Download June/July 1995

C H O P P I N G
B L O C K
Listing 1. General TRE File Format
FILE
OFFSET
DATA
SIZE
0-3
4-7
8-11
12-15
16-19
20-23
24-(xx-1)
char
char
long
long
long
long
var
xx-(yy-1)
var
yy-(zz-1)
var
Record Pointer Table. This table consists of structures
containing the file offsets and sizes of the Data Records
in this TRE file.
zz-EOF
var
Individual Data Records.
DESCRIPTION
“XTRE” char string.
Blanks. 4 bytes of 00.
Starting Offset of the Indirect Record Pointer Table.
Starting Offset of the Path Name Table.
Starting Offset of the Record Pointer Table.
Offset of the first Data Record in this TRE file.
Indirect Record Pointer Table. This table contains indirect
pointers, or “pointers to pointers”. The values in this table
point into the Path Name Table or into the Record Pointer
Table.
Path Name Table. Table of variable-length records containing
system path names and other path-related data.
NOTES:
1. var refers to data having variable sizes.
2. xx is the Starting Offset of the Path Name Table.
3. yy is the Starting Offset of the Record Pointer Table.
4. zz is the Offset of the First Data Record.
5. EOF refers to the end of the file.
and uses about 40MB of hard disk
space. A VESA driver is required for
SVGA graphics. Because Wing Commander III is such a large game and
weighs heavily on system resources, it is
no surprise that many users have had
trouble installing it. (I experienced
installation troubles because the size of
my CD-ROM drivers combined with a
Pro Audio Spectrum driver exceeded
the minimum conventional memory
requirements for my 8MB RAM system. The game features a boot disk creation routine that solved my memory
management problems.)
Wing Commander III requires a
minimum of 360K conventional RAM
plus 7,000K of either extended or
expanded memory. Access to memory
above 640K is provided by Origin’s custom JEMM.OVL protected mode
(DPMI), 32-bit driver. Virtual memory
is emulated through the use of a swap
file named SWAPFILE.$$$. The size
of the swap file is generally determined
by the amount of free hard disk space,
and it is usually 15MB to 20MB. The
swap file is created when Wing Commander III is booted and erased when
you exit the game. (This file is erased
when you exit the game via the Alt-X
keystrokes. So be sure to erase the swap
file from your Wing Commander III
hard drive subdirectory if you exit the
game using any other method such as
rebooting your PC.)
The primary Wing Commander
III executable, WC3.EXE, is about
1.1MB in size. Wing Commander III
was developed using the MetaWare
High C/C++ system and is written primarily in C++. (The MetaWare High C
system features C++ templates and a
compiler for developing 32-bit protected mode applications.) The WC3.EXE
game engine includes a built-in Examination Mode that provides general
development and debugging information while the game is running. Developers can observe the status of the
numerous game engine flags, accumulators, stack manipulation calls, and game
flow data such as act, scene, and mission information. I found several
undocumented keystrokes while exploring game operation, and I’ll summarize
them further on.
Data Storage
Essentially all of the game data is stored
in files suffixed with TRE. (I will refer
to these data files as TRE files.) TRE
files use a very flexible, open-ended
data storage format that allows for very
large files. You can store hundreds of
megabytes of data in a single TRE file.
Listing 1 gives a general summary
of the TRE file format. The first four
bytes in the file contain the XTRE
ASCII text string. The XTRE string
possibly indicates this file contains
cross-referenced data records. The bytes
at offsets 8 to 23 contain four pointers.
The first three pointers give the file offsets (offsets into the TRE file with the
first byte in the file referenced as byte
0) of three data tables. The fourth
pointer gives the offset of the first Data
Record in the file.
The first data table in the TRE
file, the Indirect Record Pointer Table,
contains pointers (file offset values) that
reference data in the other TRE file
data tables. In C nomenclature, the
Indirect Record Pointer Table contains
pointers to pointers. The Indirect
Record Pointer Table entries “point” to
various “pointers” contained in other
TRE file tables. Each entry in this table
is an 8-byte structure. The first four
bytes are bit flags and the last four bytes
contain the pointer expressed as a 32bit long value. I observed that the Indirect Record Pointer Tables of several
TRE files had “blanks” or empty data
slots. As I mentioned earlier, the TRE
file format is flexible and expandable,
and these empty data slots are probably
part of this expandable format.
The Path Name Table follows the
Indirect Record Pointer Table. Embedded in this table are the ASCII text
strings of the source paths along with
binary (possibly file offset) data related
to each path name.
The Record Pointer Table follows
the Path Name table. The Record
Pointer Table contains the file offsets
and sizes of the Data Records stored in
GAME DEVELOPER • JUNE/JULY 1995
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