Vampire 2.7 FPU

Posted by ALB42 on 3. März 2018No Comments

I just found out you can turn off the FPU with “VCONTROL FP=0” sometimes it work, sometimes not (just crashing, if this because of my sloppy power on the Vampire or just bad timing, who knows). But after it you can start FEmu as with 2.5 and the testcodes work again. Also MUIMapparium FPU Version is working :-D.
In principle that would be the better alternative to the current situation, implement single precision inside the FPGA and trap the rest, which can then be covered by FEmu. Would make Quake and Demos and so on fast, but would not violate the IEEE 754.

In the current situation Amiga (Vampire) is not anymore IEEE 754 compliant. The Double format is now something like 24 bits significant (like IEEE Single) with 11 bits exponent (like IEEE double) because it can still go to 1e308 but the significant precision is much lower as shown before. Also strange if you multiply two big numbers to produce an overflow for example 1e200 * 1e200 in double precision that would give “+Inf” or an exception, but Vampire FPU shows 1.8e+308 (something close to the max Double Value) and you can continue to calculate with that.

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