One invader

The first invader arrives: two cells on row 2, near the top, straight above where the base starts. It does not move, and a shot passes behind it; it is here to be drawn, the way everything else in the game is drawn, by a routine, from a few bytes, at a place kept in memory. The next chapters give it company, then make the company march.

What changed

Chapter 4's program, with these changes:

• ~ The comment at the top, lines 1 to 5, now describes chapter 5.
• + DRAWINV, a new routine with its comment, below FIRE: it copies two bytes to the invader's place on row 2.
• ~ The game loop's comment: its last line now ends the base, the shot, the invader.
• + Two lines in the loop, after CALL FIRE: LD HL,INVADER and CALL DRAWINV, which draw the invader every turn.
• + INVADER, after BLANK: the invader's two cells, on two lines, the label on the first.
• + INVX, after SHOT: the invader's column.

Nothing is taken out.

The invader

The invader is two graphics cells, 9EH and 0ADH:

INVADER:
        DB      9EH,0ADH        ; an invader: two cells, a head, arms and legs

As with the base, the low six bits of a graphics byte light the cell's blocks, two across and three down: bit 0 top left, 1 top right, 2 and 3 the middle, 4 and 5 the bottom (as Fugløy's screen draws them, from the TRS-80's character generator). 9EH lights bits 1 to 4, and 0ADH bits 0, 2, 3 and 5. Side by side, the top row lights the two middle blocks, a head; the middle row all four, arms; and the bottom row the two outer ones, legs. The label INVADER: is too long for the space before DB, so it stands on a line of its own, as DRAWBASE: does above its code: either way a label names the address of the next byte the program assembles.

Its place, and DRAWINV

The invader's column is a byte, as the base's is:

INVX:   DB      30              ; the invader's column, 0 to 62

DRAWINV is DRAWBASE for a thing two cells wide on row 2. Row 2 starts at 3C80H, two rows of 64 in, so the invader's left cell is 3C80H plus INVX; the column is at most 62, and 80H plus 62 is 0BEH, one byte, so the high byte is always 3CH:

DRAWINV:
        PUSH    BC              ; save BC, which LDIR counts down
        LD      A,(INVX)        ; the invader's column ...
        ADD     A,80H           ; ... plus 3C80H, row 2: the invader's left cell
        LD      E,A
        LD      D,3CH
        LD      BC,2            ; two bytes
        LDIR                    ; from (HL) to (DE), one byte at a time
        POP     BC              ; BC as the caller left it
        RET

As with DRAWBASE, HL says what to draw, so a later chapter can erase the invader with two of BLANK's spaces before moving it.

Drawn every turn

The base is drawn when it moves, and the shot when it moves. The invader does not move, yet the loop draws it every turn:

        LD      HL,INVADER      ; the invader, drawn every turn: a shot passing
        CALL    DRAWINV         ; through it erases a cell, and this mends it

That is for the shot. With the base where it starts, a shot rises in column 31, the invader's right cell. On the turn it reaches row 2, MOVESHOT draws the shot over the invader, and then DRAWINV draws the invader over the shot; on the next turn MOVESHOT erases the shot where it was, a space in the invader's right cell, and draws it on row 1, and DRAWINV mends the invader again. Between the two, a few hundred T-states, too short to see. So the shot seems to pass behind the invader. Making it hit is chapter 9's work.

Drawing it every turn costs 136 T-states a turn: LD HL,INVADER 10, CALL 17, and DRAWINV 109, of which the LDIR takes 37, 21 for the first byte, which it repeats after, and 16 for the last. A turn with no key down is now 133,470 T-states, still about 66 ms at the Model III's 2.03 MHz.

Keys go to the screen, not the editor

Click the TRS-80's screen before you press the arrows or the space bar, so that it has the keyboard.

On the Model I

The Model I has the same keyboard row and the same screen, so the program runs unchanged, a little slower at the Model I's 1.77 MHz.