Chapter 8 String Operations

8.1 Using String Instructions

String in the 80x86 Environment Contiguous collection of bytes, words, doublewords or quadwords in memory Commonly defined in a program’s data segment using such directives as response BYTE 20 DUP (?) label1 BYTE 'The results are ', 0 arrayD DWORD 60 DUP (0)

String Instructions movs (move string) –Copy a string from one location to another cmps (compare string) –Compare the contents of two strings scas (scan string) –Search a string for one particular value stos (store string) –Store a new value in some string position lods (load string) –Copies a value out of some string position

String Instruction Operation Each instruction applies to a source string, a destination string, or both The elements (bytes, words, doublewords or quadwords) are processed one at a time Register indirect addressing is used to locate the individual string elements –ESI (RSI in 64-bit mode) used for source string elements –EDI (RDI) for destination string elements

Mnemonic Variants Since ESI and EDI are automatically used, operands are unnecessary Assembler can’t tell size of string element without further information – suffix on mnemonic can be used –Example, movsb to move string of bytes

More on String Instruction Operation String instruction operates on only one string element at a time, but gets ready to operate on the next element –Changes the source index register ESI and/or the destination index register EDI to contain the address of the next element of the string(s) Can move forward or backward depending on direction flag DF –cld instruction sets forward direction –std instruction sets backward direction

Design to Copy Null-Terminated String while next source byte is not null loop copy source byte to destination; increment source index; increment destination index; end while; put null byte at end of destination string;

Implementation of String Copy mov edi,[ebp+8] ;destination mov esi,[ebp+12] ;initial source address cld ;clear direction flag whileNoNull: cmp BYTE PTR [esi],0 ;null source byte? je endWhileNoNull ;stop copying if null movsb ;copy one byte jmp whileNoNull ;go check next byte endWhileNoNull: mov BYTE PTR [edi],0 ;terminate dest string

8.2 Repeat Prefixes and More String Instructions

Repeat Prefixes Change the string instructions into versions which repeat automatically either for a fixed number of iterations or until some condition is satisfied The three repeat prefixes actually correspond to two different single-byte codes –Not themselves instructions, but supplement machine codes for the primitive string instructions, making new instructions

rep prefix Normally used with movs and with stos Causes this design to be executed: while count in ECX > 0 loop perform primitive instruction; decrement ECX by 1; end while;

Additional Repeat Prefixes repe (equivalent mnemonic repz ) –“repeat while equal” (“repeat while zero”) repne (same as repnz ) –“repeat while not equal” (“repeat while not zero”) Each appropriate for use with cmps and scas which affect the zero flag ZF

repe and repne Operation Each works the same as rep, iterating a primitive instruction while ECX is not zero Each also examines ZF after the string instruction is executed –repe and repz continue iterating while ZF=1, as it would be following a comparison where two operands were equal –repne and repnz continue iterating while ZF=0

cmps Subtracts two string elements and sets flags based on the difference If used in a loop, it is appropriate to follow cmps by a conditional jump instruction repe and repne prefixes often used with cmps instructions

scas Used to scan a string for the presence or absence of a particular string element –String which is examined is a destination string – the address of the element being examined is in the destination index register EDI –Accumulator contains the element being scanned for

stos Copies a byte, a word, a doubleword or a quadword from the accumulator to an element of a destination string Affects no flag, so only the rep prefix is appropriate for use with it –When repeated, it copies the same value into consecutive positions of a string

lods Copies a source string element to the accumulator No repeat prefix is useful with lods lods and stos are often used together in a loop –lods at the beginning of a loop to fetch an element –stos at the end after the element is manipulated

8.3 Character Translation

xlat “translate” Uses a lookup table to modify the byte in AL The table is at the address in EBX The original value in AL is used as an index into the table The byte at that index is stored as the new value in AL

Translation Code Example mov ecx, strLength ; string length lea ebx, table ; addr of translation table lea esi, string ; address of string lea edi, string ; destination also string forIndex: lodsb ; copy next character to AL xlat ; translate character stosb ; copy character back into string loop forIndex ; repeat for all characters

Building a Translation Table Normally 256 bytes long –One entry for each possible byte value

Example Table for ASCII Codes Leaves lower case letters and digits unchanged Translates upper case letters to lower case Translates all other characters to spaces table BYTE 48 DUP (' '), " ", 7 DUP (' ') BYTE "abcdefghijklmnopqrstuvwxyz", 6 DUP (' ') BYTE "abcdefghijklmnopqrstuvwxyz", 133 DUP (' ') 0 at position 48 (30 16 ) so 0 translated to 0 a at position 65 (41 16 ) so A translated to a

8.4 Converting a 2’s Complement Integer to an ASCII String

dtoa macro expansion push ebx ; save EBX lea ebx, dest ; destination address push ebx ; destination parameter mov ebx, [esp+4] ; in case source was EBX mov ebx, source ; source value push ebx ; source parameter call dtoaproc ; dtoaproc(source,dest) add esp, 8 ; remove parameters pop ebx ; restore EBX The real work is done by dtoaproc

dtoaproc algorithm determine whether source positive or negative; put 10 spaces in destination area; make EDI point at 11 th byte; repeat digit := source mod 10; convert digit to ASCII and store at [EDI]; decrement EDI; divide source by 10; until source = 0; if original source negative, append leading minus sign;

dtoaproc special case Most negative numbers are handled by processing the corresponding positive number, “remembering” the minus sign has no corresponding positive number, so the characters are stored in the destination area one at a time