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參數(shù)資料
| 型號(hào)： | AMDK86 |
| 英文描述： | AMD K86 - AMD K86 Family BIOS and Software Tools Developers Guide |
| 中文描述： | AMD的K86 - AMD的K86系列BIOS和軟件工具開發(fā)人員指南 |
| 文件頁數(shù)： | 81/144頁 |
| 文件大?。?/td> | 2179K |
| 代理商： | AMDK86 |
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AMD-K5 Processor
69
21062E/0—June 1997
AMD K86 Family BIOS and Software Tools Developers Guide
Preliminary Information
When an external interrupt occurs, the processor switches from
the Virtual-8086 program to the operating system, in the same
manner as on a 386 or 486 processor. If the operating system
determines the interrupt is for the Virtual-8086 program, it
checks the state of the VIF bit in the program’s EFLAGS image
on the stack. If VIF has been set by the processor (during an
attempt by the program to set the IF bit), the operating system
permits access to the appropriate Virtual-8086 handler via the
interrupt vector table (IVT). If VIF has been cleared, the
operating system holds the interrupt pending. The operating
system can do this by saving appropriate information (such as
the interrupt vector), setting the program's VIP flag in the
EFLAGS image on the stack, and returning to the interrupted
program. When the program subsequently attempts to set IF,
the set VIP flag causes the processor to inhibit the instruction
and generate a general-protection exception with error code
zero, thereby notifying the operating system that the program
is now prepared to accept the interrupt.
Thus, when VME extensions are enabled, the VIF and VIP bits
are set and cleared as follows:
I
VIF
—This bit is controlled by the processor and used by the
operating system to determine whether an external
maskable interrupt should be passed on to the program or
held pending. VIF is set and cleared for instructions that can
modify IF, and it is cleared during software interrupts
through interrupt gates. The original IF value is preserved
in the EFLAGS image on the stack.
VIP
—This bit is set and cleared by the operating system via
the EFLAGS image on the stack. It is set when an interrupt
occurs for a Virtual-8086 program whose VIF bit is cleared.
The bit is checked by the processor when the program
subsequently attempts to set VIF.
I
Figure 18 and Table 22 show the VIF and VIP bits in the
EFLAGS register. The VME extensions support conventional
emulation methods for passing interrupts to Virtual-8086
programs, but they make it possible for the operating system to
avoid time-consuming emulation of most instructions that write
or read the IF.
The VIF and IF flags only affect the way the operating system
deals with hardware interrupts (the INTR signal). Software
interrupts are handled like machine-generated exceptions and
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