Routines
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30822: Prepare for a cutscene
Used by the routine at 31414. On entry, A holds the LSB of the address of the screen refresh buffer byte that corresponds to the top row of the portion of the play area to be displayed during the cutscene.
Input
A Screen refresh buffer address LSB
D New y-coordinate for the topmost row of the play area on screen
E New x-coordinate for the leftmost column of the play area on screen
30822 PUSH DE Save the play area coordinates briefly
30823 PUSH AF Save the screen refresh buffer address LSB briefly
30824 CALL 64599 Hide the play area
30827 POP AF Restore the screen refresh buffer address LSB to A
30828 LD L,A Point HL at the first screen refresh buffer (SRB) byte that will need updating later
30829 POP DE Restore the play area coordinates to DE
30830 PUSH HL Save the SRB address for now
30831 AND 3 Compute Sam's new x-coordinate (x+5, where x is the x-coordinate of the leftmost column of the portion of the play area to be displayed)
30839 LD C,A C=Sam's new x-coordinate
30840 LD A,D Set the new y-coordinate of the topmost row of the play area on screen
30841 LD (32767),A
30844 LD A,L A=screen refresh buffer address LSB
30845 SUB 12 Compute Sam's new y-coordinate
30847 AND 252
30849 RRCA
30850 RRCA
30852 LD HL,58880 Point HL at byte 0 of Sam's character buffer
30855 LD (HL),0 Set Sam's animatory state to 0 (standing/walking phase 1)
30857 INC L L=1
30858 LD (HL),C Set Sam's x-coordinate
30859 INC L L=2
30860 LD (HL),A Set Sam's y-coordinate
30861 INC L L=4
30862 INC L
30863 LD (HL),1 Set Sam's z-coordinate
Next we cycle character buffer groups forwards or backwards as appropriate to correspond with the portion of the play area that will be on screen.
30865 LD HL,61879 Place a RET instruction at 61879 so that the screen will not scroll right
30868 LD (HL),201
30870 LD L,224 Place a RET instruction at 61920 so that the screen will not scroll left
30872 LD (HL),201
30874 PUSH HL
30875 LD HL,32766 Collect X (the x-coordinate of the leftmost column of the play area on screen) from 32766
30878 LD A,(HL)
30879 CP E Does X match the target x-coordinate (in E)?
30880 JR Z,30905 Jump if so
30882 PUSH DE Save the target x-coordinate briefly
30883 JR C,30892 Jump if X<E
30885 CALL 61838 Cycle a character buffer group forwards (but without scrolling the screen)
30888 LD A,248 A=-8
30890 JR 30897
30892 CALL 61882 Cycle a character buffer group backwards (but without scrolling the screen)
30895 LD A,8
30897 POP DE Restore the target x-coordinate to E
30898 LD HL,32766 Add 8 to or subtract 8 from X (the x-coordinate of the leftmost column of the play area on screen, stored at 32766)
30902 LD (HL),A
30903 JR 30879 Jump back to check whether we need to cycle any more character buffers backwards or forwards
30905 POP HL
30906 LD (HL),195 Restore the JP instruction at 61920
30908 LD L,183 Restore the JP instruction at 61879
30910 LD (HL),195
Next we determine whether any lights near Sam need to be switched on and any window blinds need to be raised.
30912 LD H,230 230=Sam
30914 CALL 63138 Collect in C the x-coordinate of the front column of Sam's sprite
30917 LD B,192 Is there a light switch next to Sam?
30919 CALL 62548
30922 JR Z,30955 Jump if not (we can reveal the play area now)
30924 BIT 5,A Set the zero flag if the light switch affects the light in only one window or window-pair
30926 LD A,(BC) A=window flags for Sam's location
30927 JR Z,30936 Jump if the light switch affects the light in only one window or window-pair
30929 BIT 5,A Is the light switch in the 'on' position?
30931 CALL NZ,62630 If not, switch the light on
30934 JR 30955 Jump forward to reveal the play area
30936 BIT 1,A Is there a right-hand window here with a blind?
30938 JR Z,30942 Jump if not
30940 RES 0,A Reset bit 0: blind raised in the right-hand window
30942 BIT 7,A Is there a left-land or single window here with a blind?
30944 JR Z,30948 Jump if not
30946 RES 6,A Reset bit 6: blind raised in the left-hand or only window
30948 RES 5,A Reset bit 5: light switch on
30950 LD (BC),A Save the new window flags (blinds raised, light switch on)
30951 XOR A Clear A to prevent the following call from changing the window flags (we've already updated them)
30952 CALL 62617 Update the SRB for the window
Finally we can reveal the portion of the play area in which Sam will walk up and down until the cutscene message has been displayed and the game resumes.
30955 POP HL Restore the SRB address to HL
30956 LD B,6 The portion is 6 rows high
30958 LD A,L Initialise A for the loop that follows
30959 LD L,A Set all bits in the relevant SRB bytes
30960 LD (HL),255