On Wed, Jan 25, 2017 at 12:23 PM, Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote: > Tasks running in virtual-8086 mode will use 16-bit addressing form > encodings as described in the Intel 64 and IA-32 Architecture Software > Developer's Manual Volume 2A Section 2.1.5. 16-bit addressing encodings > differ in several ways from the 32-bit/64-bit addressing form encodings: > the r/m part of the ModRM byte points to different registers and, in some > cases, addresses can be indicated by the addition of the value of two > registers. Also, there is no support for SiB bytes. Thus, a separate > function is needed to parse this form of addressing. > > Furthermore, virtual-8086 mode tasks will use real-mode addressing. This > implies that the segment selectors do not point to a segment descriptor > but are used to compute logical addresses. Hence, there is a need to > add support to compute addresses using the segment selectors. If segment- > override prefixes are present in the instructions, they take precedence. > > Lastly, it is important to note that when a tasks is running in virtual- > 8086 mode and an interrupt/exception occurs, the CPU pushes to the stack > the segment selectors for ds, es, fs and gs. These are accesible via the > struct kernel_vm86_regs rather than pt_regs. > > Code for 16-bit addressing encodings is likely to be used only by virtual- > 8086 mode tasks. Thus, this code is wrapped to be built only if the > option CONFIG_VM86 is selected. That's not true. It's used in 16-bit protected mode, too. And there are (ugh!) six possibilities: - Normal 32-bit protected mode. This should already work. - Normal 64-bit protected mode. This should also already work. (I forget whether a 16-bit SS is either illegal or has no effect in this case.) - Virtual 8086 mode - Normal 16-bit protected mode, used by DOSEMU and Wine. (16-bit CS, 16-bit address segment) - 16-bit CS, 32-bit address segment. IIRC this might be used by some 32-bit DOS programs to call BIOS. - 32-bit CS, 16-bit address segment. I don't know whether anything uses this. I don't know if anything you're doing cares about SS's, DS's, etc. size, but I suspect you'll need to handle 16-bit CS. -- To unsubscribe from this list: send the line "unsubscribe linux-msdos" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
