Anders Hesselbom

Let’s make the screen flash!

2011-04-16

This program will make the screen flash on the Commodore 128, and it will also run on the Commodore 64. The background color of the screen is stored at D021 (53281), so this effect is accomplished by manipulating the value at that address, multiple times. This program cannot be written in Basic, because the Basic interpreter is too slow.

Remember that the machine code monitor expects code in one format, and displays it in another, so what I show here, is what I actually type in. I will write my program at location 1000 (4096) in the memory. Type MONITOR to enter the machine code monitor, and type F 1000 1100 0 to create some free space.

To begin writing code, type . directly followed by the address you want to add an instruction to, followed by the instruction and the desired parameters. This stores what you just typed in, and expects you to type in the next command, or just hit Enter to exit the edit mode.

.1000 LDA #$01
STA $D021
NOP
NOP
NOP
LDA #$0B
STA $D021
NOP
NOP
NOP

(and so on…)

Let the final command in your program be JMP $1000.

Press Enter to exit edit mode, type X and Enter to exit the monitor. Start your program by typing SYS 4096. Your screen should now be flashing hysterically!

On the first row, .1000 tells the Commodore 128 machine code monitor that I want my program to start at address 1000 (4096). LDA loads a value to the accumulator, and STA stores current value in the accumulator to the given memory address, D021 (53281). The specific memory address D021 controls the background color. NOP means “no operation” and is used only to make the effect less hysterical. Finally, JMP jumps to the given address, creating an infinite loop. Who needs fireworks now?

Categories: Geeky

Tags: C128, C64