...the saga continues...
Here's the reference program:
Code: Select all
10 PRINT "{CLR}"
20 POKE 36879, 14
30 POKE 36878, 15
40 X = 1
50 Y = 1
60 DX = 1
70 DY = 1
80 POKE 7680 + X + 22*Y, 81
90 FOR T=1 TO 10: NEXT
100 POKE 7680 + X + 22*Y, 32
110 X = X + DX
120 IF X=0 OR X=21 THEN DX=-DX: POKE 36876, 220
130 Y = Y + DY
140 IF Y=0 OR Y=22 THEN DY=-DY: POKE 36876, 230
150 POKE 36876,0
160 GOTO 80
My approach is a bit different from Mike's, since I'm more interested in showing the wAx workflow than in duplicating the program exactly. I'll be using the KERNAL routine PLOT to position the ball.
A couple of wAx tricks are shown right at the top. The first is that wAx offers several immediate operand formats, including a character byte. The second is that there's a numeric converter tool that will help us find the hex equivalents of the decimal addresses in the BASIC program:
Next, we initialize the data. I'm doing this exactly like Mike did, with the X,Y, and direction registers at the same zero page addresses, all set to 1. Note another immediate operand option, the decimal number. Note also the -M syntax at $1818. This sets the label -M to the value $1818, which is the start of the main loop. You don't need to remember, or write down, $1818.
Here I'm setting up the PLOT call and drawing the ball:
Then I'll add a delay of 30 jiffies (more on this later). Note the use of the label -@ in the loop. The -@ label is designed to be a local looping label. Its value is set to $182A, and then the branch instruction at $182C uses that address as its operand.
Now, I write a backspace and a space, to delete the ball after the delay, and add the X and Y directions to the X and Y positions:
The pitch starts at 0, and is set to either 220 or 230, based on which direction is changed. The pitch is initialized to 0 in the Y register at $1838.
Please take note of the forward reference syntax at $1841. Sometimes, we don't know the address we're jumping to as we code. If you specify -> as the target address, this reference can be resolved later with -@. You can actually use any label as a forward reference; the special thing about -> and -@ is that it can be used as a forward reference over and over again within the program. -> is specifically designed for the kind of jumps you see here: numerous short-range decision points.
When the ball hits the wall, its direction can be reversed, from 1 to -1 ($FF), or from -1 to 1 ($01). When a direction is changed, Y is set to the pitch of the bounce.
At $186c, note that I'm using the -@ label on a line by itself. wAx lets you do this, or put an instruction after the label. In either case, the forward reference from $1864 is resolved.
Note also $186f, in which there's a JMP -M, which uses the -M address of $1818.
Now, after I've run this program, I noticed that it moves too slowly. It takes a half-second ($1e jiffies) for the ball to move. wAx helps us out with a code search tool. You may do searches by byte patterns, text, or 6502 instructions. Code search capability has turned out to be way more useful in practice than I thought it would be, enabling me to find starting points of routines by referring to the source code.
wAx is available on cartridge at
https://www.etsy.com/listing/856409783/ ... for-vic-20
