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30151-33 参数 Datasheet PDF下载

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型号: 30151-33
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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:
文件页数/大小: 244 页 / 4221 K
品牌: NSC [ National Semiconductor ]
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Processor Programming (Continued)  
3.3.1.2 Segment Registers  
The active segment register is selected according to the  
rules listed in Table 3-3 and the type of instruction being  
currently processed. In general, the DS register selector is  
used for data references. Stack references use the SS  
register, and instruction fetches use the CS register. While  
some of these selections may be overridden, instruction  
fetches, stack operations, and the destination write opera-  
tion of string operations cannot be overridden. Special  
segment-override instruction prefixes allow the use of  
alternate segment registers. These segment registers  
include the ES, FS, and GS registers.  
The 16-bit segment registers, part of the main memory  
addressing mechanism, are described in Section 3.5 “Off-  
set, Segment, and Paging Mechanisms” on page 61. The  
six segment registers are:  
CS  
DS  
SS  
ES  
FS  
GS  
-
-
-
-
-
-
Code Segment  
Data Segment  
Stack Segment  
Extra Segment  
Additional Data Segment  
Additional Data Segment  
3.3.1.3 Instruction Pointer Register  
The segment registers are used to select segments in  
main memory. A segment acts as private memory for dif-  
ferent elements of a program such as code space, data  
space, and stack space.  
The Instruction Pointer (EIP) Register contains the off-  
set into the current code segment of the next instruction to  
be executed. The register is normally incremented by the  
length of the current instruction with each instruction exe-  
cution unless it is implicitly modified through an interrupt,  
exception, or an instruction that changes the sequential  
execution flow (for example JMP and CALL).  
There are two segment mechanisms, one for real and vir-  
tual 8086 operating modes and one for protective mode.  
Initialization and transition to protective mode is described  
in Section 3.13.4 “Initialization and Transition to Protected  
Mode” on page 87. The segment mechanisms are  
described in Section 3.7 “Descriptors and Segment Mech-  
anisms” on page 62.  
Table 3-3 illustrates the code segment selection rules.  
Table 3-3. Segment Register Selection Rules  
Implied (Default)  
Segment-Override  
Prefix  
Type of Memory Reference  
Segment  
Code Fetch  
CS  
SS  
SS  
ES  
DS  
None  
Destination of PUSH, PUSHF, INT, CALL, PUSHA instructions  
Source of POP, POPA, POPF, IRET, RET instructions  
Destination of STOS, MOVS, REP STOS, REP MOVS instructions  
None  
None  
None  
Other data references with effective address using base registers of:  
EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP  
CS, ES, FS, GS, SS  
SS  
CS, DS, ES, FS, GS  
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