Routines
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621E: Control the sniper while active
The address of this interruptible subcommand routine is placed into bytes 0x0E and 0x0F of the sniper's buffer by the routine at 62DB. It controls the sniper from the moment he appears until he hides again.
Input
H 0xE3 (sniper)
621E LD L,\$02 A=sniper's y-coordinate
6220 LD A,(HL)
6221 LD L,\$11 Point HL at byte 0x11 of the sniper's buffer
6223 INC (HL) Is it zero (meaning the sniper has not fired recently)?
6224 DEC (HL)
6225 JR Z,\$6228 Jump if so
6227 DEC (HL) Otherwise decrement byte 0x11
6228 DEC L L=0x10
6229 CP \$32 The sniper's y-coordinate is 50 when he's inactive
622B JR NZ,\$6250 Jump if the sniper is currently active
The sniper is currently inactive.
622D DEC (HL) Decrement byte 0x10 of the sniper's buffer
622E JP Z,\$F284 Terminate this subcommand if byte 0x10 is now 0
6231 CALL \$66D8 Is the sniper on screen at least 8 x-coordinates away from Sam?
6234 NOP
6235 JP NZ,\$F284 Terminate this subcommand if not
6238 INC L Point HL at byte 0x02 of the sniper's buffer
6239 LD (HL),\$25 Set the sniper's y-coordinate to 37 (out of sight, but ready to emerge)
623B LD A,(\$E601) A=Sam's x-coordinate
623E DEC L L=0x01
623F SUB (HL) Subtract the sniper's x-coordinate
6240 CCF Compute the appropriate animatory state for the sniper: 0x36 if he's to the right of Sam, 0xB6 if he's to the left
6241 LD A,\$6C
6243 RRA
6244 DEC HL Set the sniper's animatory state
6245 LD (HL),A
This entry point is used by the routine at 6FE6.
6246 LD L,\$02 Collect the sniper's x- and y-coordinates in DE
6248 LD D,(HL)
6249 DEC HL
624A LD E,(HL)
624B DEC HL A=sniper's animatory state
624C LD A,(HL)
624D JP \$6670 Update the sniper's sprite tile references
The sniper is currently active (his y-coordinate is 34-37).
6250 LD A,(\$DEB6) Collect the sprite tile reference for tile 7 of animatory states 0x36/0xB6; this will be 0xEB if the sniper is firing, or 0x00, 0x6E, 0x6F or 0x71 otherwise
6253 CP H Is the sniper firing?
6254 JP NC,\$6FE6 If so, make the firing sound effect
6257 LD A,(\$7FFF) A=y-coordinate of the topmost row of the play area on screen
625A CP \$14 Is the sidewalk on screen?
625C JR NZ,\$6269 If not, deactivate the sniper and terminate this subcommand
625E DEC (HL) Decrement byte 0x10 of the sniper's buffer
625F JR NZ,\$627A Jump unless byte 0x10 is now zero
Byte 0x10 of the sniper's buffer has been decremented to zero. The apparent intention here is to make the sniper duck out of sight and terminate this subcommand; however, if the sniper is not already out of sight, he will duck briefly but remain in view, and byte 0x10 will be decremented to 0xFF on the next pass through this routine. This is a bug.
6261 CALL \$E9C8 Update the SRB for the sniper's current animatory state and location
6264 LD A,D A=sniper's y-coordinate
6265 CP \$25 Is the sniper currently out of sight?
6267 JR NZ,\$6272 Jump if not to make him duck
6269 LD L,\$0F Remove the address of this routine from bytes 0x0E and 0x0F of the sniper's buffer
626B LD (HL),\$00
626D LD L,\$02 Set the sniper's y-coordinate to 50 (inactive)
626F LD (HL),\$32
6271 RET
The sniper is in the process of ducking.
6272 INC D D=sniper's next y-coordinate (35-37)
6273 LD L,\$00 Point HL at byte 0x00 of the sniper's buffer
6275 LD A,(HL) A=sniper's animatory state
6276 AND A Reset the carry flag to indicate that the sniper is not firing
6277 JP \$6670 Update the sniper's sprite tile references
Byte 16 of the sniper's buffer is not zero.
627A CALL \$66D8 Compare the sniper's coordinates with Sam's
627D JR Z,\$628B Jump if the sniper is on screen and at least 8 x-coordinates away from Sam
627F JR NC,\$6269 Terminate this subcommand if the sniper if off screen
6281 CALL \$E9C8 Update the SRB for the sniper's current animatory state and location
6284 LD A,D A=sniper's y-coordinate
6285 CP \$25 Has the sniper already ducked out of sight?
6287 JR NZ,\$6272 Jump if not to make the sniper continue ducking
6289 JR \$626D Deactivate the sniper
The sniper is on screen and at least 8 x-coordinates away from Sam. If he's already popped his head out far enough (his y-coordinate is 34 or 35), he can consider shooting; otherwise he'll need to emerge a little further first.
628B LD L,\$02 Point HL at byte 0x02 of the sniper's buffer
628D LD A,(HL) A=sniper's y-coordinate
628E CP \$24 Is at least half of the sniper visible?
6290 JR C,\$6298 Jump if so
6292 CALL \$E9C8 Update the SRB for the sniper's current animatory state and location
6295 DEC D D=sniper's next y-coordinate (35 or 36)
6296 JR \$6276 Jump back to update the sniper's sprite tile references
At least half of the sniper is visible (his y-coordinate is 34 or 35), which means he can shoot.
6298 LD L,\$10 Collect byte 0x10 of the sniper's buffer
629A LD A,(HL)
629B AND \$03 Is bit 0 or bit 1 set?
629D JR NZ,\$62AF Jump if so
629F CALL \$F17F Get a random number in A
62A2 CP \$A0 Should the sniper consider shooting?
62A4 JR C,\$62AF Jump if so (5 times out of 8)
62A6 CALL \$E9C8 Update the SRB for the sniper's current animatory state and location
62A9 LD A,D A=sniper's y-coordinate (34 or 35)
62AA XOR \$01 Toggle between 34 and 35
62AC LD D,A D=sniper's next y-coordinate (35 or 34)
62AD JR \$6273 Jump back to update the sniper's sprite tile references
The sniper will consider shooting.
62AF LD A,(\$E604) A=Sam's z-coordinate
62B2 RRCA Is Sam indoors?
62B3 RET C Return if so
62B4 LD A,(\$7FFC) Collect Sam's status flags from 7FFC
62B7 AND \$81 Is Sam falling from a building or being carried?
62B9 RET NZ Return if so
62BA LD L,\$11 Collect byte 0x11 of the sniper's buffer
62BC LD A,(HL)
62BD AND A Has the sniper fired recently?
62BE RET NZ Return if so
62BF CALL \$F17F Get a random number in A
62C2 CP \$54 Should the sniper continue to consider shooting?
62C4 RET NC Return if not (21 times out of 64)
62C5 LD DE,\$7FC2 Point DE at the third byte of bullet buffer 1 (at 7FC0)
62C8 LD A,(DE) Pick this up in A
62C9 AND A Is bullet buffer 1 in use?
62CA JR Z,\$62D1 Jump if not
62CC LD E,\$C6 Point DE at the third byte of bullet buffer 2 (at 7FC4)
62CE LD A,(DE) Pick this up in A
62CF AND A Is bullet buffer 2 in use?
62D0 RET NZ Return if so
The sniper has decided to shoot, and a bullet buffer is available.
62D1 LD (HL),\$0C Place 0x0C into byte 0x11 of the sniper's buffer (indicating that he's just fired)
62D3 CALL \$E9C8 Update the SRB for the sniper's current animatory state and location
62D6 SCF Set the carry flag to indicate that the sniper is firing
62D7 JP \$6670 Update the sniper's sprite tile references and initialise a bullet
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