Space Invaders Bunker

The following (edited) code snippet is from Richard Degler's disassembly [.zip] of the Bally Arcade Space Invaders prototype. The LOOP section demonstrates writing a single pattern (the bunker graphic) to the screen multiple times using the WRITP UPI routine. The BUNKER section shows the referenced pattern block.

; [...]
 
; Write Four Player "Protective Bunkers"
; ---------------------------------------
;   The Four Bunker's X,Y Coordinate's are:
;      Bunker 1 (Left-Most):    10, 65
;      Bunker 2 (Middle-Left):  50, 65
;      Bunker 3 (Middle-Right): 90, 65
;      Bunker 4 (Right-Most):  130, 65
;
        LD      B,$04           ; 4 for 4 writes
        LD      E,$0A           ; 10 for LEFTmost X
LOOP:   LD      HL,BUNKER       ; point at Player's BUNKER
        XOR     A               ; Magic Register = 0 for PLOP
        LD      D,$41           ; 65 for all Y Coordinate's
        PUSH    DE              ; store DE for X Coordinate
        PUSH    BC
        SYSTEM  WRITP           ; UPI WRITe with Pattern size lookup
        ;
        POP     BC
        POP     DE              ; restore DE for X Coordinate
        LD      A,E             ; old X + 40
        ADD     A,$28
        LD      E,A             ; X Coordinate for Next Bunker
        DJNZ    LOOP            ; (-$12) to run 4 times
 
; [...]
 
; Player's BUNKER color Pattern
BUNKER: DB      $05             ; WIDTH = 5 Bytes
        DB      $0D             ; HEIGHT = 13 Rows
        ;
        DB      $03,$FF,$FF,$FF,$C0 ; . . . 3 3 3 3 3 3 3 3 3 3 3 3 3 3 . . .
        DB      $0F,$FF,$FF,$FF,$F0 ; . . 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 . .
        DB      $3F,$FF,$FF,$FF,$FC ; . 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 .
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$FF,$FF,$FF,$FF ; 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
        DB      $FF,$F0,$00,$0F,$FF ; 3 3 3 3 3 3 . . . . . . . . 3 3 3 3 3 3
        DB      $FF,$C0,$00,$03,$FF ; 3 3 3 3 3 . . . . . . . . . . 3 3 3 3 3
        DB      $FF,$00,$00,$00,$FF ; 3 3 3 3 . . . . . . . . . . . . 3 3 3 3
        DB      $FF,$00,$00,$00,$FF ; 3 3 3 3 . . . . . . . . . . . . 3 3 3 3
 
; [...]

WRITP is squarely in the middle of the screen writing routine hierarchy, meaning that it some preprocessing to execute properly.

[pattern block requirements]

[why choose WRITP vs. others]

XOR  A

is a more efficient way to perform

LD  A,0

, though the former does change flags.