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49 lines
2.4 KiB
Markdown
49 lines
2.4 KiB
Markdown
[< back to index](../doc_index.md)
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### ROM vs RAM targets
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By default, Millfork assumes that the target platform loads the program into RAM.
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Code, read-only data and preinitialized writable data are all mixed together.
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The program after loading can modify its contents, including preinitialized variables,
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almost immediately and without any extra preparation.
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When compiling for a cartridge based target, the preinitialized data cannot be writable,
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as they are not loaded into RAM, but stored in ROM.
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To make working with preinitialized data easier,
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Millfork can create a copy of preinitialized writable date in ROM,
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which can then be loaded into RAM when the program starts.
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The steps are as follows:
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* Add the `ram_init_segment=SEGMENT` option to the `[allocation]` section in your platform definition file,
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where `SEGMENT` is the ROM segment that will store the initial values.
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* Near the beginning of your program, call the `init_rw_memory` function.
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It is imported automatically, you don't need to add any import statements.
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If you are targeting both RAM-based and ROM-based platforms, wrap the call in `#if INIT_RW_MEMORY`...`#endif`,
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as the `init_rw_memory` function is not available for RAM-based targets. For example:
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void main() {
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#if INIT_RW_MEMORY
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// do a bankswitch to the SEGMENT segment if applicable
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init_rw_memory()
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#endif
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// ... rest of the code
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}
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If the default implementation of `init_rw_memory` is unsatisfactory for your needs,
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consider implementing your own and putting it in the `init_rw_memory` module
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in the same directory as your main source file, to override the standard one.
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Using the `ram_init_segment` option adds the following restrictions:
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* Preinitialized writable variables and arrays can be put only in the `default` segment.
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* Preinitialized writable variables and arrays cannot be given a fixed address.
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* On 6502-based targets, the zeropage pseudoregister has to have size of at least 4,
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unless the size of the `default` segment is 256 bytes or less (out of currently supported targets, only Atari 2600 has such small RAM),
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which uses a different, smaller and faster implementation of `init_rw_memory` that doesn't require a zeropage pseudoregister.
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You can force the compiler to use this smaller implementation by adding a preprocessor feature `TINY_RW_MEMORY = 1`.
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It works only if the total size of writable memory to initialize is 255 bytes or less. |