N_Asm Assembly strings (sol)
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This document discusses string processing in assembly language. It describes two common ways to store string lengths - explicitly storing the length or using a sentinel character. It then discusses five basic string instructions (MOVS, LODS, STOS, CMPS, SCAS) and how they can operate on bytes, words, or doublewords. Repetition prefixes like REP can be used to repeat string instructions until a counter in the CX register reaches zero, and the direction flag DF determines the direction of processing.
N_Asm Assembly strings (sol)
- 1. http://www.tutorialspoint.com/assembly_prog ramming /assembly_string s.htm Copyright © tutorialspoint.com ASSEMBLY - STRING PROCESSING We have already used variable lengths strings inour previous examples. Youmust have noticed that, the variable lengths strings canhave as many characters as required. Generally, we specify the lengthof the string by either of the two ways: Explicitly storing string length Using a sentinelcharacter We canstore the string lengthexplicitly by using the $ locationcounter symbolthat represents the current value of the locationcounter. Inthe following example: msg db 'Hello, world!',0xa ;our dear string len equ $ - msg ;length of our dear string $ points to the byte after the last character of the string variable msg. Therefore, $-msg gives the lengthof the string. We canalso write msg db 'Hello, world!',0xa ;our dear string len equ 13 ;length of our dear string Alternatively, youcanstore strings witha trailing sentinelcharacter to delimit a string instead of storing the string lengthexplicitly. The sentinelcharacter should be a specialcharacter that does not appear withina string. For example: message DB 'I am loving it!', 0 String Instructions Eachstring instructionmay require a source operand, a destinationoperand or both. For 32-bit segments, string instructions use ESI and EDI registers to point to the source and destinationoperands, respectively. For 16-bit segments, however, the SI and the DI registers are used to point to the source and destination, respectively. There are five basic instructions for processing strings. They are: MOVS - This instructionmoves 1 Byte, Word or Doubleword of data frommemory locationto another. LODS - This instructionloads frommemory. If the operand is of one byte, it is loaded into the AL register, if the operand is one word, it is loaded into the AX register and a doubleword is loaded into the EAX register. STOS - This instructionstores data fromregister (AL, AX, or EAX) to memory. CMPS - This instructioncompares two data items inmemory. Data could be of a byte size, word or doubleword. SCAS - This instructioncompares the contents of a register (AL, AX or EAX) withthe contents of anitem inmemory. Eachof the above instructionhas a byte, word and doubleword versionand string instructions canbe repeated by using a repetitionprefix. These instructions use the ES:DI and DS:SI pair of registers, where DI and SI registers containvalid offset addresses that refers to bytes stored inmemory. SI is normally associated withDS (data segment) and DI is always associated withES (extra segment). The DS:SI (or ESI) and ES:DI (or EDI) registers point to the source and destinationoperands, respectively.
- 2. The source operand is assumed to be at DS:SI (or ESI) and the destinationoperand at ES:DI (or EDI) in memory. For 16-bit addresses, the SI and DI registers are used, and for 32-bit addresses, the ESI and EDI registers are used. The following table provides various versions of string instructions and the assumed space of the operands. Basic Instruction Operands at Byte Operation Word Operation Double word Operation MOVS ES:DI, DS:EI MOVSB MOVSW MOVSD LODS AX, DS:SI LODSB LODSW LODSD STOS ES:DI, AX STOSB STOSW STOSD CMPS DS:SI, ES: DI CMPSB CMPSW CMPSD SCAS ES:DI, AX SCASB SCASW SCASD Repetition Prefixes The REP prefix, whenset before a string instruction, for example - REP MOVSB, causes repetitionof the instructionbased ona counter placed at the CX register. REP executes the instruction, decreases CX by 1, and checks whether CX is zero. It repeats the instructionprocessing untilCX is zero. The DirectionFlag (DF) determines the directionof the operation. Use CLD (Clear DirectionFlag, DF = 0) to make the operationleft to right. Use STD (Set DirectionFlag, DF = 1) to make the operationright to left. The REP prefix also has the following variations: REP: it is the unconditionalrepeat. It repeats the operationuntilCX is zero. REPE or REPZ: It is conditionalrepeat. It repeats the operationwhile the zero flag indicates equal/zero. It stops whenthe ZF indicates not equal/zero or whenCX is zero. REPNE or REPNZ: It is also conditionalrepeat. It repeats the operationwhile the zero flag indicates not equal/zero. It stops whenthe ZF indicates equal/zero or whenCX is decremented to zero.