428 lines
7.7 KiB
HTML
428 lines
7.7 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<HTML
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><HEAD
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><TITLE
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>Producing Commodore 64 programs</TITLE
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><META
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NAME="GENERATOR"
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CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK
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REL="HOME"
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TITLE="Programming with Ophis"
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HREF="book1.html"><LINK
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TITLE="The basics"
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HREF="c35.html"><LINK
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TITLE="The basics"
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TITLE="Related commands and options"
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HREF="x119.html"></HEAD
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SUMMARY="Header navigation table"
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><TR
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><TH
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COLSPAN="3"
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ALIGN="center"
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>Programming with Ophis</TH
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></TR
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><TR
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><TD
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WIDTH="10%"
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ALIGN="left"
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VALIGN="bottom"
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><A
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HREF="c35.html"
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ACCESSKEY="P"
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><<< Previous</A
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><TD
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WIDTH="80%"
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ALIGN="center"
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VALIGN="bottom"
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>The basics</TD
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><TD
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WIDTH="10%"
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ALIGN="right"
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VALIGN="bottom"
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><A
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HREF="x119.html"
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ACCESSKEY="N"
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>Next >>></A
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></TD
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><HR
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="SECTION"
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><H1
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CLASS="SECTION"
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><A
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NAME="AEN51"
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>Producing Commodore 64 programs</A
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></H1
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><P
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> Commodore 64 programs are stored in
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the <TT
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CLASS="FILENAME"
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>PRG</TT
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> format on disk. Some emulators
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(such as CCS64 or VICE) can run <TT
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CLASS="FILENAME"
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>PRG</TT
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>
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programs directly; others need them to be transferred to
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a <TT
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CLASS="FILENAME"
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>D64</TT
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> image first.
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</P
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><P
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> The <TT
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CLASS="FILENAME"
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>PRG</TT
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> format is ludicrously simple. It
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has two bytes of header data: This is a little-endian number
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indicating the starting address. The rest of the file is a
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single continuous chunk of data loaded into memory, starting at
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that address. BASIC memory starts at memory location 2048, and
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that's probably where we'll want to start.
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</P
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><P
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> Well, not quite. We want our program to be callable from BASIC,
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so we should have a BASIC program at the start. We guess the
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size of a simple one line BASIC program to be about 16 bytes.
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Thus, we start our program at memory location 2064 ($0810), and
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the BASIC program looks like this:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>10 SYS 2064
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</PRE
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></TD
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></TR
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></TABLE
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><P
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> We <KBD
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CLASS="USERINPUT"
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>SAVE</KBD
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> this program to a file, then
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study it in a debugger. It's 15 bytes long:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="SCREEN"
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>1070:0100 01 08 0C 08 0A 00 9E 20-32 30 36 34 00 00 00
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</PRE
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></TD
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></TR
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></TABLE
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><P
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> The first two bytes are the memory location: $0801. The rest of
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the data breaks down as follows:
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</P
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><DIV
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CLASS="TABLE"
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><A
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NAME="AEN65"
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></A
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><P
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><B
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>Table 1. BASIC program breakdown</B
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></P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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CELLSPACING="0"
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CELLPADDING="4"
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CLASS="CALSTABLE"
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><THEAD
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><TR
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><TH
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ALIGN="CENTER"
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>Memory Locations</TH
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><TH
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ALIGN="CENTER"
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>Value</TH
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></TR
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></THEAD
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><TBODY
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><TR
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><TD
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>$0801-$0802</TD
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><TD
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>2-byte pointer to the next line of BASIC code ($080C).</TD
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></TR
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><TR
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><TD
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>$0803-$0804</TD
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><TD
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>2-byte line number ($000A = 10).</TD
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></TR
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><TR
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><TD
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>$0805</TD
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><TD
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>Byte code for the <KBD
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CLASS="USERINPUT"
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>SYS</KBD
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> command.</TD
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></TR
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><TR
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><TD
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>$0806-$080A</TD
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><TD
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>The rest of the line, which is just the string <SPAN
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CLASS="QUOTE"
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>" 2064"</SPAN
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>.</TD
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></TR
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><TR
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><TD
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>$080B</TD
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><TD
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>Null byte, terminating the line.</TD
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></TR
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><TR
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><TD
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>$080C-$080D</TD
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><TD
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>2-byte pointer to the next line of BASIC code ($0000 = end of program).</TD
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></TR
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></TBODY
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></TABLE
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></DIV
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><P
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> That's 13 bytes. We started at 2049, so we need 2 more bytes of
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filler to make our code actually start at location 2064. These
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17 bytes will give us the file format and the BASIC code we need
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to have our machine language program run.
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</P
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><P
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> These are just bytes—indistinguishable from any other sort of
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data. In Ophis, bytes of data are specified with
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the <TT
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CLASS="LITERAL"
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>.byte</TT
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> command. We'll also have to tell
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Ophis what the program counter should be, so that it knows what
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values to assign to our labels. The <TT
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CLASS="LITERAL"
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>.org</TT
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>
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(origin) command tells Ophis this. Thus, the Ophis code for our
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header and linking info is:
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>.byte $01, $08, $0C, $08, $0A, $00, $9E, $20
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.byte $32, $30, $36, $34, $00, $00, $00, $00
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.byte $00, $00
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.org $0810
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</PRE
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></TD
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></TR
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></TABLE
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><P
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> This gets the job done, but it's completely incomprehensible,
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and it only uses two directives—not very good for a
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tutorial. Here's a more complicated, but much clearer, way of
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saying the same thing.
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</P
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><TABLE
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BORDER="0"
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BGCOLOR="#E0E0E0"
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WIDTH="100%"
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><TR
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><TD
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><PRE
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CLASS="PROGRAMLISTING"
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>.word $0801
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.org $0801
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.word next, 10 ; Next line and current line number
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.byte $9e," 2064",0 ; SYS 2064
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next: .word 0 ; End of program
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.advance 2064
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</PRE
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></TD
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></TR
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></TABLE
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><P
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> This code has many advantages over the first.
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<P
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></P
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><UL
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><LI
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><P
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> It describes better what is actually
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happening. The <TT
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CLASS="LITERAL"
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>.word</TT
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> directive at the
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beginning indicates a 16-bit value stored in the typical
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65xx way (small byte first). This is followed by
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an <TT
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CLASS="LITERAL"
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>.org</TT
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> statement, so we let the
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assembler know right away where everything is supposed to
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be.
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</P
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></LI
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><LI
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><P
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> Instead of hardcoding in the value $080C, we
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instead use a label to identify the location it's pointing
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to. Ophis will compute the address
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of <TT
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CLASS="LITERAL"
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>next</TT
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> and put that value in as data.
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We also describe the line number in decimal since BASIC
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line numbers generally <I
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CLASS="EMPHASIS"
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>are</I
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> in decimal.
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Labels are defined by putting their name, then a colon, as
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seen in the definition of <TT
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CLASS="LITERAL"
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>next</TT
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>.
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</P
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></LI
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><LI
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><P
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>
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Instead of putting in the hex codes for the string part of
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the BASIC code, we included the string directly. Each
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character in the string becomes one byte.
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</P
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></LI
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><LI
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><P
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>
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Instead of adding the buffer ourselves, we
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used <TT
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CLASS="LITERAL"
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>.advance</TT
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>, which outputs zeros until
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the specified address is reached. Attempting
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to <TT
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CLASS="LITERAL"
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>.advance</TT
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> backwards produces an
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assemble-time error.
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</P
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></LI
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><LI
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><P
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>
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It has comments that explain what the data are for. The
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semicolon is the comment marker; everything from a semicolon
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to the end of the line is ignored.
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</P
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></LI
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></UL
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>
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</P
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></DIV
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><DIV
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CLASS="NAVFOOTER"
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><HR
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ALIGN="LEFT"
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WIDTH="100%"><TABLE
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SUMMARY="Footer navigation table"
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>The basics</TD
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><TD
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WIDTH="34%"
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VALIGN="top"
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>Related commands and options</TD
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