In this first part of the tutorial we will create a simple Hello World program to run on the Commodore 64. This will cover: How to make programs run on a Commodore 64 Writing simple code with labels Numeric and string data Invoking the assembler

Throughout these tutorials, I will be using a lot of both decimal and hexadecimal notation. Hex numbers will have a dollar sign in front of them. Thus, 100 = $64, and $100 = 256.

Commodore 64 programs are stored in the PRG format on disk. Some emulators (such as CCS64 or VICE) can run PRG programs directly; others need them to be transferred to a D64 image first. The PRG format is ludicrously simple. It has two bytes of header data: This is a little-endian number indicating the starting address. The rest of the file is a single continuous chunk of data loaded into memory, starting at that address. BASIC memory starts at memory location 2048, and that's probably where we'll want to start. Well, not quite. We want our program to be callable from BASIC, so we should have a BASIC program at the start. We guess the size of a simple one line BASIC program to be about 16 bytes. Thus, we start our program at memory location 2064 ($0810), and the BASIC program looks like this: 10 SYS 2064 We SAVE this program to a file, then study it in a debugger. It's 15 bytes long: 1070:0100 01 08 0C 08 0A 00 9E 20-32 30 36 34 00 00 00 The first two bytes are the memory location: $0801. The rest of the data breaks down as follows: Memory Locations Value $0801-$08022-byte pointer to the next line of BASIC code ($080C).$0803-$08042-byte line number ($000A = 10).$0805Byte code for the SYS command.$0806-$080AThe rest of the line, which is just the string 2064.$080BNull byte, terminating the line.$080C-$080D2-byte pointer to the next line of BASIC code ($0000 = end of program). That's 13 bytes. We started at 2049, so we need 2 more bytes of filler to make our code actually start at location 2064. These 17 bytes will give us the file format and the BASIC code we need to have our machine language program run. These are just bytes—indistinguishable from any other sort of data. In Ophis, bytes of data are specified with the .byte command. We'll also have to tell Ophis what the program counter should be, so that it knows what values to assign to our labels. The .org (origin) command tells Ophis this. Thus, the Ophis code for our header and linking info is: .byte $01, $08, $0C, $08, $0A, $00, $9E, $20 .byte $32, $30, $36, $34, $00, $00, $00, $00 .byte $00, $00 .org $0810 This gets the job done, but it's completely incomprehensible, and it only uses two directives—not very good for a tutorial. Here's a more complicated, but much clearer, way of saying the same thing. .word $0801 .org $0801 .word next, 10 ; Next line and current line number .byte $9e," 2064",0 ; SYS 2064 next: .word 0 ; End of program .advance 2064 This code has many advantages over the first. It describes better what is actually happening. The .word directive at the beginning indicates a 16-bit value stored in the typical 65xx way (small byte first). This is followed by an .org statement, so we let the assembler know right away where everything is supposed to be. Instead of hardcoding in the value $080C, we instead use a label to identify the location it's pointing to. Ophis will compute the address of next and put that value in as data. We also describe the line number in decimal since BASIC line numbers generally are in decimal. Labels are defined by putting their name, then a colon, as seen in the definition of next. Instead of putting in the hex codes for the string part of the BASIC code, we included the string directly. Each character in the string becomes one byte. Instead of adding the buffer ourselves, we used .advance, which outputs zeros until the specified address is reached. Attempting to .advance backwards produces an assemble-time error. It has comments that explain what the data are for. The semicolon is the comment marker; everything from a semicolon to the end of the line is ignored.

This code includes constants that are both in decimal and in hex. It is also possible to specify constants in octal, binary, or with an ASCII character. To specify decimal constants, simply write the number. To specify hexadecimal constants, put a $ in front. To specify octal constants, put a 0 (zero) in front. To specify binary constants, put a % in front. To specify ASCII constants, put an apostrophe in front. Example: 65 = $41 = 0101 = %1000001 = 'A There are other commands besides .byte and .word to specify data. In particular, the .dword command specifies four-byte values which some applications will find useful. Also, some linking formats (such as the SID format) have header data in big-endian (high byte first) format. The .wordbe and .dwordbe directives provide a way to specify multibyte constants in big-endian formats cleanly.

Now that we have our header information, let's actually write the Hello world program. It's pretty short—a simple loop that steps through a hardcoded array until it reaches a 0 or outputs 256 characters. It then returns control to BASIC with an RTS statement. Each character in the array is passed as an argument to a subroutine at memory location $FFD2. This is part of the Commodore 64's BIOS software, which its development documentation calls the KERNAL. Location $FFD2 prints out the character corresponding to the character code in the accumulator. ldx #0 loop: lda hello, x beq done jsr $ffd2 inx bne loop done: rts hello: .byte "HELLO, WORLD!", 0 The complete, final source is available in the file.

The Ophis assembler is a collection of Python modules, controlled by a master script. On Windows, this should all have been combined into an executable file ophis.exe; on other platforms, the Ophis modules should be in the library and the ophis script should be in your path. Typing ophis with no arguments should give a summary of available command line options. Ophis takes a list of source files and produces an output file based on assembling each file you give it, in order. You can add a line to your program like this to name the output file: .outfile "hello.prg" Alternately, you can use the -o option on the command line. This will override any .outfile directives. If you don't specify any name, it will put the output into a file named ophis.bin. If you are using Ophis as part of some larger toolchain, you can also make it run in pipe mode. If you give a dash - as an input file or as the output target, Ophis will use standard input or output for communication. Option Effect -o FILEOverrides the default filename for output.-uAllows the 6510 undocumented opcodes as listed in the VICE documentation.-cAllows opcodes and addressing modes added by the 65C02.-qQuiet operation. Only reports warnings and errors.-vVerbose operation. Reports files as they are loaded. The only options Ophis demands are an input file and an output file. Here's a sample session, assembling the tutorial file here: localhost$ ophis -v hello1.oph Loading hello1.oph Assembly complete: 45 bytes output (14 code, 29 data, 2 filler) This will produce a file named hello.prg. If your emulator can run PRG files directly, this file will now run (and print HELLO, WORLD!) as many times as you type RUN. Otherwise, use a D64 management utility to put the PRG on a D64, then load and run the file off that.