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I'm just creating my own .prg file using NCurses Hexeditor in Ubuntu which I load using load"file.prg",8 in Vice. I followed the information in this link explaining how to create a .prg file. It says basic starts at 2048 ($0800) but loads the .prg code starting at $0801:

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Is there any reason for this? I tried it at $0800 and all works fine (see below). I guess it wouldn't matter because the Assembly code starts at 2064 ($0810), but just thought I might be missing something as to why they chose $0801 and not $0800.

What I tried

First of all, according to the C64 Programmer's Reference Guide (C64PRM, p.59):

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Tests show that no matter what PRG header I use my bytes are always loaded into 2048 when using ,8 as per C64PRM. But let's just suppose I use a header for $0800, then I have the following bytes in Hexeditor:

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and I don't get an error, e.g.:

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  • I think that if you look at the monitor after loading your program you will see that it is actually loaded at $0801. – Brian H Dec 27 '20 at 16:09
  • @BrianH yes the monitor seems to show everything stored at $0801 and the next BASIC instruction pointer is automatically incremented. Strange, but there it is. – Pixel Dec 27 '20 at 19:32
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It is not strictly true that "all works fine" for ML programs with a standard BASIC header starting at $0800 vice $0801. The idea of the BASIC header is that your program is easily loaded and then started using the "RUN" command:

LOAD"MYPROG",8 REM NOTE ABSENCE OF ",1"
RUN

If you move the header one-byte down to $0800, then BASIC no longer parses it correctly. Your line number will be messed up, and RUN'ing your program will likely return "?SYNTAX ERROR". It is true that your actual ML code is unaffected and you could still start it with the appropriate "SYS" command manually entered. But the desire is for it to work by just using the standard LOAD & RUN. Screen shot below shows this for a quick & dirty test program I created to load at $0800. The list of the program should read as:

2020 SYS 4864

Looking at the disassembly of Commodore BASIC v2, this assumption of the actual first line being at $0801 is probably related to the fact that the RUN command is a special case of the GOTO command. Because of this, I think the first byte is skipped just to simplify the coding of the interpreter.

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  • Thanks @BrianH I'm a little uncertain as the C64 Programmers Reference Manual says that programs will LOAD into 2048 unless the secondary address ,8,1 is used. And if I change my PRG header to anything I like, it always loads into 2048 when I use ,8 and I don't get the corrupt line number or crash you show above. – Pixel Dec 27 '20 at 9:18
  • No it does not load to $0800 (2048) if the secondary address is not given, it loads to $0801 (2049), always. – Retrograde Dec 27 '20 at 19:06
  • @Retrograde yes, the monitor seems show everything stored at $0801 and the next BASIC instruction pointer is automatically incremented. Strange, but there it is. – Pixel Dec 27 '20 at 19:32
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    It seems the C64 Programmers Reference Manual is wrong in saying that programs will LOAD to 2048. Many other publications describes it correctly (with load start at $0801 or 2049). There is some confusion about the start of the BASIC program area which is actually $0800, but on the first location a 0 is needed because the RUN command starts with the BASIC text pointer pointing to the last read character. This 0 byte provides an end-of-line condition, which let the interpreter search for the next line (an implicit GOTO 0) and advance to $801 as the first line to interpret. – Johann Klasek Jan 20 at 19:35
  • @JohannKlasek Thanks. That explanation makes sense with what I was also seeing in the BASIC interpreter source code. – Brian H Jan 21 at 0:01

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