Specifications

Intel
®
Quark SoC X1000 Core
October 2013 Developer’s Manual
Order Number: 329679-001US 111
Protected Mode Architecture—Intel
®
Quark Core
An Intel
®
Quark SoC X1000 Core operating system can provide a Virtual 8086
environment that is totally transparent to the application software by intercepting and
then emulating the legacy operating system's calls, and intercepting IN and OUT
instructions.
6.5.6 Entering and Leaving Virtual 8086 Mode
A Intel
®
Quark SoC X1000 Core is executing in Protected Mode can be switched to
Virtual 8086 Mode by executing an IRET instruction (at CPL=0), or task switch (at any
CPL) to a Intel
®
Quark SoC X1000 Core task whose TSS has a FLAGS image containing
a 1 in the VM bit position. That is, one way to enter Virtual 8086 Mode is to switch to a
task with a Intel
®
Quark SoC X1000 Core TSS that has a 1 in the VM bit in the EFLAGS
image. The other way is to execute a 32-bit IRET instruction at privilege level 0, where
the stack has a 1 in the VM bit in the EFLAGS image. POPF does not affect the VM bit,
even if the Intel
®
Quark SoC X1000 Core is in Protected Mode or level 0, and so cannot
be used to enter Virtual 8086 Mode. PUSHF always pushes a 0 in the VM bit, even if the
Intel
®
Quark SoC X1000 Core is in Virtual 8086 Mode, so that a program cannot tell
whether it is executing in Real Mode or in Virtual 8086 Mode.
Figure 49. Virtual 8086 Environment Interrupt and Call Handling
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