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Blades of Steel (NES, Famicom Disk System)/ROM map

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Chip tiny.png The following article is a ROM map for Blades of Steel (NES, Famicom Disk System).

PPU data bundles

This game has a simple RLE compressed format for PPU data. The ROM stores 15 PPU data bundles, decoded by a subroutine at $7:C818, taking X as an index (x2) to tables that locate the data:

$7:C89D - pointer table to packet data (2-byte address, little-endian, X as index)
$7:C8BB - bank table for packet data (1-byte UNROM bank numbers, X/2 as index)

After setting the specified bank, and loading the pointer, the data stream encoded at this pointer is as follows:

1. 2-byte PPU write address (little endian)
2. Control byte:
   2.1. $FF     - end of bundle
                  stop decoding
   2.2. $7F     - new write address
                  return to step 1
   2.3. $01-$7E - run-length for next byte
        $xx     - byte to be repeated for the duration of the run length
                  return to step 2
   2.4. $81-$FE - & $7F = length of uncompressed data that follows
        ...     - uncompressed data (length given above)
                  return to step 2
   2.4. $00     - run-length value of 256 (never used)
        $xx     - byte to be repeated 256 times
                  return to step 2
   2.5. $80     - invalid uncompressed length value (causes hang)
        ...     - 255 bytes of uncompressed data
                  $80 is written as the 256th byte
                  data pointer doesn't advance, return to step 2.5

Python utility for decoding/encoding: Gist