DLL System

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Dinosaur Planet uses a DLL ([[Dynamic-Link Library>>https://en.wikipedia.org/wiki/Dynamic-link_library]]) system to reduce the runtime memory required for the game's code.

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By swapping relocatable code modules in-and-out, the game can make sure only the code necessary for the core game as well as the currently used game objects is loaded.

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There are a total of 796 DLLs in the ROM, which are separated in to 4 banks.

= DLL Format =

(% border="0" %)

|=Offset|=Size|=Description

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|0x00|4|Header size

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|0x04|4|DATA offset (relative start of header, or 0xFFFFFFFF if section not present)

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|0x08|4|RODATA offset (relative start of header, or 0xFFFFFFFF if section not present)

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|0x0C|2|Export count

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|0x0E|2|Padding

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|0x10|4|Constructor offset (relative end of header)

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|0x14|4|Destructor offset (relative end of header)

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|0x18|4|Unused

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|0x1C|4 * count|Export offsets (relative end of header)

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|0x1C + 4 * count|4|Unused

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|0x20 + 4 * count|*|Body

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The export count is often checked by the code responsible for loading any specific DLL as a method of determining compatibility. If the DLL does not export enough functions to satisfy the requesting code, it is immediately unloaded.

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== DLL Relocations ==

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The RODATA segment - if it exists - must begin with a DLL relocation table, which has the following structure.

(% border="0" %)

|=Value|=Description

|*|(((

Descriptors for RODATA segment.

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If the MSB (bit 31) is set, this is an index into DLLSIMPORTTAB to resolve a pointer to the base executable.

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If the MSB is not set, this is an offset relative end of header to something within the DLL.

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)))

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|0xFFFFFFFE|Terminates the above section.

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|*|(((

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Descriptors for TEXT segment.

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These are offsets relative end of header to the start of DLL entrypoints for patching the $gp initializer stub.

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)))

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|0xFFFFFFFD|Terminates the above section.

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|*|(((

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Descriptors for DATA segment.

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These are offsets relative the start of the DATA segment, to offsets stored in that segment that are to be replaced with pointers.

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)))

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|0xFFFFFFFF|Terminates the above section.

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If a section within the relocation table is unused, it can be immediately terminated, but the table structure must exist.

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DLL entrypoint functions usually begin with a $gp initializer stub which is patched by the loader to allow the DLL to locate its data segments. The value assigned is a pointer to the start of the RODATA segment.

lui gp, 0x0

ori gp, gp, 0x0

nop

DLLs may only reference other DLLs that they load themselves, or ones that are loaded by the base executable during startup. The latter are made accessible through global variables which have an entry in the DLLSIMPORTTAB.

== DLL Table ==

The DLLS.tab describes the start and end offsets of each DLL within the DLLS.bin, but also the BSS size. It begins with a 16-byte header describing the 4 DLL banks.

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(% border="0" %)

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|=Offset|=Size|=Description

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|0x00|4|Bank 0 (engine) last index

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|0x04|4|Bank 1 (modgfx) last index

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|0x08|4|Unused

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|0x0C|4|Bank 2 (projgfx) last index

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The rest of the file is an array of the following, for each DLL.

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(% border="0" %)

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|=Offset|=Size|=Description

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|0x00|4|Start offset

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|0x04|4|BSS segment size

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The end offset is calculated using the next DLL's start offset.

= Loaded DLL List =

The list at address {{code}}0x800A7D10{{/code}} describes which DLLs are currently loaded by the game, with {{code}}0x800A7D14{{/code}} being the number of currently loaded DLLs.

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Each entry looks like this:

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(% border="0" %)

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|=Offset|=Size|=Description

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|0x00|4|DLL ID

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|0x04|4|Reference count

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|0x08|4|Pointer to exports table

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|0x0C|4|Pointer to end of DLL memory

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Once the reference count becomes zero the DLL is overwritten with {{code}}0x0007000D{{/code}} (break instructions) and is subsequently freed. This is presumably to assist debugging use-after-frees, but would only be useful until the memory is reallocated to something else.

author	version	line-number	content
		1	Dinosaur Planet uses a DLL ([[Dynamic-Link Library>>https://en.wikipedia.org/wiki/Dynamic-link_library]]) system to reduce the runtime memory required for the game's code.
		2
		3	By swapping relocatable code modules in-and-out, the game can make sure only the code necessary for the core game as well as the currently used game objects is loaded.
		4
		5	There are a total of 796 DLLs in the ROM, which are separated in to 4 banks.
		6
		7	{{toc numbered="true"/}}
		8
		9	= DLL Format =
		10
		11	(% border="0" %)
		12	\|=Offset\|=Size\|=Description
		13	\|0x00\|4\|Header size
		14	\|0x04\|4\|DATA offset (relative start of header, or 0xFFFFFFFF if section not present)
		15	\|0x08\|4\|RODATA offset (relative start of header, or 0xFFFFFFFF if section not present)
		16	\|0x0C\|2\|Export count
		17	\|0x0E\|2\|Padding
		18	\|0x10\|4\|Constructor offset (relative end of header)
		19	\|0x14\|4\|Destructor offset (relative end of header)
		20	\|0x18\|4\|Unused
		21	\|0x1C\|4 * count\|Export offsets (relative end of header)
		22	\|0x1C + 4 * count\|4\|Unused
		23	\|0x20 + 4 * count\|*\|Body
		24
		25	The export count is often checked by the code responsible for loading any specific DLL as a method of determining compatibility. If the DLL does not export enough functions to satisfy the requesting code, it is immediately unloaded.
		26
		27	== DLL Relocations ==
		28
		29	The RODATA segment - if it exists - must begin with a DLL relocation table, which has the following structure.
		30
		31	(% border="0" %)
		32	\|=Value\|=Description
		33	\|*\|(((
		34	Descriptors for RODATA segment.
		35	If the MSB (bit 31) is set, this is an index into DLLSIMPORTTAB to resolve a pointer to the base executable.
		36	If the MSB is not set, this is an offset relative end of header to something within the DLL.
		37	)))
		38	\|0xFFFFFFFE\|Terminates the above section.
		39	\|*\|(((
		40	Descriptors for TEXT segment.

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