Invaders drop bombs, and the base has lives

Now the invaders fight back. One of them, picked with the R register as lesson 4 picked a move, drops a bomb, a zigzag that falls a row every turn. A bomb that falls into the base costs a life; the lives, 3 to begin with, are at the top right, LIVES 3. With none left, GAME OVER appears in the top line and the program stops. There is one bomb at a time, as there is one shot, and the next drops as soon as the last is gone. The invaders no longer stop above the base: one that comes down to row 13 has landed, and that ends the game too, as in the arcade game. And now that the game can be lost, it waits for you: PRESS SPACE TO START.

What changed

Chapter 10's program, with these changes:

• ~ The comment at the top, lines 1 to 5, now describes chapter 11.
• ~ MARCH's comment: line 142 is changed and line 143 is new; an invader that comes down to row 13 ends the game.
• − Six lines of MARCH, from EX DE,HL to LD (INVROW),A, are taken out: the invaders no longer stop at a floor. Below the program, under lines taken out, they show struck through, before line 157.
• + In MARCH, line 159: CALL LANDED, after CALL MOVEROW.
• ~ SHOWSCORE's comment: lines 240 and 241 are changed and line 242 is new; it now prints the lives too.
• + Seven lines at the start of SHOWSCORE, lines 244 to 250: they print LIVES and the lives at the end of row 0.
• + MOVEBOMB, DROP and LANDED, new routines with their comments, after TWODIG, lines 276 to 354: the bomb falls, hits the base, or a new one drops; and an invader on row 13 ends the game.
• + At the end of START, lines 365 to 380: the top line and the invaders are drawn, and the program waits for the space bar under PRESS SPACE TO START, then erases the words.
• ~ The game loop's comment: line 383 is changed and line 384 is new; a turn now moves the bomb and draws the top line too.
• ~ At the loop's end, lines 392 and 393 are changed and 394 to 406 are new: CALL MOVEBOMB comes in before CALL SHOWSCORE, whose comment now names the lives, JR LOOP's comment says the loop goes round until the game is over, and after the loop come OVER and STOP, the end of the game.
• − INVROW is taken out, with the floor; it shows struck through before line 426.
• + LIVESTXT, LIVES, BOMBCH, BOMB, OVERTXT and GOTXT, after SCORE, lines 429 to 436: the lives' word, the lives, the bomb's character, where the bomb is, and the words GAME OVER and PRESS SPACE TO START.

A bomb falls as the shot rises

BOMB holds the bomb's screen address, or 0 for none, as SHOT does for the shot, and MOVEBOMB is MOVESHOT turned upside down: it adds 64 instead of -64. The test for leaving the screen is the same BIT 2,H, because the address after the bottom row, 4000H, has bit 2 of its high byte clear, as 3BFFH before the top row does.

The bomb's character is 99H, BOMBCH, a graphics byte lighting bits 0, 3 and 4: the top left block, the middle right and the bottom left, a zigzag (as Fugløy's screen draws them). EQU names it, as it named NINV.

The bomb has to get past things the shot never met: it starts under an invader on row 2, 4 or 6, and the rows' invaders stand in the same columns, so a bomb from row 2 falls through rows 4 and 6. So MOVEBOMB is careful in two ways. It erases the bomb only if the bomb is still what is in its cell, and it draws the bomb only in a blank cell:

        LD      A,(HL)          ; is the bomb still what is in its cell?
        CP      BOMBCH
        JR      NZ,FALL         ; no: something was drawn over it, leave that
        LD      (HL),20H        ; yes: erase it
...
        LD      A,(HL)          ; what is in the cell the bomb falls into?
        CP      20H             ; a blank: draw the bomb there
        JR      Z,BOMBDRAW

So a bomb that falls into an invader's cell is not drawn, and the next turn, finding the invader there and not the bomb, it leaves the invader alone and falls on: it passes behind. The same happens with the shot. A shot and a bomb pass through each other; here a shot cannot stop a bomb. CALL MOVEBOMB comes after CALL DRAWROW in the loop, so the invaders are on the screen when MOVEBOMB looks.

A bomb in the base

If the cell the bomb falls into holds 0BCH, a wing, or 0BFH, the middle, the bomb has hit the base: nothing else on the screen uses those two characters, so, as in chapter 9, the screen says what was hit:

        CP      0BCH            ; the base's wing
        JR      Z,BASEHIT
        CP      0BFH            ; or its middle
        JR      Z,BASEHIT
...
BASEHIT:
        LD      HL,0            ; the bomb is gone
        LD      (BOMB),HL
        LD      HL,LIVES        ; and a life
        DEC     (HL)
        RET     NZ              ; lives left: play on
        JP      OVER            ; none: the game is over

DEC (HL) takes 1 from the byte in memory and sets Z when it reaches 0, as DEC (HL) on TICK does in MARCH. The base is not damaged; only the lives show the hit. A bomb is a hit when it falls into the base. A bomb that lands on the bottom row beside the base, and that the base then moves onto, is not: MOVEBASE draws the base over it, and the next turn the bomb falls off the screen. SHOWSCORE prints the lives as one digit, '0' plus LIVES, the way TWODIG prints a digit.

Which invader: the R register

When no bomb is falling, DROP picks an invader. R, as in lesson 4, is the Z80's refresh register: the processor adds 1 to its low seven bits at every opcode fetch, once for most instructions and twice for those with a prefix byte, as the ones using IX have. Bit 7 is left alone, and it is 0 here, so LD A,R gives a number from 0 to 127. Lesson 4's remainder loop, with NINV for 3, makes it a place in the table, 0 to 23:

DROP:   LD      A,R             ; R counts instruction fetches: 0 to 127 here
WHICH:  CP      NINV            ; the remainder by NINV: carry when A is less
        JR      C,PICKED
        SUB     NINV            ; otherwise take NINV away and compare again
        JR      WHICH
PICKED: ADD     A,A             ; times 2: a word a place
        LD      E,A
        LD      D,0
        LD      IX,INVS
        ADD     IX,DE           ; IX points at the invader's place

ADD IX,DE adds a pair to IX as ADD HL,DE adds one to HL. If that place is 0, the invader is shot down and there is no bomb this turn; R will be different the next. Otherwise HL is the invader's address, and JR FALL jumps into MOVEBOMB to move it down a row and draw the bomb there, so a new bomb starts under the invader's left cell. As the invaders are shot down, more picks find a 0, and bombs come less often.

None of this is random: R is a count. What makes it change from game to game is you. Before the game starts, START waits for the space bar a turn at a time, so how many turns you take to press it changes R when the first bomb drops; after that, every key you press changes how many instructions run, and so which invader R picks next. Fugløy's checks on this chapter press the space bar at the same moment every time, and see the same game every time.

Waiting for the space bar

Before the first turn, START draws the top line and the invaders, prints PRESS SPACE TO START on row 10, and waits:

GO:     CALL    WAIT            ; a turn's pause, then look at the space bar
        LD      A,(3840H)       ; the space bar is bit 7 of the arrows' row
        BIT     7,A
        JR      Z,GO            ; not down: wait another turn

It is lesson 4's wait for a key, with CALL WAIT in it, so it looks once a turn. Then a DJNZ loop writes 20 spaces over the words, and the game starts. If you are still holding the space bar when the first turn comes round, FIRE fires a shot at once.

Landing

Chapter 10's invaders stopped coming down when the top row reached row 9, and marched along there for ever. Now there is no floor: MARCH loses the six lines that kept INVROW, and INVROW goes too, so every step at an edge goes down a row. After every step, LANDED walks the table and looks for an invader on row 13:

        LD      A,(IX+1)        ; the high byte: 3FH is rows 12 to 15
        CP      3FH
        JR      NZ,LANDNEXT
        LD      A,(IX+0)        ; the low byte: 40H or more is row 13 or lower
        CP      40H
        JP      NC,OVER         ; landed: the game is over

Rows 12 to 15 are 3F00H to 3FFFH, and row 13 starts at 3F40H, so an address on row 13 has the high byte 3FH and the low byte 40H or more. CP 40H sets the carry when A is less than 40H, so JP NC jumps when it is not. A place that is 0 has the high byte 0 and is never on row 13, so the shot-down invaders need no test of their own. It is the lowest invader still standing that lands: shoot the bottom row away and the two above it have further to come.

The end of the game

The game ends two ways: BASEHIT with no lives left, and LANDED. Both jump to OVER, which prints the top line once more, so that it shows the lives, draws the invaders, because MARCH erases them before it moves them and LANDED runs before they are drawn again, then copies GAME OVER into the middle of row 0, and stops:

STOP:   JR      STOP            ; the game is over: wait here for good

JR STOP jumps to itself, for ever, as lesson 1's program parked at its end. MOVEBOMB and LANDED are called, and jump to OVER with JP, not CALL, so return addresses are left on the stack; nothing will ever return, so they do no harm. To play again, press Run ▶: after GAME OVER the program stays where it stopped.

The cost

A falling bomb adds about 200 T-states to a turn, and printing the lives 178, so a turn with no key down is about 137,400 T-states. LANDED runs only when the invaders step.

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. Fugløy runs the Model I at 1.77 MHz and the Model III at 2.03 MHz, so the game runs a little more slowly there.