Move the base with the arrow keys

Hold the left or right arrow and the base slides along the bottom row, a cell a turn; at the screen's edges it stops. The game loop's turn has its first piece of work: each turn it looks at the arrows, and if one is down, moves the base.

What changed

Chapter 2's program, with these changes:

• ~ The comment at the top, lines 1 to 5, now describes chapter 3.
• + MOVEBASE, a new routine with its comment, below DRAWBASE: it reads the arrows and moves the base.
• ~ The game loop's comment: its last line now ends this one moves the base in place of this one only waits.
• + CALL MOVEBASE in the loop, after CALL WAIT.
• + BLANK, three spaces, after BASE: what MOVEBASE draws to erase the base.

Nothing is taken out.

The keyboard is memory too

The keyboard is read as memory, from 3800H: its keys are wired in rows, and each row is read at its own address, 3801H to 3840H. A key that is down reads as a 1 bit in its row's byte, a key that is up as a 0. The row read at 3840H holds ENTER, CLEAR, BREAK, the four arrows and the space bar, and the left and right arrows are its bits 5 and 6: that order within the row is the one Fugløy's emulator uses, from the ROM's own keyboard routine, not a page of the manuals.

MOVEBASE:
        LD      A,(3840H)       ; the arrows' row: bit 5 left, 6 right
        AND     60H             ; keep those two bits (0110 0000)
        RET     Z               ; neither is down: nothing to do
        LD      C,A             ; keep the keys in C, so A is free

AND 60H keeps bits 5 and 6 and clears the rest, and sets the zero flag when what is left is 0, when neither arrow is down. RET Z returns only when the zero flag is set; otherwise the routine runs on. So on most turns, with no key down, MOVEBASE is three instructions and back. If you did lesson 3, this is its read of the arrows.

Erase, move, draw

Nothing on the screen moves by itself. To move the base, MOVEBASE draws three spaces where it is, changes BASEX, and draws the base again at the new column. Chapter 2's DRAWBASE does both draws: HL says what to draw, BLANK or BASE.

BLANK:  DB      '   '           ; three spaces, to erase it

Between the two draws the column changes by one:

        LD      A,(BASEX)       ; the base's column, from memory
        BIT     5,C             ; left?
        JR      Z,NOTLT
        CP      0               ; column 0, the left edge: stay
        JR      Z,NOTLT
        DEC     A
NOTLT:
        BIT     6,C             ; right?
        JR      Z,NOTRT
        CP      61              ; column 61, the right edge for three cells: stay
        JR      Z,NOTRT
        INC     A
NOTRT:
        LD      (BASEX),A       ; the new column, back to memory

BIT 5,C sets the zero flag when bit 5 of C is 0, so JR Z,NOTLT skips the move left when the left arrow is up. CP 0 compares A with 0: it sets the flags as A minus 0 would, zero when A is 0, and leaves A as it was. At column 0 the base is at the left edge and stays; at 61 its right wing is in column 63, the last, and it stays. That is the whole of kept inside the screen.

Why DRAWBASE keeps BC

MOVEBASE keeps the keys in C, and calls DRAWBASE to erase the base before it tests them. DRAWBASE's LDIR counts BC down to 0, so without its PUSH BC and POP BC, C would be 0 when MOVEBASE came to BIT 5,C, and the base would never move. The routine leaves BC as its caller had it.

The speed

MOVEBASE runs once a turn, after WAIT, so a held arrow moves the base one cell a turn. With no key down it adds 48 T-states to the turn, CALL 17, LD A,(3840H) 13, AND 7 and RET Z 11, for 133,233 in all; Fugløy runs the Model III at 2.03 MHz, so that is about 66 ms, and the base crosses the screen at about 15 cells a second. Moving costs a few hundred T-states more, too few to see.

Keys go to the screen, not the editor

Click the TRS-80's screen before you press the arrows, 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 its Z80 at 1.77 MHz, so a turn lasts about 75 ms there and the base moves about 13 cells a second.