Al2ed chapter10

String Processing Chapter 10 S. Dandamudi

Outline String representation Using string length Using a sentinel character String instructions Repetition prefixes Direction flag String move instructions String compare instructions String scan instructions Illustrative examples LDS and LES instructions Examples str_len str-cpy str_cat str_cmp str_chr str_cnv Indirect procedure call Performance: Advantage of string instructions

String Representation Two types Fixed-length Variable-length Fixed length strings Each string uses the same length Shorter strings are padded (e.g. by blank characters) Longer strings are truncated Selection of string length is critical Too large ==> inefficient Too small ==> truncation of larger strings

String Representation (cont’d) Variable-length strings Avoids the pitfalls associated with fixed-length strings Two ways of representation Explicitly storing string length (used in PASCAL) string DB ‘Error message’ str_len DW $-string $ represents the current value of the location counter $ points to the byte after the last character of string Using a sentinel character (used in C) Uses NULL character Such NULL-terminated strings are called ASCIIZ strings

String Instructions Five string instructions LODS LOaD String source STOS STOre String destination MOVS MOVe String source & destination CMPS CoMPare String source & destination SCAS SCAn String destination Specifying operands 32-bit segments: DS:ESI = source operand ES:EDI = destination operand 16-bit segments: DS:SI = source operand ES:DI = destination operand

String Instructions (cont’d) Each string instruction Can operate on 8-, 16-, or 32-bit operands Updates index register(s) automatically Byte operands: increment/decrement by 1 Word operands: increment/decrement by 2 Doubleword operands: increment/decrement by 4 Direction flag DF = 0: Forward direction (increments index registers) DF = 1: Backward direction (decrements index registers) Two instructions to manipulate DF std set direction flag (DF = 1) cld clear direction flag (DF = 0)

Repetition Prefixes String instructions can be repeated by using a repetition prefix Two types Unconditional repetition rep REPeat Conditional repetition repe/repz REPeat while Equal REPeat while Zero repne/repnz REPeat while Not Equal REPeat while Not Zero

Repetition Prefixes (cont’d) rep while (ECX  0) execute the string instruction ECX := ECX  1 end while ECX register is first checked If zero, string instruction is not executed at all More like the JECXZ instruction

Repetition Prefixes (cont’d) repe/repz while (ECX  0) execute the string instruction ECX := ECX  1 if (ZF = 0) then exit loop end if end while Useful with cmps and scas string instructions

Repetition Prefixes (cont’d) repne/repnz while (ECX  0) execute the string instruction ECX := ECX  1 if (ZF = 1) then exit loop end if end while

String Move Instructions Three basic instructions movs, lods, and stos Move a string (movs) Format movs dest_string,source_string movsb ; operands are bytes movsw ; operands are words movsd ; operands are doublewords First form is not used frequently Source and destination are assumed to be pointed by DS:ESI and ES:EDI, respectively

String Move Instructions (cont’d) movsb --- move a byte string ES:EDI:= (DS:ESI) ; copy a byte if (DF=0) ; forward direction then ESI := ESI+1 EDI := EDI+1 else ; backward direction ESI := ESI  1 EDI := EDI  1 end if Flags affected: none

String Move Instructions (cont’d) Example .DATA string1 db 'The original string',0 strLen EQU $ - string1 .UDATA string2 resb 80 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen ; strLen includes NULL mov ESI,string1 mov EDI,string2 cld ; forward direction rep movsb

String Move Instructions (cont’d) Load a String (LODS) Copies the value from the source string at DS:ESI to AL ( lodsb ) AX ( lodsw ) EAX ( lodsd ) Repetition prefix does not make sense It leaves only the last value in AL, AX, or EAX register

String Move Instructions (cont’d) lodsb --- load a byte string AL := (DS:ESI) ; copy a byte if (DF=0) ; forward direction then ESI := ESI+1 else ; backward direction ESI := ESI  1 end if Flags affected: none

String Move Instructions (cont’d) Store a String (STOS) Performs the complementary operation Copies the value in AL ( lodsb ) AX ( lodsw ) EAX ( lodsd ) to the destination string at ES:EDI Repetition prefix can be used if you want to initialize a block of memory

String Move Instructions (cont’d) stosb --- store a byte string ES:EDI := AL ; copy a byte if (DF=0) ; forward direction then EDI := EDI+1 else ; backward direction EDI := EDI  1 end if Flags affected: none

String Move Instructions (cont’d) Example: Initializes array1 with -1 .UDATA array1 resw 100 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,100 mov EDI,array1 mov AX,-1 cld ; forward direction rep stosw

String Move Instructions (cont’d) In general, repeat prefixes are not useful with lods and stos Used in a loop to do conversions while copying mov ECX,strLen mov ESI,string1 mov EDI,string2 cld ; forward direction loop1: lodsb or AL,20H stosb loop loop1 done:

String Compare Instruction cmpsb --- compare two byte strings Compare two bytes at DS:ESI and ES:EDI and set flags if (DF=0) ; forward direction then ESI := ESI+1 EDI := EDI+1 else ; backward direction ESI := ESI  1 EDI := EDI  1 end if Flags affected: As per cmp instruction (DS:ESI)  (ES:EDI)

String Compare Instruction (cont’d) .DATA string1 db 'abcdfghi',0 strLen EQU $ - string1 string2 db 'abcdefgh',0 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen mov ESI,string1 mov EDI,string2 cld ; forward direction repe cmpsb dec ESI dec EDI ; ESI & EDI pointing to the last character that differs

String Compare Instruction (cont’d) .DATA string1 db 'abcdfghi',0 strLen EQU $ - string1 - 1 string2 db 'abcdefgh',0 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov EECX,strLen mov ESI,string1 + strLen - 1 mov EDI,string2 + strLen - 1 std ; backward direction repne cmpsb inc ESI ; E SI & EDI pointing to the first character that matches inc EDI ; in the backward direction

String Scan Instruction scasb --- Scan a byte string Compare AL to the byte at ES:EDI & set flags if (DF=0) ; forward direction then EDI := EDI+1 else ; backward direction EDI := EDI  1 end if Flags affected: As per cmp instruction (DS:ESI)  (ES:EDI) scasw uses AX and scasd uses EAX instead of AL

String Scan Instruction (cont’d) Example 1 .DATA string1 db 'abcdefgh',0 strLen EQU $ - string1 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen mov EDI,string1 mov AL,'e' ; character to be searched cld ; forward direction repne scasb dec EDI ; leaves EDI pointing to e in string1

String Scan Instruction (cont’d) Example 2 .DATA string1 db ' abc',0 strLen EQU $ - string1 .CODE .STARTUP mov AX,DS ; set up ES mov ES,AX ; to the data segment mov ECX,strLen mov EDI,string1 mov AL,' ' ; character to be searched cld ; forward direction repe scasb dec EDI ; EDI pointing to the first non-blank character a

Illustrative Examples LDS and LES instructions String pointer can be loaded into DS/SI or ES/DI register pair by using lds or les instructions Syntax lds register,source les register,source register should be a 32-bit register source is a pointer to a 48-bit memory operand register is typically ESI in lds and EDI in les

Illustrative Examples (cont’d) Actions of lds and les lds register := (source) DS := (source+4) les register := (source) ES := (source+4) Pentium also supports lfs , lgs , and lss to load the other segment registers

Illustrative Examples (cont’d) Seven popular string processing routines are given as examples str_len str-cpy str_cat str_cmp str_chr str_cnv

Indirect Procedure Call Direct procedure calls specify the offset of the first instruction of the called procedure In indirect procedure call, the offset is specified through memory or a register If BX contains pointer to the procedure, we can use call EBX If the word in memory at target_proc_ptr contains the offset of the called procedure, we can use call [target_proc_ptr] These are similar to direct and indirect jumps

Performance: Advantage of String Instructions Two chief advantages of string instructions Index registers are automatically updated Can operate two operands in memory Example: Copy data from array1 to array2 cld rep movsd Assumes: DS:ESI points to array1 ES:EDI points to array2 ECX contains the array size

Performance: Advantage of String Instructions (cont’d) No string instructions With string instructions 50,000-element array-to-array copy Last slide

Al2ed chapter10

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Al2ed chapter10