Disassembly of Dragon 32 game Invader's Revenge

;Sub_InitWarmReset

;Turn off cassette motor; configure for warm reset at Sub_WarmReset; jump to

;next phase of boot.

Sub_InitWarmResetJSRD32_CasMotorOff2D2DBD BD DC       '...'

ORCC#$502D301A 50          '.P'

LDA#$552D3286 55          '.U'

STAD32_RestartFlag2D3497 71          '.q'

LDX#Sub_WarmReset2D368E 2D 3E       '.->'

STXD32_RestartVector2D399F 72          '.r'

JMPSub_ShowIntroQryNPlayers2D3B7E 39 03       '~9.'

;Sub_WarmReset

;First instruction must be NOP to allow warm reset.  Clear screen and wipe

;most code and data.  (I don't know why it wipes everything; to make it

;harder to cheat?)  Then enter infinite loop.

Sub_WarmResetNOP2D3E12             '.'

JSRD32_TextCls2D3FBD BA 77       '..w'

LDX#WarmRestart_WipeStart2D428E 2D 4F       '.-O'

WR_L1CLR,X+2D456F 80          'o.'

CMPX#$3FFC2D478C 3F FC       '.?.'

BLEWR_L12D4A2F F9          '/.'

WR_L2JMPWR_L22D4C7E 2D 4C       '~-L'

;Sub_InitDefenderPatrol

;Start a new defender on a patrol across the screen.

Sub_InitDefenderPatrolLDY#Arr_DefendersData2D8C10 8E 2E B2    '....'

LDA#112D9086 0B          '..'

STAVar_b_IDP_NRemaining2D92B7 2D 8A       '.-.'

;Search for free slot (one with Location == 0) within Arr_DefendersData

IDP_L1LDD,Y++2D95EC A1          '..'

LDX,Y++2D97AE A1          '..'

BNEIDP_L92D9926 4E          '&N'

;Found a free slot (Y points just past it); get velocity of new patrol:

;A = $01 or $FF; negate for next time so patrols alternate direction

LDAVar_b_IDP_NextXVelocity2D9BB6 2D 89       '.-.'

NEGVar_b_IDP_NextXVelocity2D9E70 2D 89       'p-.'

;phase += 1; if phase == 17...

INCVar_b_IDP_NextKindPhase2DA17C 2D 8B       '|-.'

LDB#172DA4C6 11          '..'

CMPBVar_b_IDP_NextKindPhase2DA6F1 2D 8B       '.-.'

BNEIDP_L22DA926 05          '&.'

;...reset phase, and turn L-to-R normal defender into special

CLRVar_b_IDP_NextKindPhase2DAB7F 2D 8B       '.-.'

ORA#$402DAE8A 40          '.@'

;defender->KindAndVelocity = A (one of: $01, $41, $FF)

IDP_L2STA-$03,Y2DB0A7 3D          '.='

;Compute XColumn field — clear "special" bit, leaving $01 or $BF; if L-to-R

;(KindAndVelocity is either $01 or $41), then $01 is the correct starting

;value for XColumn...

ANDA#$BF2DB284 BF          '..'

;...but if we got $BF...

BPLIDP_L32DB42A 02          '*.'

;...then defender is R-to-L, so XColumn should be $1F

LDA#$1F2DB686 1F          '..'

;defender->XColumn = $01 (L-to-R) or $1F (R-to-L)

IDP_L3STA-$04,Y2DB8A7 3C          '.<'

;Search for patrol start location in unoccupied row

IDP_L4LDX[Var_p_IDP_LaunchLoc]2DBAAE 9F 2E 09    '....'

;Scan every other byte of row two rows down for evidence of existing

;defender (any blue or red pixel)

LEAX$40,X2DBE30 88 40       '0.@'

LDA#162DC186 10          '..'

IDP_L5LDB,X++2DC3E6 81          '..'

ANDB#$AA2DC5C4 AA          '..'

BEQIDP_L62DC727 04          ''.'

;Row contains blue or red; try next patrol-start location, by using the

;"cyclically advance pointer" logic at the end of this subroutine as a

;subroutine in its own right

BSRIDP_L72DC98D 0B          '..'

BRAIDP_L42DCB20 ED          ' .'

;Keep looking for blue or red

IDP_L6DECA2DCD4A             'J'

BNEIDP_L52DCE26 F3          '&.'

;Scan completed without finding blue or red; we have found a suitable

;patrol-start location; store to defender->Location

LDX[Var_p_IDP_LaunchLoc]2DD0AE 9F 2E 09    '....'

STX-$02,Y2DD4AF 3E          '.>'

;Cyclically advance launch-location pointer; the CMPD operand is as it is

;because the pointer lives just beyond the array into which it points

IDP_L7LDDVar_p_IDP_LaunchLoc2DD6FC 2E 09       '...'

ADDD#$00022DD9C3 00 02       '...'

CMPD#Var_p_IDP_LaunchLoc2DDC10 83 2E 09    '....'

BCSIDP_L82DE025 03          '%.'

LDDCon_P_IDP_LaunchLocs_02DE2FC 2E 0B       '...'

IDP_L8STDVar_p_IDP_LaunchLoc2DE5FD 2E 09       '...'

RTS2DE839             '9'

;Continue search for free slot

IDP_L9DECVar_b_IDP_NRemaining2DE97A 2D 8A       'z-.'

BNEIDP_L12DEC26 A7          '&.'

RTS2DEE39             '9'

;Sub_EndDfrPatrolStartNew

;Mark the entry of the Arr_DefendersData array immediately before the one

;pointed to by Y as unused (by setting its location field to zero), and

;launch a new patrol.  (The search for an available array slot is therefore

;guaranteed to succeed because of the slot we just marked as free.)

;

;This could be considered part of Sub_MoveDefenderCheckHit; its only use

;is being tail-called from there.

Sub_EndDfrPatrolStartNewPSHSY2E0D34 20          '4 '

CLR-$02,Y2E0F6F 3E          'o>'

CLR-$01,Y2E116F 3F          'o?'

JSRSub_InitDefenderPatrol2E13BD 2D 8C       '.-.'

PULSY2E1635 20          '5 '

RTS2E1839             '9'

;Sub_MoveDefenderCheckHit

;Move one defender one step.  Check whether it collides with the player or

;with a player's shot.  If so, award points, and initiate an explosion and

;sound effect.

;

;Arguments (data about defender being processed) in registers:

;  B = struct[0] = XColumn

;  A = velocity ($01 L-R, $FF R-L)

;  X = struct[2:3] = Location

;  U → graphics data according to kind of defender

;  Y → just past struct of defender to be moved (elt of Arr_DefendersData)

;      (Y is preserved)

Sub_MoveDefenderCheckHitPSHSX,A2E1934 12          '4.'

;Determine whether this defender has just started its patrol; the product of

;its XColumn and velocity will tell us this: the only product with low byte

;$01 is $01 × $01, and the only product with low byte $E1 is $1F × $FF

MUL2E1B3D             '='

;If L-to-R defender at far left...

CMPB#$012E1CC1 01          '..'

;...skip hit check and erase

LBEQMDCH_L072E1E10 27 00 53    '.'.S'

;If R-to-L defender at far right...

CMPB#$E12E22C1 E1          '..'

;...skip hit check and erase

LBEQMDCH_L072E2410 27 00 4D    '.'.M'

;Prepare for "erase" loop

LDA#72E2886 07          '..'

STAVar_b_MDCH_BltRows2E2AB7 2D 53       '.-S'

;Get two-byte sample within defender's bounding box — five rows down from

;top row (recall: 0=G, 1=Y, 2=B, 3=R) — in order to test whether the

;defender has been turned yellow from being hit by a player shot

LDD$00A0,X2E2DEC 89 00 A0    '....'

;If neither byte has any red or yellow pixels...

BITB#$552E31C5 55          '.U'

BNEMDCH_L012E3326 04          '&.'

BITA#$552E3585 55          '.U'

;...skip down to "erase, move, redraw" logic

BEQMDCH_L052E3727 2F          ''/'

;...or, if right-hand byte has any red...

MDCH_L01ANDB#$AA2E39C4 AA          '..'

;...skip down to "erase, move, redraw"

BNEMDCH_L052E3B26 2B          '&+'

;Defender is drawn in yellow; it got turned yellow because it got hit by

;player shot

STXVar_s_LastDfrExpln2E3DBF 39 E0       '.9.'

;Overwrite X value on stack with zero to signal this defender gone

LDD#$00002E40CC 00 00       '...'

STD$01,S2E43ED 61          '.a'

PSHSU,Y,X2E4534 70          '4p'

;Determine what kind of defender was destroyed

LDA#12E4786 01          '..'

LDB-$03,Y2E49E6 3D          '.='

BMIMDCH_L032E4B2B 0D          '+.'

ANDB#$402E4DC4 40          '.@'

BNEMDCH_L022E4F26 02          '&.'

BRAMDCH_L032E5120 07          ' .'

;Special (red) defender destroyed

MDCH_L02JSRSub_InitSpclDfrExpln2E53BD 2F 74       './t'

LDA#102E5686 0A          '..'

BRAMDCH_L042E5820 03          ' .'

;Normal (blue) defender destroyed

MDCH_L03JSRSub_InitNormalDfrExpln2E5ABD 2F 29       './)'

;Tasks arising from destroyed defender

MDCH_L04STAVar_b_LastDfrExplnKind2E5DB7 39 DF       '.9.'

JSRSub_AwardPoints2E60BD 37 BB       '.7.'

JSRSub_InitDfrExplosion2E63BD 39 E4       '.9.'

PULSU,Y,X2E6635 70          '5p'

;Erase an 8-wide, 7-high block of pixels (i.e., 2 bytes wide), with the

;top-left byte pointed to by X; this erases the defender

MDCH_L05LDD#$00002E68CC 00 00       '...'

MDCH_L06STD,X2E6BED 84          '..'

LEAX$20,X2E6D30 88 20       '0. '

DECVar_b_MDCH_BltRows2E707A 2D 53       'z-S'

BNEMDCH_L062E7326 F6          '&.'

;Retrieve location (X) and location/column increment (A); if this is an

;active slot, compute new location

MDCH_L07PULSX,A2E7535 12          '5.'

CMPX#$00002E778C 00 00       '...'

LBEQSub_EndDfrPatrolStartNew2E7A10 27 FF 8F    '.'..'

LEAXA,X2E7E30 86          '0.'

;A defender moving along the top row does so at double speed; update

;location and increment in this case

CMPX#Loc_x000_y0432E808C 0B 60       '..`'

BGTMDCH_L082E832E 03          '..'

LEAXA,X2E8530 86          '0.'

ASLA2E8748             'H'

;Compute new column; if that takes the defender to or beyond the leftmost

;column, or to one beyond the rightmost column, mark this patrol as over and

;launch a new one

MDCH_L08ADDA-$04,Y2E88AB 3C          '.<'

LBEQSub_EndDfrPatrolStartNew2E8A10 27 FF 7F    '.'..'

LBMISub_EndDfrPatrolStartNew2E8E10 2B FF 7B    '.+.{'

CMPA#$212E9281 21          '.!'

LBEQSub_EndDfrPatrolStartNew2E9410 27 FF 75    '.'.u'

;Still on screen; store new location and column to the structure, and draw

;defender in new position

STA-$04,Y2E98A7 3C          '.<'

LDA#72E9A86 07          '..'

STAVar_b_MDCH_BltRows2E9CB7 2D 53       '.-S'

STX-$02,Y2E9FAF 3E          '.>'

MDCH_L09LDD,X2EA1EC 84          '..'

ORA,U+2EA3AA C0          '..'

ORB,U+2EA5EA C0          '..'

STD,X2EA7ED 84          '..'

LEAX$20,X2EA930 88 20       '0. '

DECVar_b_MDCH_BltRows2EAC7A 2D 53       'z-S'

BNEMDCH_L092EAF26 F0          '&.'

RTS2EB139             '9'

;Sub_StepDfrsMovement

;If we're at the correct phase (when Var_b_SDM_Phase is 1 on entry to this

;subroutine), reset the phase to the DefenderMovementPeriod slot of the

;active player, and do one step of movement for all active defenders.

Sub_StepDfrsMovementDECVar_b_SDM_MovePhase2EE27A 2E DE       'z..'

BNESDM_L42EE526 31          '&1'

;MovePhase = active_player->DfrMvPeriod

LDUVar_p_ActivePlayerData2EE7FE 37 B9       '.7.'

LDA$01,U2EEAA6 41          '.A'

STAVar_b_SDM_MovePhase2EECB7 2E DE       '...'

LDY#Arr_DefendersData2EEF10 8E 2E B2    '....'

LDA#112EF386 0B          '..'

STAVar_b_SDM_NRemaining2EF5B7 2E E1       '...'

;Prepare to blt normal defender gfx; might be overridden below

SDM_L1LDU#GFX_NormalDefender2EF8CE 2D 7A       '.-z'

;B = defender->XColumn

LDB,Y+2EFBE6 A0          '..'

;A = defender->KindAndVelocity

LDA,Y+2EFDA6 A0          '..'

;R-to-L normal defender?

CMPA#$012EFF81 01          '..'

BEQSDM_L22F0127 09          ''.'

;L-to-R normal defender?

CMPA#$FF2F0381 FF          '..'

BEQSDM_L22F0527 05          ''.'

;Must be L-R special defender; A becomes $01

ANDA#$BF2F0784 BF          '..'

;Prepare to blt special defender gfx instead

LDU#GFX_SpecialDefender2F09CE 2D 6C       '.-l'

;X = defender->location

SDM_L2LDX,Y++2F0CAE A1          '..'

;If defender active (location != 0)...

BEQSDM_L32F0E27 03          ''.'

;...move it and check for collisions

JSRSub_MoveDefenderCheckHit2F10BD 2E 19       '...'

SDM_L3DECVar_b_SDM_NRemaining2F137A 2E E1       'z..'

BNESDM_L12F1626 E0          '&.'

SDM_L4RTS2F1839             '9'

;Sub_InitNormalDfrExpln

;Blt the initial "normal defender" explosion graphic to X, and play a short

;(c.60ms) chirp consisting of 48 cycles of linearly increasing period (so

;gently falling pitch).

Sub_InitNormalDfrExplnPSHSU,Y,X,D2F2934 76          '4v'

;Blt 7 rows of explosion gfx to X

LDU#GFX_NormalDefenderExpln2F2BCE 2F 1A       './.'

LDA#72F2E86 07          '..'

STAVar_b_INDE_Rows2F30B7 2F 19       './.'

INDE_L1LDD,U++2F33EC C1          '..'

STD,X2F35ED 84          '..'

LEAX$20,X2F3730 88 20       '0. '

DECVar_b_INDE_Rows2F3A7A 2F 19       'z/.'

BNEINDE_L12F3D26 F4          '&.'

;Set initial half-period complement of falling-tone beep

LDB#$AF2F3FC6 AF          '..'

STBVar_b_INDE_BeepHPeriodC2F41F7 2F 28       './('

;Play one cycle of tone

INDE_L2LDA#$FF2F4486 FF          '..'

STAD32_PIA_1_A_Data2F46B7 FF 20       '.. '

LDBVar_b_INDE_BeepHPeriodC2F49F6 2F 28       './('

INDE_L3INCB2F4C5C             '\'

BNEINDE_L32F4D26 FD          '&.'

CLRD32_PIA_1_A_Data2F4F7F FF 20       '.. '

LDBVar_b_INDE_BeepHPeriodC2F52F6 2F 28       './('

INDE_L4INCB2F555C             '\'

BNEINDE_L42F5626 FD          '&.'

;Decrease half-period complement, thereby increasing half-period

DECVar_b_INDE_BeepHPeriodC2F587A 2F 28       'z/('

;Keep going if half-period complement is at least (in unsigned sense) $80

BMIINDE_L22F5B2B E7          '+.'

;Playing the sound takes time; no need to explicitly delay

CLRVar_b_PostDrawDelay2F5D7F 30 DE       '.0.'

PULSU,Y,X,D2F6035 76          '5v'

RTS2F6239             '9'

;Sub_InitSpclDfrExpln

;Blt the initial "special defender" explosion graphic to X, and play two

;equal short rising-pitch chirps.

Sub_InitSpclDfrExplnPSHSU,Y,X,D2F7434 76          '4v'

;If about to draw explosion too high up screen...

CMPX#Loc_x048_y0392F768C 0A EC       '...'

;...don't

BLEISDE_L22F792F 14          '/.'

;Blt 7 rows of explosion gfx to X

LDU#GFX_BlueYellowSparks2F7BCE 2F 64       './d'

LDA#72F7E86 07          '..'

STAVar_b_ISDE_Rows2F80B7 2F 63       './c'

ISDE_L1LDD,U++2F83EC C1          '..'

STD,X2F85ED 84          '..'

LEAX$20,X2F8730 88 20       '0. '

DECVar_b_ISDE_Rows2F8A7A 2F 63       'z/c'

BNEISDE_L12F8D26 F4          '&.'

;Play two chirps

ISDE_L2LDB#22F8FC6 02          '..'

ISDE_L3LDACon_B_ISDE_InitPeriod2F91B6 2F 72       './r'

STAVar_b_ISDE_Period2F94B7 2F 73       './s'

ISDE_L4LDAVar_b_ISDE_Period2F97B6 2F 73       './s'

ISDE_L5DECA2F9A4A             'J'

BNEISDE_L52F9B26 FD          '&.'

COMD32_PIA_1_A_Data2F9D73 FF 20       's. '

DECVar_b_ISDE_Period2FA07A 2F 73       'z/s'

BNEISDE_L42FA326 F2          '&.'

DECB2FA55A             'Z'

BNEISDE_L32FA626 E9          '&.'

;Playing the sound takes time; no need to explicitly delay

CLRVar_b_PostDrawDelay2FA87F 30 DE       '.0.'

PULSU,Y,X,D2FAB35 76          '5v'

RTS2FAD39             '9'

;Sub_DisplayPauseMsg

;Cycle between displaying two variants of the PAUSE text until the player

;presses something other than "P", at which point clear the message and

;resume play.

Sub_DisplayPauseMsgLDU#GFX_Text_PAUSE_Blue3025CE 30 65       '.0e'

;Check for completion of 256-iteration cycle; if done...

DPM_L1INCB30285C             '\'

;...switch text colour

BEQDPM_L2302927 12          ''.'

;If no key pressed...

JSR[D32_ReadKbd_Vec]302BAD 9F 80 07    '....'

;...loop

BEQDPM_L1302F27 F7          ''.'

;...or if "p" key pressed...

CMPA#'P303181 50          '.P'

;...loop

BEQDPM_L1303327 F3          ''.'

;Exit pause: clear text from screen and resume play loop

LDX#Loc_x052_y02330358E 08 ED       '...'

BSRSub_EraseSixBySix30388D 15          '..'

JMPSub_MainPlayLoop303A7E 30 DF       '~0.'

;Blt text, leaving U pointing one beyond just-blt'd text

DPM_L2LDX#Loc_x052_y023303D8E 08 ED       '...'

PSHSB304034 04          '4.'

JSRSub_DrawTextCharCells3042BD 37 04       '.7.'

PULSB304535 04          '5.'

;If U is just beyond second (yellow) text...

CMPU#Sub_SetGraphicsMode304711 83 30 A3    '..0.'

;...loop, resetting U to point to first (blue) text

BGESub_DisplayPauseMsg304B2C D8          ',.'

;...otherwise, loop, with U pointing to second (yellow) text

BRADPM_L1304D20 D9          ' .'

;Sub_EraseSixBySix

;Erase a 6×6 block of bytes (i.e., a 24-wide and 6-high rectangle of pixels)

;with the top-left such byte pointed to by X.

Sub_EraseSixBySixLDB#6304FC6 06          '..'

LDY#$0000305110 8E 00 00    '....'

ESBS_L1STY,X++305510 AF 81       '...'

STY,X++305810 AF 81       '...'

STY,X305B10 AF 84       '...'

LEAX$1C,X305E30 88 1C       '0..'

DECB30615A             'Z'

BNEESBS_L1306226 F1          '&.'

RTS306439             '9'

;Sub_SetGraphicsMode

;Configure the desired graphics mode, play an introductory sound effect,

;then chain to the subroutine which asks the player to choose settings.

;Set registers for graphics mode

Sub_SetGraphicsModeLDA#$E030A386 E0          '..'

STAD32_PIA_1_B_Data30A5B7 FF 22       '.."'

STAD32_SAM_Set_V130A8B7 FF C3       '...'

STAD32_SAM_Set_V230ABB7 FF C5       '...'

STAD32_SAM_Set_F030AEB7 FF C7       '...'

;Play sound effect

LDA#2530B186 19          '..'

STAVar_b_IntroSoundPDDelay30B3B7 30 C9       '.0.'

SGM_L1STAVar_b_PostDrawDelay30B6B7 30 DE       '.0.'

JSRSub_FallingPitchBeep30B9BD 3B B4       '.

DECVar_b_IntroSoundPDDelay30BC7A 30 C9       'z0.'

LDAVar_b_IntroSoundPDDelay30BFB6 30 C9       '.0.'

CMPA#830C281 08          '..'

BNESGM_L130C426 F0          '&.'

;Chain to next phase of boot

JMPSub_QueryShotsSpeed30C67E 3D A6       '~=.'

;Sub_SetTextMode

;Configure the VDG and SAM for text mode.  Some usages, to handle an error

;condition, jump to this subroutine, which I think will terminate the game

;immediately.

;

;Unknown: Why set A to $34?

Sub_SetTextModeCLRA30CA4F             'O'

STAD32_PIA_1_B_Data30CBB7 FF 22       '.."'

STAD32_SAM_Clr_V230CEB7 FF C4       '...'

STAD32_SAM_Clr_V130D1B7 FF C2       '...'

STAD32_SAM_Clr_F030D4B7 FF C6       '...'

LDA#$3430D786 34          '.4'

RTS30D939             '9'

;Sub_MainPlayLoop

;Infinite loop, performing all tasks for playing the game:

;

;Move all the player's shots down, checking for hitting something

;Check for keypresses moving the player or firing a player shot

;Every Nth loop, move the defender base

;Move the player, checking for collisions

;Do one step of animation of player explosion (if active)

;Play a little bit of sound

;Do one step of animation of any active defender explosions

;Do one step of animation of defender's shot hitting something

;Do one step of animation of defender's shot

;Every Nth loop, move the defenders

;Delay to keep game at proper pace

;

;In the above, "Nth" is used to denote that the action occurs once in a

;period which gets shorter as the player gets more points.

Sub_MainPlayLoopJSRSub_MovePlayerShotsDown30DFBD 32 E9       '.2.'

;If the player's ship is exploding...

TSTVar_b_PlrExplnUnderway30E27D 3C C4       '}<.'

;...do not allow the player to move the ship

LBNEMPL_L430E510 26 00 4A    '.&.J'

;If key "P" pressed...

LDA#$FE30E986 FE          '..'

STAD32_PIA_0_B_Data30EBB7 FF 02       '...'

LDAD32_PIA_0_A_Data30EEB6 FF 00       '...'

BITA#$1030F185 10          '..'

;...enter flashing PAUSE loop

LBEQSub_DisplayPauseMsg30F310 27 FF 2E    '.'..'

;Three bytes at MPL_MovePlrFromKbd are overwritten by "JMP $4100"

;if joysticks are enabled.

;If key "right arrow" pressed...

MPL_MovePlrFromKbdLDA#$BF30F786 BF          '..'

STAD32_PIA_0_B_Data30F9B7 FF 02       '...'

LDAD32_PIA_0_A_Data30FCB6 FF 00       '...'

BITA#$2030FF85 20          '. '

BNEMPL_L1310126 03          '&.'

;...update position to move right

JSRSub_MovePlayerRight3103BD 31 8E       '.1.'

;If key "left arrow" pressed...

MPL_L1LDA#$DF310686 DF          '..'

STAD32_PIA_0_B_Data3108B7 FF 02       '...'

LDAD32_PIA_0_A_Data310BB6 FF 00       '...'

BITA#$20310E85 20          '. '

BNEMPL_L2311026 03          '&.'

;...update position to move left

JSRSub_MovePlayerLeft3112BD 31 A9       '.1.'

;If key "down arrow" pressed...

MPL_L2LDA#$EF311586 EF          '..'

STAD32_PIA_0_B_Data3117B7 FF 02       '...'

LDAD32_PIA_0_A_Data311AB6 FF 00       '...'

BITA#$20311D85 20          '. '

BNEMPL_L3311F26 03          '&.'

;...update position to move down

JSRSub_MovePlayerDown3121BD 32 6D       '.2m'

;If key "up arrow" pressed...

MPL_L3LDA#$F7312486 F7          '..'

STAD32_PIA_0_B_Data3126B7 FF 02       '...'

LDAD32_PIA_0_A_Data3129B6 FF 00       '...'

BITA#$20312C85 20          '. '

BNEMPL_L4312E26 03          '&.'

;...update position to move up

JSRSub_MovePlayerUp3130BD 32 5D       '.2]'

MPL_L4JSRSub_MoveDfrBaseCheckHit3133BD 33 91       '.3.'

;If key "space" pressed...

LDA#$7F313686 7F          '..'

STAD32_PIA_0_B_Data3138B7 FF 02       '...'

LDAD32_PIA_0_A_Data313BB6 FF 00       '...'

ANDA#$20313E84 20          '. '

BNEMPL_L5314026 05          '&.'

;...fire player shot

JSRSub_LaunchPlayerShot3142BD 32 98       '.2.'

;...and leave flag alone

BRAMPL_L6314520 03          ' .'

;...otherwise ("space" not pressed), clear flag

MPL_L5CLRVar_b_ShotInProgress31477F 32 97       '.2.'

MPL_L6JSRSub_RedrawPlayer314ABD 31 EB       '.1.'

JSRSub_StepPlayerExpln314DBD 3C C8       '.<.'

JSRSub_PlaySoundSlice3150BD 3B 6D       '.

JSRSub_StepDefenderExplns3153BD 3A 3C       '.:<'

JSRSub_StepDfrShotHitAnimn3156BD 34 23       '.4#'

JSRSub_StepDfrShotMovement3159BD 34 7B       '.4{'

JSRSub_StepDfrsMovement315CBD 2E E2       '...'

;If a delay is required...

LDBVar_b_PostDrawDelay315FF6 30 DE       '.0.'

BEQMPL_L8316227 06          ''.'

;...do so, for time proportional to Var_b_PostDrawDelay

MPL_L7DECA31644A             'J'

BNEMPL_L7316526 FD          '&.'

DECB31675A             'Z'

BNEMPL_L7316826 FA          '&.'

MPL_L8LDAVar_b_Init_PostDrawDelay316AB6 31 73       '.1s'

STAVar_b_PostDrawDelay316DB7 30 DE       '.0.'

JMPSub_MainPlayLoop31707E 30 DF       '~0.'

;Sub_ClearLowerScreen

;Clear from the location in Var_ClearLowerScreen_Start to the end of the

;screen, then set Var_ClearLowerScreen_Start to 5/24 down from the top of

;the screen.  On boot, Var_ClearLowerScreen_Start points to the very

;top-left of the screen.

Sub_ClearLowerScreenLDXVar_s_ClrLowerScreen_03176BE 31 74       '.1t'

CLS_L1CLR,X+31796F 80          'o.'

CMPX#D32_GraphicsPage5317B8C 1E 00       '...'

BNECLS_L1317E26 F9          '&.'

LDX#Loc_x000_y04031808E 0B 00       '...'

STXVar_s_ClrLowerScreen_03183BF 31 74       '.1t'

LDX#$000131868E 00 01       '...'

CLS_L2LEAX-$01,X318930 1F          '0.'

BNECLS_L2318B26 FC          '&.'

RTS318D39             '9'

;Sub_MovePlayerRight

;Move the player right one half-byte, i.e., two pixels.  The condition for

;no update to whole (whole + fraction == $1E) can in theory happen in two

;ways: (whole, fraction) == ($1E, $00) or ($1F, $FF).  But the former will

;always happen before the latter can become true, so the rightmost the

;player can be is at ($1E, $00), i.e., in right-position with centre byte in

;column $1E.

;If player already at right-end limit of travel...

Sub_MovePlayerRightLDAVar_b_ActivePlr_Xwhole318EB6 31 E6       '.1.'

ADDAVar_b_ActivePlr_Xfrac3191BB 31 E5       '.1.'

CMPA#$1E319481 1E          '..'

;...do not move (see comment above for detail)

BEQMPR_L2319627 10          ''.'

LDXVar_s_ActivePlayer3198BE 31 E7       '.1.'

;If toggling fraction results in new value being $00=right...

COMVar_b_ActivePlr_Xfrac319B73 31 E5       's1.'

;...we can leave byte-column and screen-loc as they are

BEQMPR_L1319E27 05          ''.'

;...otherwise (new value $FF=left), increase byte-column and screen-loc

LEAX$01,X31A030 01          '0.'

INCVar_b_ActivePlr_Xwhole31A27C 31 E6       '|1.'

;Other player movement subroutines jump here too

MPR_L1STXVar_s_ActivePlayer31A5BF 31 E7       '.1.'

MPR_L2RTS31A839             '9'

;Sub_MovePlayerLeft

;Move the player left one half-byte, i.e., two pixels.  The condition for no

;update to whole (whole + fraction == $00) can in theory happen in two ways:

;if (whole, fraction) == ($01, $FF) or ($00, $00).  But the former will

;always happen before the latter can become true, so the leftmost the player

;can be is at ($01, $FF), i.e., left-position, centre byte in column $01.

;If player already at left-end limit of travel...

Sub_MovePlayerLeftLDBVar_b_ActivePlr_Xwhole31A9F6 31 E6       '.1.'

ADDBVar_b_ActivePlr_Xfrac31ACFB 31 E5       '.1.'

;...do not move (see comment above for detail)

BEQMPR_L231AF27 F7          ''.'

LDXVar_s_ActivePlayer31B1BE 31 E7       '.1.'

;If toggling fraction results in new value being $FF=left...

COMVar_b_ActivePlr_Xfrac31B473 31 E5       's1.'

;...we can leave byte-column and screen-loc as they are

BNEMPR_L131B726 EC          '&.'

;...otherwise (new value $00=right), decrease byte-column and screen-loc

LEAX-$01,X31B930 1F          '0.'

DECVar_b_ActivePlr_Xwhole31BB7A 31 E6       'z1.'

BRAMPR_L131BE20 E5          ' .'

;Sub_RedrawPlayer

;Erase the player (checking for collision with blue/red) from previous

;location, and then redraw at current location, taking account of fractional

;position (i.e., whether at "left" or "right" position within byte).

;If the player is mid-explosion...

Sub_RedrawPlayerTSTVar_b_PlrExplnUnderway31EB7D 3C C4       '}<.'

;...skip all this

LBNERP_L531EE10 26 00 39    '.&.9'

;Prepare for erase and hit check; if player is in "left" position...

TSTVar_b_ActivePlr_Xfrac31F27D 31 E5       '}1.'

;...use "left position" ship graphic for hit-detect mask

BNERP_L631F526 35          '&5'

;...else use "right position" ship graphic

LDU#GFX_PlayerRightPosn31F7CE 31 C1       '.1.'

RP_L1JSRSub_ErasePlayerCheckHit31FABD 32 32       '.22'

;Prepare for drawing; assume "right position" ship graphic

LDU#GFX_PlayerRightPosn31FDCE 31 C1       '.1.'

;But if player is in "left" position...

TSTVar_b_ActivePlr_Xfrac32007D 31 E5       '}1.'

BEQRP_L2320327 03          ''.'

;...use "left position" ship graphic

LDU#GFX_PlayerLeftPosn3205CE 31 D3       '.1.'

;If both players have zero lives...

RP_L2LDAPlrData_0_b_NLives3208B6 37 A3       '.7.'

ADDAPlrData_1_b_NLives320BBB 37 AF       '.7.'

;...do not redraw player ship

BEQRP_L5320E27 1B          ''.'

;Blt (with "OR") 6 rows and 3 bytes

LDXVar_s_ActivePlayer3210BE 31 E7       '.1.'

LDA#6321386 06          '..'

STAVar_b_RP_Rows3215B7 31 C0       '.1.'

RP_L3LDA#3321886 03          '..'

RP_L4LDB,U+321AE6 C0          '..'

ORB,X321CEA 84          '..'

STB,X+321EE7 80          '..'

DECA32204A             'J'

BNERP_L4322126 F7          '&.'

LEAX$1D,X322330 88 1D       '0..'

DECVar_b_RP_Rows32267A 31 C0       'z1.'

BNERP_L3322926 ED          '&.'

RP_L5RTS322B39             '9'

RP_L6LDU#GFX_PlayerLeftPosn322CCE 31 D3       '.1.'

BRARP_L1322F20 C9          ' .'

;Sub_ErasePlayerCheckHit

;Erase yellow from a 6-high, 3-byte-wide section of screen whose top-left

;byte is in Var_s_ActivePlayer_Prev.  While doing so, use the bitmask

;pointed to by U for hit-detection against red or blue pixels.  If a red

;or blue pixel is found, then this means that the player ship has crashed

;into something, and we jump to HandlePlayerCrash.

;

;I'm not fully clear on the choice of mask, because we use the mask

;corresponding to the _current_ ship position but the display-RAM address

;for the _previous_ ship position.  Maybe this doesn't matter; the fact that

;defenders are byte-aligned might be relevant.

Sub_ErasePlayerCheckHitLDXVar_s_ActivePlayer_Prev3232BE 31 E9       '.1.'

;Prepare to loop over 6 rows of bitmask

LDA#6323586 06          '..'

STAVar_b_EP_Rows3237B7 32 31       '.21'

;Prepare to loop over 3 byte-columns of bitmask (12 pixels)

EPCH_L1LDA#3323A86 03          '..'

;Turn next mask byte into red/blue detect mask (recall: 0=G, 1=Y, 2=B, 3=R)

EPCH_L2LDB,U+323CE6 C0          '..'

ASLB323E58             'X'

;If the player overlaps with something red or blue...

ANDB,X323FE4 84          '..'

;...they have hit a defender or defender shot; chain to handler sub

LBNESub_HandlePlayerCrash324110 26 03 5C    '.&.\'

;Mask out yellow from destination byte (Y->G, R->B)

LDB,X3245E6 84          '..'

ANDB#$AA3247C4 AA          '..'

STB,X+3249E7 80          '..'

;Loop over byte-columns

DECA324B4A             'J'

BNEEPCH_L2324C26 EE          '&.'

;Loop over rows

LEAX$1D,X324E30 88 1D       '0..'

DECVar_b_EP_Rows32517A 32 31       'z21'

BNEEPCH_L1325426 E4          '&.'

LDXVar_s_ActivePlayer3256BE 31 E7       '.1.'

STXVar_s_ActivePlayer_Prev3259BF 31 E9       '.1.'

RTS325C39             '9'

;Sub_MovePlayerUp

;Compute screen location two rows higher

Sub_MovePlayerUpLDXVar_s_ActivePlayer325DBE 31 E7       '.1.'

LEAX-$40,X326030 88 C0       '0..'

;If proposed new location is too high up screen...

CMPX#Loc_x000_y04032638C 0B 00       '...'

;...do not store new value

LBLTMPR_L2326610 2D FF 3E    '.-.>'

;...otherwise (not too high up), DO store new value

JMPMPR_L1326A7E 31 A5       '~1.'

;Sub_MovePlayerDown

;Compute screen location two rows lower

Sub_MovePlayerDownLDXVar_s_ActivePlayer326DBE 31 E7       '.1.'

LEAX$40,X327030 88 40       '0.@'

;If player is above double-stripe...

CMPX#Loc_x000_y03532738C 0A 60       '..`'

BGEMPD_L132762C 04          ',.'

;...move down six more rows to enter main play area

LEAX$0120,X327830 89 01 20    '0.. '

;If proposed new location is too low down screen...

MPD_L1CMPX#Loc_x000_y175327C8C 1B E0       '...'

;...do not store new value

LBGTMPR_L2327F10 2E FF 25    '...%'

;...otherwise (not too low down), DO store new value

JMPMPR_L132837E 31 A5       '~1.'

;Sub_LaunchPlayerShot

;Attempt to launch a shot from the player's ship.  A free struct is sought,

;and if one is found, it is populated with the location of the new shot, and

;a bitmask for drawing it, depending on the fractional horizontal position

;of the player's ship.  Also play a beep.  If a shot is already being

;launched, do not launch another one, but instead prolong the beep, lowering

;its pitch.

;If a shot is already in the process of being fired...

Sub_LaunchPlayerShotLDAVar_b_ShotInProgress3298B6 32 97       '.2.'

;...do not launch another, but do (perhaps) extend sound effect

BNELPS_L3329B26 36          '&6'

;If player is above double-stripe...

LDXVar_s_ActivePlayer329DBE 31 E7       '.1.'

CMPX#Loc_x000_y04032A08C 0B 00       '...'

;...do not launch a shot — TODO check if sound still happens

BCSLPS_L332A325 2E          '%.'

;Scan for available shot struct...

LDY#Arr_PlayerShots32A510 8E 32 86    '..2.'

LPS_L1LDB,Y+32A9E6 A0          '..'

LDX,Y++32ABAE A1          '..'

BNELPS_L532AD26 33          '&3'

;... leaving Y just past struct to populate

;Compute screen-loc 6 rows down, one byte to right of player's location,

;i.e., under the centre byte-column of the three-wide ship image

LDXVar_s_ActivePlayer32AFBE 31 E7       '.1.'

LEAX$00C1,X32B230 89 00 C1    '0...'

;Start by assuming right-posn shot mask...

LDA#$0132B686 01          '..'

;...but if fraction is $FF=left...

TSTVar_b_ActivePlr_Xfrac32B87D 31 E5       '}1.'

BEQLPS_L232BB27 02          ''.'

;...use left-posn mask instead

LDA#$1032BD86 10          '..'

;Store to Bitmask field of being-populated struct

LPS_L2STA-$03,Y32BFA7 3D          '.='

;Draw bottom pixel of shot; rest will be drawn as it starts moving

ORA,X32C1AA 84          '..'

STA,X32C3A7 84          '..'

;Store to ScreenLoc field of being-populated struct

STX-$02,Y32C5AF 3E          '.>'

;High-pitched beep

LDA#132C786 01          '..'

STAVar_b_ShotInProgress32C9B7 32 97       '.2.'

STAVar_b_ShotSoundProgress32CCB7 32 E1       '.2.'

JSRSub_FallingPitchBeep32CFBD 3B B4       '.

RTS32D239             '9'

;Extend the beep for up to five calls, lowering the pitch

LPS_L3LDAVar_b_ShotSoundProgress32D3B6 32 E1       '.2.'

CMPA#632D681 06          '..'

BEQLPS_L432D827 06          ''.'

JSRSub_FallingPitchBeep32DABD 3B B4       '.

INCVar_b_ShotSoundProgress32DD7C 32 E1       '|2.'

LPS_L4RTS32E039             '9'

;Loop if within bound set by max n.shots

LPS_L5CMPYVar_p_ShotPtrLimit32E210 BC 32 95    '..2.'

BLTLPS_L132E62D C1          '-.'

RTS32E839             '9'

;Sub_MovePlayerShotsDown

;Iterate over all potential player shots, processing each "active" one.  A

;shot is "active" if its Location is not $0000.  Processing consists of the

;following.  Move down the screen.  Check for collision with defender

;(patrol or base), turning that defender yellow and marking the shot slot

;"inactive" if so.  Erase shot once it reaches the bottom, marking that

;array slot "inactive" if so.

Sub_MovePlayerShotsDownLDY#Arr_PlayerShots32E910 8E 32 86    '..2.'

;B = shot->Bitmask; X = shot->ScreenLoc

MPSD_L1LDB,Y+32EDE6 A0          '..'

LDX,Y++32EFAE A1          '..'

;If this shot not active, move on to next

BEQMPSD_L232F127 2F          ''/'

;Erase both possible shot pixels from rows two above and one above

;current shot location

LDA-$40,X32F3A6 88 C0       '...'

ANDA#$EE32F684 EE          '..'

STA-$40,X32F8A7 88 C0       '...'

LDA-$20,X32FBA6 88 E0       '...'

ANDA#$EE32FE84 EE          '..'

STA-$20,X3300A7 88 E0       '...'

;Compute potential new shot location, two rows down

LEAX$40,X330330 88 40       '0.@'

;If shot has reached bottom of screen...

CMPX#Loc_x000_y18933068C 1D A0       '...'

;...it is finished with

BGEMPSD_L333092C 1E          ',.'

;A = current screen byte of four pixels

LDA,X330BA6 84          '..'

;Convert drawing bitmask into red/blue detection bitmask (which will be

;either $20 for a left-posn shot or $02 for a right-posn one)

ASLB330D58             'X'

STB-$02,S330EE7 7E          '.~'

;If screen byte contains red/blue at the shot's position...

BITA-$02,S3310A5 7E          '.~'

;...the shot has hit a defender (or base or shot); turn defender yellow

BNEMPSD_L4331226 2C          '&,'

;...else draw shot pixel into new bottom row and row above it

ORA-$03,Y3314AA 3D          '.='

STA,X3316A7 84          '..'

LDA-$20,X3318A6 88 E0       '...'

ORA-$03,Y331BAA 3D          '.='

STA-$20,X331DA7 88 E0       '...'

;Update shot->ScreenLoc

STX-$02,Y3320AF 3E          '.>'

;Loop while more shots might be active

MPSD_L2CMPYVar_p_ShotPtrLimit332210 BC 32 95    '..2.'

BCSMPSD_L1332625 C5          '%.'

RTS332839             '9'

;Mark this array entry as unused

MPSD_L3CLR-$02,Y33296F 3E          'o>'

CLR-$01,Y332B6F 3F          'o?'

;Erase both possible shot pixels from rows one above and two above

;current shot location

LDA-$20,X332DA6 88 E0       '...'

ANDA#$EE333084 EE          '..'

STA-$20,X3332A7 88 E0       '...'

LDA-$40,X3335A6 88 C0       '...'

ANDA#$EE333884 EE          '..'

STA-$40,X333AA7 88 C0       '...'

;If more potential shots, process next one

BRAMPSD_L2333D20 E3          ' .'

;Find the defender which the player's shot has hit, and turn it yellow.

;Unclear why we process 8 rows, when the flying defenders are 7 rows high

;and the base is 9 rows high.

MPSD_L4LDA#8334086 08          '..'

STAVar_b_PSH_Rows3342B7 33 3F       '.3?'

PSHSX334534 10          '4.'

;Point one row higher, then test the byte one to the right of where we

;originally found the collision; if there are no defender pixels there...

LEAX-$20,X334730 88 E0       '0..'

LDB$21,X334AE6 88 21       '..!'

BNEMPSD_L5334D26 02          '&.'

;...point one byte (four pixels) to the left

LEAX-$01,X334F30 1F          '0.'

;We are now pointing somewhere in the left half of the defender; scan

;upwards until the pixel above us has no blue or red (or until we are

;pointing at the row below the bottom stripe of the double stripe); we will

;then be pointing to the top-left byte of the defender

MPSD_L5LDA-$20,X3351A6 88 E0       '...'

CMPX#Loc_x000_y04033548C 0B 00       '...'

BLEMPSD_L633572F 09          '/.'

ANDA#$AA335984 AA          '..'

BEQMPSD_L6335B27 05          ''.'

LEAX-$20,X335D30 88 E0       '0..'

BRAMPSD_L5336020 EF          ' .'

;(Recall: bit patterns: 00=G, 01=Y, 10=B, 11=R); blue and red pixels become

;yellow; green and yellow pixels become green; this works because any set

;bits overflowing from one pixel into the one to its right get masked off

MPSD_L6LDD,X3362EC 84          '..'

LSRA336444             'D'

ANDA#$55336584 55          '.U'

LSRB336754             'T'

ANDB#$553368C4 55          '.U'

STD,X336AED 84          '..'

;Process next row up; the "ANDA #$AA; BEQ" branch above will be taken

;because of the ANDing with $55 we just did

LEAX$20,X336C30 88 20       '0. '

DECVar_b_PSH_Rows336F7A 33 3F       'z3?'

BNEMPSD_L5337226 DD          '&.'

;Clear this shot and process any others

PULSX337435 10          '5.'

BRAMPSD_L3337620 B1          ' .'

;Sub_MoveDfrBaseCheckHit

;Move the defender base and check whether it's been hit, launching the

;appropriate actions (starting new base patrol, initiating explosion and

;sound effect) if so.

Sub_MoveDfrBaseCheckHitDECVar_b_MDBCH_MovePhase33917A 33 8D       'z3.'

LBNEMDBCH_L5339410 26 00 80    '.&..'

;MovePhase = active_player->BaseMovePeriod

LDUVar_p_ActivePlayerData3398FE 37 B9       '.7.'

LDA,U339BA6 C4          '..'

STAVar_b_MDBCH_MovePhase339DB7 33 8D       '.3.'

;Look for any yellow pixels in a sample of three bytes within the defender

;base bounding box (left byte six rows down; right byte six rows down; left

;byte two rows down)

LDXVar_s_DefenderBase33A0BE 33 8A       '.3.'

LDA#933A386 09          '..'

LDB$00C0,X33A5E6 89 00 C0    '....'

ORB$00C1,X33A9EA 89 00 C1    '....'

ORB$40,X33ADEA 88 40       '..@'

ANDB#$5533B0C4 55          '.U'

;If no yellow, move base as normal

BEQMDBCH_L233B227 2D          ''-'

;Defender base hit; record its location...

LDUVar_s_DefenderBase33B4FE 33 8A       '.3.'

STUVar_s_LastDfrExpln33B7FF 39 E0       '.9.'

PSHSY,X,D33BA34 36          '46'

;...and the fact that a base was just destroyed

LDA#1233BC86 0C          '..'

STAVar_b_LastDfrExplnKind33BEB7 39 DF       '.9.'

;...and initiate the explosion

JSRSub_InitDfrExplosion33C1BD 39 E4       '.9.'

PULSY,X,D33C435 36          '56'

;Start by assuming defender base will spawn at left edge...

CLRVar_b_DfrBase_X_bytes33C67F 33 8F       '.3.'

LDU#Loc_x000_y18133C9CE 1C A0       '...'

;...but if SpawnSide == $00 == RIGHT...

TSTVar_b_MDBCH_SpawnSide33CC7D 33 90       '}3.'

BNEMDBCH_L133CF26 08          '&.'

;...arrange to spawn at right edge instead

LDU#Loc_x120_y18133D1CE 1C BE       '...'

LDA#3033D486 1E          '..'

STAVar_b_DfrBase_X_bytes33D6B7 33 8F       '.3.'

;Store screen-loc of to-be-spawned base

MDBCH_L1STUVar_s_DefenderBase33D9FF 33 8A       '.3.'

;Get ready to spawn from other side next time

COMVar_b_MDBCH_SpawnSide33DC73 33 90       's3.'

;Erase the defender base, by clearing an 8-wide, 9-high block of pixels

;(i.e., 2 bytes wide) with the top-left byte pointed to by X; panic if this

;takes us beyond screen memory

LDA#933DF86 09          '..'

MDBCH_L2CMPX#D32_GraphicsPage533E18C 1E 00       '...'

LBGESub_SetTextMode33E410 2C FC E2    '.,..'

CLR,X+33E86F 80          'o.'

CLR,X33EA6F 84          'o.'

LEAX$1F,X33EC30 88 1F       '0..'

DECA33EF4A             'J'

BNEMDBCH_L233F026 EF          '&.'

;Adjust the base's location and byte-column to try to track the player's

;ship, by moving base left if player is strictly to the left of base,

;otherwise moving base right

LDXVar_s_DefenderBase33F2BE 33 8A       '.3.'

LDAVar_b_DfrBase_X_bytes33F5B6 33 8F       '.3.'

CMPAVar_b_ActivePlr_Xwhole33F8B1 31 E6       '.1.'

BCSMDBCH_L633FB25 1C          '%.'

LEAX-$01,X33FD30 1F          '0.'

DECVar_b_DfrBase_X_bytes33FF7A 33 8F       'z3.'

;Store new defender location

MDBCH_L3STXVar_s_DefenderBase3402BF 33 8A       '.3.'

;Blt defender-base graphic to new location

LDU#GFX_BlueDefenderBase3405CE 33 78       '.3x'

LDA#9340886 09          '..'

MDBCH_L4LDB,U+340AE6 C0          '..'

STB,X+340CE7 80          '..'

LDB,U+340EE6 C0          '..'

STB,X3410E7 84          '..'

LEAX$1F,X341230 88 1F       '0..'

DECA34154A             'J'

BNEMDBCH_L4341626 F2          '&.'

MDBCH_L5RTS341839             '9'

;Part of above; move base right if player not to our left

MDBCH_L6LEAX$01,X341930 01          '0.'

INCVar_b_DfrBase_X_bytes341B7C 33 8F       '|3.'

BRAMDBCH_L3341E20 E2          ' .'

;Sub_StepDfrShotHitAnimn

;When a defender base shot has hit something, wait three frames then draw the

;red asterisk small explosion graphic, and leave it there for a duration of

;four frames before erasing it; on the next frame, launch a new shot and

;become idle.  Progression through this sequence is tracked by the variable

;Var_b_DefenderShot_State, which can acquire a non-idle state externally by:

;

;  Sub_StepDfrShotMovement setting DefenderShot_State to 0 when the

;  shot reaches the top of the play area, causing a new shot to be launched

;

;or

;

;  Sub_InitDefenderShotHit (which is called when a hit by the defender shot

;  is detected) setting DefenderShot_State to 8, causing the explosion

;  sequence to run.

;

;In terms of the value of Var_b_DefenderShot_State on entry:

;

;  >0 (except 5 or 1): do nothing, decrement state value

;  5: draw mini explosion; move to state 4

;  1: erase mini explosion; move to state 0

;  0: launch new shot; move to state -1

;  -1: idle; do nothing; stay in state -1

;Branch according to state; if zero...

Sub_StepDfrShotHitAnimnLDAVar_b_DefenderShot_State3423B6 34 22       '.4"'

;...launch new shot

BEQSDSHA_L3342627 28          ''('

;...else if negative (in practice only ever -1), do nothing

BMISDSHA_L434282B 32          '+2'

;Move to logically "next" state; if state was 1...

DECVar_b_DefenderShot_State342A7A 34 22       'z4"'

;...erase explosion

BEQSDSHA_L5342D27 2E          ''.'

;If state on entry was not 5...

CMPA#5342F81 05          '..'

;...nothing more to do

BNESDSHA_L4343126 29          '&)'

;State-on-entry 5 — draw red asterisk (bottom up)

LDXVar_s_DefenderShot3433BE 34 20       '.4 '

LDA#5343686 05          '..'

STAVar_b_DefenderShot_Rows3438B7 34 BB       '.4.'

LDU#GFX_RedAsterisk343BCE 34 BC       '.4.'

SDSHA_L1LDD,U++343EEC C1          '..'

CMPX#Loc_x000_y04034408C 0B 00       '...'

BLESDSHA_L234432F 02          '/.'

STD,X3445ED 84          '..'

SDSHA_L2LEAX-$20,X344730 88 E0       '0..'

DECVar_b_DefenderShot_Rows344A7A 34 BB       'z4.'

BNESDSHA_L1344D26 EF          '&.'

RTS344F39             '9'

;State-on-entry 0 — launch new shot; move to state -1 (idle)

SDSHA_L3LDXVar_s_DefenderBase3450BE 33 8A       '.3.'

LEAX-$60,X345330 88 A0       '0..'

STXVar_s_DefenderShot3456BF 34 20       '.4 '

DECVar_b_DefenderShot_State34597A 34 22       'z4"'

SDSHA_L4RTS345C39             '9'

;State-on-entry 1 — erase shot / asterisk (bottom up)

SDSHA_L5LDXVar_s_DefenderShot345DBE 34 20       '.4 '

LEAX$60,X346030 88 60       '0.`'

LDA#9346386 09          '..'

STAVar_b_DefenderShot_Rows3465B7 34 BB       '.4.'

LDD#$00003468CC 00 00       '...'

SDSHA_L6CMPX#Loc_x000_y040346B8C 0B 00       '...'

BLESDSHA_L7346E2F 02          '/.'

STD,X3470ED 84          '..'

SDSHA_L7LEAX-$20,X347230 88 E0       '0..'