Unfinished Bitness

Continuing with my simple PREFS program, I made a simple modification to learn about the JMP (JuMP) instruction. This also gave me the opportunity to learn about labels, not that there is much to learn, and how defines work in Mac/65.  These are not permanent, just experimental.

Now the program jumps around doing different steps in a non-linear order.  It also utilizes the random number memory location and sets the background color to whatever value it contains at runtime.  The Atari 8 bit computer random number location is 53770 ($D20A).  It will always have a number between 0 ($00) and 255 ($FF) in it.

Instructions

The definition of the JMP instruction is:

• JMP (JuMP): Alters program execution to the jump address specified.

Adding It In

Rather than drive straight through, I’ve added labels and a series of JuMPs to alter the order in which the settings are made.  I also renumbered the first couple of line to make room for modifications.

10    *= $3800
20    .OPT OBJ
50 ; Define memory location 53770 as RAND
51 RAND = $D20A
105 ;
106 ; Set left margin to 0
110    LDA #0
120    STA 82
121 JMP COLOR
125 ;
126 ; Set key repeat rate
130 KEYS   LDA #2
140    STA 730
145 ; Set key delay
150    LDA #14
160    STA 729
161 JMP CLICK
165 ;
166 ; Set random background color
170 COLOR   LDA RAND
180    STA 710
185 ; Set text color
190    LDA #$0C
200    STA 709
201 JMP KEYS
205 ;
206 ; No key click
210 CLICK   LDA #1
220    STA 731
225 ;
230    RTS
240    .END

Breaking It Down

Changes in source from last time are bold.

• 10: Specify the load address of $3800
• 20: Tell Mac/65 to product object code
• 51: Tell Mac/65 to replace occurrences of RAND with $D20A
• 110: Load the accumulator with decimal value 0
• 120: Store the accumulators contents (0) into memory location 82 (the right margin)
• 121: Jump to the address referenced by the label COLOR
• 130: Assign location to label KEYS. Load the accumulator with decimal value 2
• 140: Store the accumulators contents (2) into memory location 730 (the key repeat rate)
• 150: Load the accumulator with decimal value 14
• 160: Store the accumulators contents (14) into memory location 729 (the key repeat delay)
• 161: Jump to the address referenced by the label CLICK
• 170: Assign location to label COLOR. Load the accumulator with the value from the memory location identified by RAND.
• 180: Store the accumulators contents (random #) into memory location 710 (the background color for graphics 0 display mode)
• 190: Load the accumulator with hex value 0C
• 200: Store the accumulators contents (decimal 12) into memory location 709 (the text color for graphics 0 display mode)
• 201: Jump to the address referenced by the label KEYS
• 210: Assign location to label CLICK. Load the accumulator with decimal value 1
• 220: Store the accumulators contents (1) into memory location 731 (the key click; > 0 is off)
• 230: Return
• 240: Tell the assembler we’re done.

Trying It Out

After compiling with “ASM ,,#D:PREFS.COM”, drop back to DOS and run it with “D:PREFS.COM”.  My data drive is D2 so you’ll see D2 instead of D in the screen shots.  At DOS, about to press [RETURN] for the first run:

After the first run:

I ran it again to validate I got a different random color:

Success!  Now I want it to print something on the screen.  Hopefully that isn’t too complex.