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30140-23 参数 Datasheet PDF下载

30140-23图片预览
型号: 30140-23
PDF下载: 下载PDF文件 查看货源
内容描述: 的Geode ™ GXM处理器与MMX支持集成的x86解决方案 [Geode⑩ GXm Processor Integrated x86 Solution with MMX Support]
分类和应用: 微控制器和处理器外围集成电路微处理器
文件页数/大小: 244 页 / 4221 K
品牌: NSC [ National Semiconductor ]
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Processor Programming (Continued)  
3.14 VIRTUAL 8086 MODE  
Both real mode and virtual 8086 (V86) modes are sup-  
ported by the GXm processor, allowing execution of 8086  
application programs and 8086 operating systems. V86  
mode allows the execution of 8086-type applications, yet  
still permits use of the paging and protection mechanisms.  
V86 tasks run at privilege level 3. Before entry, all seg-  
ment limits must be set to FFFFh (64K) as in real mode.  
3.14.3 Interrupt Handling  
To fully support the emulation of an 8086-type machine,  
interrupts in V86 mode are handled as follows. When an  
interrupt or exception is serviced in V86 mode, program  
execution transfers to the interrupt service routine at privi-  
lege level 0 (i.e., transition from V86 to protected mode  
occurs). The VM bit in the EFLAGS register (bit 17) is  
cleared. The protected mode interrupt service routine  
then determines if the interrupt came from a protected  
mode or V86 application by examining the VM bit in the  
EFLAGS image stored on the stack. The interrupt service  
routine may then choose to allow the 8086 operating sys-  
tem to handle the interrupt or may emulate the function of  
the interrupt handler. Following completion of the interrupt  
service routine, an IRET instruction restores the EFLAGS  
register (restores VM = 1) and segment selectors and  
control returns to the interrupted V86 task.  
3.14.1 Memory Addressing  
While in V86 mode, segment registers are used in an  
identical fashion to real mode. The contents of the Seg-  
ment register are multiplied by 16 and added to the offset  
to form the Segment Base Linear Address. The GXm pro-  
cessor permits the operating system to select which pro-  
grams use the V86 address mechanism and which  
programs use protected mode addressing for each task.  
The GXm processor also permits the use of paging when  
operating in V86 mode. Using paging, the 1 MB address  
space of the V86 task can be mapped to any region in the  
4 GB linear address space.  
3.14.4 Entering and Leaving Virtual 8086 Mode  
V86 mode is entered from protected mode by either exe-  
cuting an IRET instruction at CPL = 0 or by task switching.  
If an IRET is used, the stack must contain an EFLAGS  
image with VM = 1. If a task switch is used, the TSS must  
contain an EFLAGS image containing a 1 in the VM bit  
position. The POPF instruction cannot be used to enter  
V86 mode since the state of the VM bit is not affected.  
V86 mode can only be exited as the result of an interrupt  
or exception. The transition out must use a 32-bit trap or  
interrupt gate that must point to a non-conforming privi-  
lege level 0 segment (DPL = 0), or a 32-bit TSS. These  
restrictions are required to permit the trap handler to IRET  
back to the V86 program.  
The paging hardware allows multiple V86 tasks to run  
concurrently, and provides protection and operating sys-  
tem isolation. The paging hardware must be enabled to  
run multiple V86 tasks or to relocate the address space of  
a V86 task to physical address space other than 0.  
3.14.2 Protection  
All V86 tasks operate with the least amount of privilege  
(level 3) and are subject to all CPU protected mode protec-  
tion checks. As a result, any attempt to execute a privi-  
leged instruction within a V86 task results in a general  
protection fault.  
In V86 mode, a slightly different set of instructions are  
sensitive to the I/O privilege level (IOPL) than in protected  
mode. These instructions are: CLI, INT n, IRET, POPF,  
PUSHF, and STI. The INT3, INTO and BOUND variations  
of the INT instruction are not IOPL sensitive.  
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