mirror of
https://github.com/KarolS/millfork.git
synced 2024-12-27 04:29:34 +00:00
49 lines
2.4 KiB
Markdown
49 lines
2.4 KiB
Markdown
[< back to index](../doc_index.md)
|
|
|
|
### ROM vs RAM targets
|
|
|
|
By default, Millfork assumes that the target platform loads the program into RAM.
|
|
Code, read-only data and preinitialized writable data are all mixed together.
|
|
The program after loading can modify its contents, including preinitialized variables,
|
|
almost immediately and without any extra preparation.
|
|
|
|
When compiling for a cartridge based target, the preinitialized data cannot be writable,
|
|
as they are not loaded into RAM, but stored in ROM.
|
|
To make working with preinitialized data easier,
|
|
Millfork can create a copy of preinitialized writable date in ROM,
|
|
which can then be loaded into RAM when the program starts.
|
|
|
|
The steps are as follows:
|
|
|
|
* Add the `ram_init_segment=SEGMENT` option to the `[allocation]` section in your platform definition file,
|
|
where `SEGMENT` is the ROM segment that will store the initial values.
|
|
|
|
* Near the beginning of your program, call the `init_rw_memory` function.
|
|
It is imported automatically, you don't need to add any import statements.
|
|
If you are targeting both RAM-based and ROM-based platforms, wrap the call in `#if INIT_RW_MEMORY`...`#endif`,
|
|
as the `init_rw_memory` function is not available for RAM-based targets. For example:
|
|
|
|
void main() {
|
|
#if INIT_RW_MEMORY
|
|
// do a bankswitch to the SEGMENT segment if applicable
|
|
init_rw_memory()
|
|
#endif
|
|
// ... rest of the code
|
|
}
|
|
|
|
If the default implementation of `init_rw_memory` is unsatisfactory for your needs,
|
|
consider implementing your own and putting it in the `init_rw_memory` module
|
|
in the same directory as your main source file, to override the standard one.
|
|
|
|
|
|
Using the `ram_init_segment` option adds the following restrictions:
|
|
|
|
* Preinitialized writable variables and arrays can be put only in the `default` segment.
|
|
|
|
* Preinitialized writable variables and arrays cannot be given a fixed address.
|
|
|
|
* On 6502-based targets, the zeropage pseudoregister has to have size of at least 4,
|
|
unless the size of the `default` segment is 256 bytes or less (out of currently supported targets, only Atari 2600 has such small RAM),
|
|
which uses a different, smaller and faster implementation of `init_rw_memory` that doesn't require a zeropage pseudoregister.
|
|
You can force the compiler to use this smaller implementation by adding a preprocessor feature `TINY_RW_MEMORY = 1`.
|
|
It works only if the total size of writable memory to initialize is 255 bytes or less. |