El dom, 31-05-2009 a las 21:35 +0200, Christian Franke escribió: > Vladimir 'phcoder' Serbinenko wrote: > >> > >> Do not do this. Some BIOS functions (like ah=08h) return data in dl. > >> Clients should not expect data in registers to be preserved across > >> interrupt calls. I don't know if there is something like a 8086/PC-BIOS > >> ABI document that we can find to confirm the non-preservation of dl, but > >> the FreeDOS MBR should be fixed then. > >> > > Thank you for pointing ah=8 function > > AFAIK there is no normative reference. The source which was mainly > > used for long years is helppc and it states following: > > - registers DS, BX, CX and DX are preserved > > (http://heim.ifi.uio.no/~stanisls/helppc/int_13.html) > > But now this reference is terribly outdated. > > SeaBIOS preserves %dl too > > > > T13 EDD provides a probably more up to date documentation of int13. > > There was no requirement to preserve registers is in EDD (2000) and > EDD-2 (2002). It appeared in EDD-3 (2004) and is still in first EDD-4 > draft (2009): > > "The values in all registers that are not explicitly defined in the > following sections shall be preserved at the completion of each function > call" > From Section 8 of: BIOS Enhanced Disk Drive Services - 3 (T13/1572D > Revision 3) > > Fortunately, T13 docs are (unlike T10 and SATA-IO) still publicly available: > http://www.t13.org/Documents/MinutesDefault.aspx?DocumentType=4&DocumentStage=2 >
Hmm... from those docs, and accepting that we ignore TSRs, we need to save %ah and %dl at handler entry, then check the saved %ah at exit, like the old handler from GRUB Legacy did - by the way, when writing the new handler, I asked what that code did and noone was able to tell me ¬¬ The only functions in the standard that return in %dl are 08h and 15h, so the check should be simple. If we want to be even more extensible, we could have a 32-byte bitmap, one bit per %ah function, and restore %dl depending on the value of the particular bit. However, I think that would be going too far. -- -- Lazy, Oblivious, Recurrent Disaster -- Habbit
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