xex(5): add examples.

1 files changed, 153 insertions, 1 deletions

diff --git a/xex.5 b/xex.5
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+++ b/xex.5
@@ -27,7 +27,7 @@ level margin: \\n[rst2man-indent\\n[rst2man-indent-level]]
 .\" new: \\n[rst2man-indent\\n[rst2man-indent-level]]
 .in \\n[rst2man-indent\\n[rst2man-indent-level]]u
 ..
-.TH "XEX" 5 "2024-04-30" "0.2.1" "Urchlay's Atari 8-bit Tools"
+.TH "XEX" 5 "2024-05-01" "0.2.1" "Urchlay's Atari 8-bit Tools"
 .SH NAME
 xex \- Atari 8-bit executable file format.
 .\" RST source for xex(5) man page. Convert with:
@@ -102,7 +102,159 @@ exit.
 .sp
 The run address segment is the one that loads the 2\-byte run address
 into address \fB$02E0\fP, aka \fBRUNAD\fP\&.
+.SS Raw Data Blocks
+.sp
+When init code runs, the file is still being loaded. It\(aqs open on IOCB
+#1, and the init code can read some or all of the rest of the file
+itself, before returning to DOS. The part of the file read by an init
+segment doesn\(aqt have to conform to the regular XEX file structure, so
+they\(aqre referred to here as raw data blocks.
+.sp
+Raw data blocks usually occur in files created with "packer" or
+"compressor" programs, or occasionally in other large programs (Turbo
+BASIC is an example). Raw data blocks are generally found just after
+an init address segment. If you have an executable that loads just
+fine on a real Atari or emulator, but fails with \fBxexcat\fP and
+\fBxexamine\fP, a raw data block is usually the reason why.
+.sp
+Files with raw data blocks also can\(aqt be loaded by simple bootloaders
+such as Fenders 3\-sector loader.
 .SH EXAMPLES
+.SS Assembly
+.sp
+Here is a simple assembly language program that changes the background
+of the GRAPHICS 0 text screen to black, and the text to high\-intensity white:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+1000     *=  $0600
+1010 START
+1020     LDA #0
+1030     STA 710
+1040     LDA #$0F
+1050     STA 709
+1060     RTS
+2000     *=  $02E0   ; AKA RUNAD
+2010     .WORD START
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+To assemble this with either Atari\(aqs Assembler/Editor or OSS Mac/65,
+the command is:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+ASM ,,#D:COLORS.XEX
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+This will create a binary file that looks like this:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+offset  0: FF FF 00 06 0A 06 A9 00
+offset  8: 8D C6 02 A9 0F 8D C5 02
+offset 16: 60 E0 02 E1 02 00 06
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+This is a complete (though short) XEX file, and it can be loaded from
+the DOS menu (or D1: prompt, if you use a command\-line\-based DOS).
+.sp
+The first 2 bytes ($FF, $FF) are the signature for the initial
+segment header.
+.sp
+The next 2 bytes ($00, $06) are the load address of the first segment
+($0600, in 6502\-style LSB\-first notation).
+.sp
+The next 2 bytes ($0A, $06) are the end address ($060A).
+.sp
+Since the header says to load data from $0600 to $060A, there are 11
+data bytes in the segment, beginning with $A9, $00 (the 6502 object
+code for the \fILDA #0\fP instruction), and extending to the $60 (RTS
+opcode) at offset 16.
+.sp
+The data from the first segment is immediately followed by the header
+of the next segment, at offset 17. A $FF, $FF signature would be allowed
+here, but in the example, the 2nd segment uses the 4\-byte header.
+.sp
+At offset 17, the $E0, $02 (aka $02E0) is the load address. $E1, $02
+($02E1) is the end address. $02E0/$02E1 is known as \fIRUNAD\fP in the
+Atari world, and it\(aqs the address where DOS will find the entry point
+to the program when it\(aqs done being loaded.
+.sp
+The next (and last) 2 bytes are $00, $06 (aka $0600), which is the run
+address itself (to be deposited at \fIRUNAD\fP).
+.sp
+There are no more segments, since we\(aqve reached end of file.
+.SS Data Only
+.sp
+Since a XEX file can load arbitrary data at arbitrary addresses, there\(aqs
+simpler way to accomplish the color changes. Instead of writing code to
+do the job, we just create a XEX file that loads the colors we want
+directly into the color shadow registers.
+.sp
+In BASIC, you\(aqd say:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+POKE 709,15
+POKE 710,0
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+\&...but BASIC doesn\(aqt have an easy way to create a XEX file.
+An assembler is a more convenient tool. The code is:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+1000  *= $02C5
+1010  .BYTE $0F,$00
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+The binary looks like this:
+.INDENT 0.0
+.INDENT 3.5
+.sp
+.nf
+.ft C
+ff ff C5 02 C6 02 0F 00
+.ft P
+.fi
+.UNINDENT
+.UNINDENT
+.sp
+The file begins with $FF, $FF header, then the 2\-byte load address
+($C5, $02 for address $02C5), then the 2\-byte end address ($C6, $02,
+aka $02C6), then the 2 bytes of data to be stored there ($0F, $00).
+.sp
+When loaded, this will store $0F at address $02C5 (aka \fICOLOR0\fP,
+the text luminance in \fIGRAPHICS 0\fP) and $00 at $02C6 (\fICOLOR1\fP, the
+\fIGRAPHICS 0\fP background color).
+.sp
+Notice that there\(aqs no run or init address. That\(aqs because no code is
+executed; it\(aqs just 2 bytes of raw data.
 .SH HISTORY
 .sp
 The first Atari 8\-bit binary load format was defined by Atari DOS 1.0,