Routines
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82D4: Animate a sprite
Used by the routine at 8277.
Input
HL Pointer to the sprite being updated, as an entry into the buffer at 8029
82D4 LD E,(HL) Put the on-screen address in DE
82D5 INC HL
82D6 LD D,(HL)
82D7 INC HL
82D8 LD C,(HL) Put the frame counter in BC
82D9 INC HL
82DA LD B,$00
82DC LD A,C
82DD BIT 0,(HL) Are we going left or right?
82DF JR Z,$82E8 Jump forward if we're going left
82E1 ADD A,$20 Get the next frame to the right
82E3 LD C,A
82E4 LD A,E
82E5 ADC A,B
82E6 JR $82EF
82E8 SUB $20 Get the next frame to the left
82EA LD C,A
82EB LD A,E
82EC SBC A,B
82ED LD B,$01
82EF LD E,A Store the next frame
82F0 CPL Flip all the bits, but only keep the bottom 5
82F1 AND $1F
82F3 JR NZ,$82FD Move on if we don't need to change direction
A sprite needs changing direction. Don't get the next frame counter, position and direction, but instead just reuse these values and go the other way.
82F5 LD (HL),B Current direction
82F6 DEC HL
82F7 LD C,(HL) Current position
82F8 DEC HL
82F9 DEC HL
82FA LD E,(HL) Current frame counter
82FB JR $8305
Otherwise, get the next set of values to animate with
82FD LD B,(HL) Next direction
82FE DEC HL
82FF LD (HL),C Next position
8300 DEC HL
8301 DEC HL
8302 LD (HL),E Next frame counter
8303 CP $00
Put the sprite on screen
8305 LD HL,$8077 Base address of sprites
8308 ADD HL,BC Add 100 for right-facing, otherwise 0
8309 EX DE,HL Swap this with the screen offset to write to
830A LD C,$01 Repeat this twice for 16 lines
830C LD B,$08 Writing 8 lines at a time
830E LD A,(DE) Get the first half of the line
830F LD (HL),A Put it on screen
8310 INC L Move onto next half on screen
8311 INC DE Move onto next half in sprite buffer
8312 LD A,(DE) Get the second half of the line
8313 LD (HL),A Put it on screen
8314 DEC L Move back to left half on screen
8315 INC DE Move to next line in sprite buffer
8316 INC H Move onto next line on screen
8317 DJNZ $830E Loop round to write the next line
8319 DEC C Done the second half yet?
831A LD BC,$F820 Point to second half of sprite
831D ADD HL,BC
831E JR Z,$830C Loop back if second half not yet done
At this point we are done with all animation and need to resync the tape signal
8320 POP BC Restore flags
8321 POP DE
8322 EXX
8323 LD B,$B0 Timing constant
8325 CALL $8362 Get a edge change from tape
8328 LD A,$16 Loop round again
832A DEC A
832B JR NZ,$832A
832D CALL C,$8362 If we got an edge change, there shouldn't be another one within this time frame
8330 JP NC,$0000 If there was an edge change, the tape is out of sync. Reset.
8333 LD B,$02 Wait for a bit
8335 DJNZ $8335
8337 EXX Increment the animation change flag to compensate for drawing
8338 INC C
8339 INC C
833A INC DE Is anything else queued?
833B LD A,(DE)
833C AND A
833D JP NZ,$8277 Jump back if so
8340 RET Otherwise all done
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