"special" chip-mem allocation 68K-ASM

[Herpes]

Hi there everybody!

I want to allocate memory to us it as bit plane memory. That usually not a problem but I have certain requirements which must be satisfied.
I wonder if there is an elegant solution ...

* I want to keep it as legal as possible - so I need to allocate the memory with AllocAbs or AllocMem.
* I need a mem-block much less than 64K - lets say 24K.
* I do not want to cross 64K boundaries - therefore the the memory should be positioned in the chip area - just to be safe on most Amigas - below $80000. So typically I would like to have my memblock either before or after a 64K boundary.

Do I need to try to iteratively AllocAbs from e.g. ($20000+-MyBlocksize) up to ($80000-MyBlocksize) until I get a valid block? or is there a better way?


Thanks for your help!

[britelite]

I'm guessing allocating a larger block than the 24k needed is not an option?

[StingRay]

Alloc a bit more memory than you need and align it then. For example, if you want to have your memory aligned to a 64k boundary you would alloc YOURMEMSIZE+65536 bytes and after allocating you'd do this:

; d0.l: allocated memory
add.l #65536,d0
and.l #$3ff0000,d0 ; align to 64k boundary

Another option (if you're going to use AllocAbs() anyway), save the memory block you're going to destroy in a temporary buffer and then just use it. At the end you need to restore it of course.

[Toni Wilen]

Ugly but working method which does not waste memory (Enforcer uses this method to get MMU page aligned memory which means it can't be too bad):

AllocMem(65536 + memory size you need) normally.
Forbid()
Free whole allocated block
AllocAbs(allocated block aligned to next 65536 byte boundary, size you need)
Permit()

[Herpes]

Gyus - thank you very much - these are very nice ideas - and for me still "legal enough" - I don't know which I like most