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--- supercross_2000:ovln [2019/07/04 19:28]
David created
+++ supercross_2000:ovln [2019/07/14 01:53] (current)
David Updated information.
@@ Line 1: / Line 1: @@
 ====== OVLN ======
-This is a compression format used by EA Games
+This is a format for compressing game code and other misc data in the ROM. There are 4 OVLN blocks in Supercross 2000.
+This format uses [[supercross_2000:eahd|EAHD]] for the compression, which starts at offset 0x10 of the OVLN block.
 ----
 ===== Header =====
-The header takes up the first 0x15 bytes of the compressed file
+The OVLN header takes up the first 0x10 bytes of the compressed file. After the header is a EAHD compressed block.
 ^Offset^#Bytes^Description^
@@ Line 13: / Line 15: @@
 |0x08|4|RAM location to decompress|
 |0x0C|4|RAM location to JAL to when done|
-|0x10|2|Magic Number. If it is 0x10FB then decompress, otherwise directly DMA with uncompressed size.|
-|0x12|3|Uncompressed size|
-----
-===== Decompression Code =====
-==== Python ====
-<code python [enable_line_numbers="true"] ovln_parse.py>
-def bytes_to_int24(arr, offset):
-    return (arr[offset + 0] << 16) | (arr[offset + 1] << 8) | arr[offset + 2]
-# get 4 bytes from the array and arrange them in little endian order.
-def bytes_to_int_reversed(arr, offset):
-    arr_boundary = len(arr) - offset # for safety
-    if arr_boundary >= 4:
-        return (arr[offset + 3] << 24) | (arr[offset + 2] << 16) | (arr[offset + 1] << 8) | arr[offset]
-    elif arr_boundary == 3:
-        return (arr[offset + 2] << 16) | (arr[offset + 1] << 8) | arr[offset]
-    elif arr_boundary == 2:
-        return (arr[offset + 1] << 8) | arr[offset]
-    else:
-        return arr[offset]
-def ovln_memset(dst, dst_offset, value, amount):
-    for i in range(amount):
-        dst[dst_offset + i] = value
-def ovln_memcpy(dst, dst_offset, dst_backwards_offset, amount):
-    src_offset = dst_offset - dst_backwards_offset
-    for i in range(amount):
-        dst[dst_offset + i] = dst[src_offset + i]
-def ovln_fetch_bytes(dst, dst_offset, dst_backwards_offset, amount):
-    if dst_backwards_offset > 1:
-        ovln_memcpy(dst, dst_offset, dst_backwards_offset, amount)
-    else:
-        ovln_memset(dst, dst_offset, dst[dst_offset - dst_backwards_offset], amount)
-    return dst_offset + amount
-def ovln_copy_literal(dst, dst_offset, src, src_offset, amount):
-    for i in range(amount):
-        dst[dst_offset + i] = src[src_offset + i]
-# Input: src = compressed data list
-# Output: returns decompressed data list
-def ovln_parse(src):
-    decompressed_size = bytes_to_int24(src, 0x12)
-    dst = [0] * decompressed_size # Decompressed data list
-    src_offset = 0x15
-    dst_offset = 0
-    while True:
-        cmd = bytes_to_int_reversed(src, src_offset)
-        if cmd & 0x80 == 0: # Covers commands 00 to 7F
-            src_offset += 2
-            # Copy bytes from the compressed data array
-            amount_to_copy = cmd & 0x3
-            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
-            src_offset += amount_to_copy
-            dst_offset += amount_to_copy
-            # Fetch bytes from previous decompressed data
-            backward_fetch_offset = ((cmd << 3) & 0x300) + ((cmd >> 8) & 0xFF) + 1
-            amount_of_bytes_to_fetch = ((cmd >> 2) & 0x7) + 3
-            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
-            continue
-        if cmd & 0x40 == 0: # Covers commands 0x80 to 0xBF
-            # Copy bytes from the compressed data array
-            src_offset += 3
-            amount_to_copy = ((cmd >> 8) >> 6) & 0x3
-            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
-            src_offset += amount_to_copy
-            dst_offset += amount_to_copy
-            # Fetch bytes from previous decompressed data
-            backward_fetch_offset = (((cmd >> 8) << 8) & 0x3F00) + ((cmd >> 16) & 0xFF) + 1
-            amount_of_bytes_to_fetch = (cmd & 0x3F) + 4
-            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
-            continue
-        if cmd & 0x20 == 0: # Covers commands 0xC0 to 0xDF
-            src_offset += 4
-            amount_to_copy = cmd & 0x3
-            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
-            src_offset += amount_to_copy
-            dst_offset += amount_to_copy
-            # Fetch bytes from previous decompressed data
-            backward_fetch_offset = ((cmd << 12) & 0x10000) + (((cmd >> 8) << 8) & 0xFF00) + 1 + ((cmd >> 16) & 0xFF)
-            amount_of_bytes_to_fetch = ((cmd << 6) & 0x300) + ((cmd >> 24) & 0xFF) + 5
-            dst_offset = ovln_fetch_bytes(dst, dst_offset, backward_fetch_offset, amount_of_bytes_to_fetch)
-            continue
-        src_offset += 1
-        if (cmd & 0xFF) < 0xFC: # Covers commands 0xE0 to 0xFB
-            # Copy bytes from the compressed data array
-            amount_to_copy = ((cmd & 0x1F) + 1) * 4
-            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
-            src_offset += amount_to_copy
-            dst_offset += amount_to_copy
-            continue
-        if cmd & 3 != 0: # Copy any left-over bytes
-            # Copy bytes from the compressed data array
-            amount_to_copy = cmd & 0x3
-            ovln_copy_literal(dst, dst_offset, src, src_offset, amount_to_copy)
-            src_offset += amount_to_copy
-            dst_offset += amount_to_copy
-        break # A command of 0xFC, 0xFD, 0xFE, or 0xFF will end the decompression routine.
-    return dst
-</code>
 ----

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