📄 s03_06.htm
字号:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 3.6</TITLE></HEAD><BODY><B>up:</B> <A HREF="c03.htm">Chapter 3 -- Applications Instruction Set</A><BR><B>prev:</B> <A HREF="s03_05.htm">3.5 Control Transfer Instructions</A><BR><B>next:</B> <A HREF="s03_07.htm">3.7 Instructions for Block-Structured Languages</A><P><HR><P><H1>3.6 String and Character Translation Instructions</H1>The instructions in this category operate on strings rather than on logicalor numeric values. Refer also to the section on I/O for information aboutthe string I/O instructions (also known as block I/O).<P>The power of 80386 string operations derives from the following features ofthe architecture:<OL><LI> A set of primitive string operations<UL><LI><A HREF="MOVS.htm">MOVS</A> -- Move String<LI><A HREF="CMPS.htm">CMPS</A> -- Compare string<LI><A HREF="SCAS.htm">SCAS</A> -- Scan string<LI><A HREF="LODS.htm">LODS</A> -- Load string<LI><A HREF="STOS.htm">STOS</A> -- Store string</UL><P><LI> Indirect, indexed addressing, with automatic incrementing ordecrementing of the indexes.<DL><DT>Indexes:<DD>ESI -- Source index register<BR>EDI -- Destination index register<DT>Control flag:<DD>DF -- Direction flag<DT>Control flag instructions:<DD><A HREF="CLD.htm">CLD</A> -- Clear direction flag instruction<BR><A HREF="STD.htm">STD</A> -- Set direction flag instruction</DL><P><LI> Repeat prefixes<UL><LI><A HREF="REP.htm">REP</A> -- Repeat while ECX not xero<LI><A HREF="REP.htm">REPE/REPZ</A> -- Repeat while equal or zero<LI><A HREF="REP.htm">REPNE/REPNZ</A> -- Repeat while not equal or not zero</UL></OL><P>The primitive string operations operate on one element of a string. Astring element may be a byte, a word, or a doubleword. The string elementsare addressed by the registers ESI and EDI. After every primitive operationESI and/or EDI are automatically updated to point to the next element of thestring. If the direction flag is zero, the index registers are incremented;if one, they are decremented. The amount of the increment or decrement is1, 2, or 4 depending on the size of the string element.<H2>3.6.1 Repeat Prefixes</H2>The repeat prefixes <A HREF="REP.htm">REP</A> (Repeat While ECX Not Zero), <A HREF="REP.htm">REPE/REPZ</A> (RepeatWhile Equal/Zero), and <A HREF="REP.htm">REPNE/REPNZ</A> (Repeat While Not Equal/Not Zero) specifyrepeated operation of a string primitive. This form of iteration allows theCPU to process strings much faster than would be possible with a regularsoftware loop.<P>When a primitive string operation has a repeat prefix, the operation isexecuted repeatedly, each time using a different element of the string. Therepetition terminates when one of the conditions specified by the prefix issatisfied.<P>At each repetition of the primitive instruction, the string operation maybe suspended temporarily in order to handle an exception or externalinterrupt. After the interruption, the string operation can be restartedagain where it left off. This method of handling strings allows operationson strings of arbitrary length, without affecting interrupt response.<P>All three prefixes causes the hardware to automatically repeat theassociated string primitive until ECX=0. The differences among the repeatprefixes have to do with the second termination condition. <A HREF="REP.htm">REPE/REPZ</A> and<A HREF="REP.htm">REPNE/REPNZ</A> are used exclusively with the <A HREF="SCAS.htm">SCAS</A> (Scan String) and <A HREF="CMPS.htm">CMPS</A>(Compare String) primitives. When these prefixes are used, repetition of thenext instruction depends on the zero flag (ZF) as well as the ECX register.ZF does not require initialization before execution of a repeated stringinstruction, because both <A HREF="SCAS.htm">SCAS</A> and <A HREF="CMPS.htm">CMPS</A> set ZF according to the results ofthe comparisons they make. The differences are summarized in theaccompanying table.<PRE>Prefix Termination TerminationCondition 1 Condition 2REP ECX = 0 (none)REPE/REPZ ECX = 0 ZF = 0REPNE/REPNZ ECX = 0 ZF = 1</PRE><H2>3.6.2 Indexing and Direction Flag Control</H2>The addresses of the operands of string primitives are determined by theESI and EDI registers. ESI points to source operands. By default, ESI refersto a location in the segment indicated by the DS segment register. Asegment-override prefix may be used, however, to cause ESI to refer to CS,SS, ES, FS, or GS. EDI points to destination operands in the segmentindicated by ES; no segment override is possible. The use of two differentsegment registers in one instruction allows movement of strings betweendifferent segments.<P>This use of ESI and DSI has led to the descriptive names source index anddestination index for the ESI and EDI registers, respectively. In allcases other than string instructions, however, the ESI and EDI registers maybe used as general-purpose registers.<P>When ESI and EDI are used in string primitives, they are automaticallyincremented or decremented after to operation. The direction flag determineswhether they are incremented or decremented. The instruction <A HREF="CLD.htm">CLD</A> puts zeroin DF, causing the index registers to be incremented; the instruction <A HREF="STD.htm">STD</A>puts one in DF, causing the index registers to be decremented. Programmersshould always put a known value in DF before using string instructions in aprocedure.<H2>3.6.3 String Instructions</H2><A HREF="MOVS.htm">MOVS</A> (Move String) moves the string element pointed to by ESI to thelocation pointed to by EDI. <A HREF="MOVS.htm">MOVSB</A> operates on byte elements, <A HREF="MOVS.htm">MOVSW</A> operates on word elements, and <A HREF="MOVS.htm">MOVSD</A> operates on doublewords. The destination segmentregister cannot be overridden by a segment override prefix, but the sourcesegment register can be overridden.<P>The <A HREF="MOVS.htm">MOVS</A> instruction, when accompanied by the <A HREF="REP.htm">REP</A> prefix, operates as amemory-to-memory block transfer. To set up for this operation, the programmust initialize ECX and the register pairs ESI and EDI. ECX specifies thenumber of bytes, words, or doublewords in the block.<P>If DF=0, the program must point ESI to the first element of the sourcestring and point EDI to the destination address for the first element. IfDF=1, the program must point these two registers to the last element of thesource string and to the destination address for the last element,respectively.<P><A HREF="CMPS.htm">CMPS</A> (Compare Strings) subtracts the destination string element (at ES:EDI)from the source string element (at ESI) and updates the flags AF, SF, PF, CFand OF. If the string elements are equal, ZF=1; otherwise, ZF=0. If DF=0,the processor increments the memory pointers (ESI and EDI) for the twostrings. <A HREF="CMPS.htm">CMPSB</A> compares bytes, <A HREF="MOVS.htm">MOVSW</A> compares words, and <A HREF="MOVS.htm">MOVSD</A> compares doublewords. The segment register used for the source address can be changedwith a segment override prefix while the destination segment registercannot be overridden.<P><A HREF="SCAS.htm">SCAS</A> (Scan String) subtracts the destination string element at ES:EDI fromEAX, AX, or AL and updates the flags AF, SF, ZF, PF, CF and OF. If thevalues are equal, ZF=1; otherwise, ZF=0. If DF=0, the processor incrementsthe memory pointer (EDI) for the string. <A HREF="SCAS.htm">SCASB</A> scans bytes; <A HREF="SCAS.htm">SCASW</A> scans words; <A HREF="SCAS.htm">SCASD</A> scans doublewords. The destination segment register (ES) cannotbe overridden.<P>When either the <A HREF="REP.htm">REPE</A> or <A HREF="REP.htm">REPNE</A> prefix modifies either the <A HREF="SCAS.htm">SCAS</A> or <A HREF="CMPS.htm">CMPS</A>primitives, the processor compares the value of the current string elementwith the value in EAX for doubleword elements, in AX for word elements, orin AL for byte elements. Termination of the repeated operation depends onthe resulting state of ZF as well as on the value in ECX.<P><A HREF="LODS.htm">LODS</A> (Load String) places the source string element at ESI into EAX fordoubleword strings, into AX for word strings, or into AL for byte strings.<A HREF="LODS.htm">LODS</A> increments or decrements ESI according to DF.<P><A HREF="STOS.htm">STOS</A> (Store String) places the source string element from EAX, AX, or ALinto the string at ES:DSI. <A HREF="STOS.htm">STOS</A> increments or decrements EDI according toDF.<P><HR><P><B>up:</B> <A HREF="c03.htm">Chapter 3 -- Applications Instruction Set</A><BR><B>prev:</B> <A HREF="s03_05.htm">3.5 Control Transfer Instructions</A><BR><B>next:</B> <A HREF="s03_07.htm">3.7 Instructions for Block-Structured Languages</A></BODY>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -