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Improved Compression Doc, Issue #500

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Co-Authored-By: PauldB <169483608+p42db@users.noreply.github.com>
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Wayne Warthen
2025-10-14 17:52:41 -07:00
parent 6121e80310
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@@ -1252,26 +1252,42 @@ ROM image.
#### Compressed ROM Image File: RCZ80_ez512_std_64k.rom
A 64K ROM image is supported. As there were many areas of $00 and $FF
bytes in RomWBW, it is possible to compress the 128K RomWBW *.upd file
to fit into 64K ROM. The compression routine looks for two or more
consecutive bytes of the same value (any values of $00 to $FF). If it
finds duplicates, it would leave two duplicate bytes followed by a byte
count, n-1, (with a max byte count of $FF) where n is the total number
of duplicates.
The RomWBW utility program 'compress' is designed to squeeze the
compiled RomWBW 128K file 'RCZ80_ez512_std.upd' into a 64K ROM. As there
are many areas in RomWBW with repeating bytes of the same value, it is
possible to compress the 128K file to fit into a 64K ROM.
A stand-alone program written in C creates a ready-to-flash binary file
for the 64K ROM. The 64K binary file is in the following format. Layout
of the 64K ROM:
##### How Compression Works
The compression program looks for two or more consecutive bytes of the
same value (any values of $00 to $FF). If it finds duplicates, it leaves
two of the duplicate bytes followed by a byte count, n-1 (n <= $FF),
where n is the total number of duplicates. If the program succeeds in
compressing the input file to fit into the available space, the file
'RCZ80_ez512_std_64k.rom' is constructed, along with a short Z80
decompression program that will be stored in the last 256 bytes of the
ROM. The constructed 64K file is saved and the unused storage space is
output in bytes. Should compression fail to fit the input file into
available space, only an error message and the overrun in bytes is
output.
##### How the 64K ROM Code Works
The decompression program, located at $FF00, is executed at startup via
the 3-byte jump at location $0000, decompressing the stored code in ROM
into the computer's RAM. When decompression finishes, control is passed
to RAM location $0000, which in turn starts execution of RomWBW.
##### The 64K ROM Layout
* The first 3 bytes are always $C3 $00 $FF, a jump to the Z80
decompression code located at $FF00 in the ROM.
* Locations $0003 up to, and including, $FEFF are then available to
store the compressed 128K *.upd file.
* Locations $0003 up to, and including, $FEFF are available to store
the compressed 128K input file.
* Locations $FF00 up to $FFFF, a seperate Z80 decompression routine that
executes at startup to decompress the the 64K ROM into RAM.
* Location $FF00 up to, and including, $FFFF, is where the Z80
decompression program is stored.
#### Supported Hardware