LD ($C3DD),A Keep the high byte of the row step.
C390 LD A,(IY+$02) Read the shape's second step.
C393 LD ($C3E1),A Keep it as the low byte of the other row step.
C396 RLA Sign-extend it when its top bit is set.
C397 LD A,$00
C399 JR NC,DrawBlockShape_High2
C39B DEC A The high byte is FF.
DrawBlockShape_High2 C39C LD ($C3E2),A Keep that high byte.
C39F LD A,(IY+$04) Read the shape's fourth byte.
C3A2 LD ($C3EA),A Keep it as the low byte of a column step.
C3A5 RLA Sign-extend it when its top bit is set.
C3A6 LD A,$00
C3A8 JR NC,DrawBlockShape_High3
C3AA DEC A The high byte is FF.
DrawBlockShape_High3 C3AB LD ($C3EB),A Keep that high byte.
C3AE LD A,(IY+$03) Read the shape's third byte.
C3B1 LD ($C3F0),A Keep it as the low byte of the other column step.
C3B4 RLA Sign-extend it when its top bit is set.
C3B5 LD A,$00
C3B7 JR NC,DrawBlockShape_High4
C3B9 DEC A The high byte is FF.
DrawBlockShape_High4 C3BA LD ($C3F1),A Keep that high byte.
C3BD LD A,$00 Read the shape number. The byte is patched.
C3BF CP $09 A shape below 09 is drawn here.
C3C1 JR C,DrawBlockShape_Second
C3C3 CP $11 Shapes 09 to 10 use the third shape.
C3C5 JP C,DrawBlockShapeThree
DrawBlockShape_Second C3C8 CP $12 Shape 11 is drawn here.
C3CA JR C,DrawBlockShape_Rows
C3CC CP $16 Shapes 12 to 15 use the second shape.
C3CE JP C,DrawBlockShapeTwo
DrawBlockShape_Rows C3D1 PUSH BC Remember the column count and both addresses.
C3D2 PUSH HL
C3D3 PUSH DE
C3D4 LD B,$00 Take the patched row count.
DrawBlockShape_Loop C3D6 PUSH BC Remember how many rows remain.
C3D7 CALL DrawBlockRow Draw this row.
This entry point is used by the routine at DrawBlockShapeTwo.
DrawBlockShape_Resume C3DA EXX Step both addresses on by the row steps.
C3DB LD BC,$0000
C3DE ADD HL,BC
C3DF EX DE,HL
C3E0 LD BC,$0000
C3E3 ADD HL,BC
C3E4 EX DE,HL
C3E5 POP BC Bring the row count back.
C3E6 DJNZ DrawBlockShape_Loop Repeat for the remaining rows.
C3E8 POP HL Restore the screen address.
C3E9 LD BC,$0000 Step on by the column step.
C3EC ADD HL,BC
C3ED EX DE,HL Switch the addresses.
C3EE POP HL Restore the sprite address.
C3EF LD BC,$0000 Step it on by the other column step.
C3F2 ADD HL,BC
C3F3 POP BC Bring the column count back.
C3F4 DJNZ DrawBlockShape_Rows Repeat for the remaining columns.
C3F6 RET Return.
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