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Coding > Long Longs

#14454 - animension - Tue Jan 06, 2004 11:49 pm

Quick question: how does the GBA handle long longs (64-bit integers)?
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#14455 - jma - Wed Jan 07, 2004 12:21 am

No 32-bit processor can handle 64-bit math. The compiler you are using can, though, by coding some assembly routines to do the operations, stores, loads, etc.

Well, check that some processors have a few opcodes that can handle addition and multiplication... but typically this is very limited.

Jeff
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#14457 - DekuTree64 - Wed Jan 07, 2004 12:32 am

You have to sort of emulate a 64-bit processor for it. Basically you just use 2 regs to store each 64-bit number, and for addition, add the 2 bottom halves, and then adc (add with carry) the upper halves. Multiplication is trickier and takes a bunch of multiplyingand adding. It's the same theory behind how you do multiplication on paper. I think it would be destLo = lo1 * lo2, destHi = lo1 * hi1 + lo2 * hi1 + lo1 * hi2 + lo2 * hi2. hi1 * hi2 would naturally overflow 64 bits, so there's no point in doing it. Not sure how to do division, but GBA doesn't have a hardware divide anyway, so you could just write a 64-bit divider yourself.
To make life easy, let the compiler take care of all that dirty work for you.
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#14459 - Miked0801 - Wed Jan 07, 2004 12:53 am

64-bit Multiplies are handled with mul/mla commands across 2 registers. I found DevKitAdv to have very limited 64-bit support and after having it yell at me a bit, have since stopped trying to use it.

Mike

#14461 - poslundc - Wed Jan 07, 2004 1:57 am

Miked0801 wrote:
I found DevKitAdv to have very limited 64-bit support and after having it yell at me a bit, have since stopped trying to use it.


I never had any problem with it, so long as I typecast all of my multiply ops as (long long), and then made sure to reduce the result to 32 bits before doing any other ops on it.

Dan.