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scasb example

rep movsb

repne scasb instruction in 8086

scasb instruction example

x86 repne

repnz

stosb

repne scasb

Opcode. Instruction. Description. AE. SCAS m8. Compare AL with byte at ES:(E)DI and set status flags. AF. SCAS m16. Compare AX with word at ES:(E)DI and set status flags. AF. SCAS m32. Compare EAX with doubleword at ES(E)DI and set status flags. AE. SCASB. Compare AL with byte at ES:(E)DI and set status flags.
25 Nov 2015 against the current value pointed at by. ES:[EDI] . When used together with the. REPNE. prefix (REPeat while Not Equal),. SCAS. scans the string searching for the first string element which is equal to the value in the accumulator. The instruction. scasb. searches the memory for the byte in. EAX. , starting at.
I'll try to explain it by reversing the code back into C. Intel's Instruction Set Reference (Volume 2 of Software Developer's Manual) is invaluable for this kind of reverse engineering. REPNE SCASB. The logic for REPNE and SCASB combined: while (ecx != 0) { temp = al - (BYTE )edi; SetStatusFlags(temp); if
To work on an entire string at a time, each string instruction can be accompanied by a repeat prefix, either REP or one of REPE and REPNE (or their synonyms ;Increment DI after each character REPNE SCASB ;Scan string for NUL, decrementing CX for each char MOV AX, -2 ;CX will be -2 for length 0, -3 for length 1, .
At the assembly-code level, two forms of this instruction are allowed: the "explicit-operands" form and the "no-operands" form. The explicit-operand form (specified with the SCAS mnemonic) allows the memory operand to be specified explicitly. Here, the memory operand should be a symbol that indicates the size and
Opcode, Instruction, Clocks, Description, Example. AE, scasb, 7, Compare bytes AL-ES:[DI], update (E)DI, scasb. AF, scasw, 7, Compare words AX-ES:[DI], update (E)DI, scasw. AF, scasl, 7, Compare dwords EAX-ES:[DI], update (E)DI, scasl. Operation. IF AddressSize = 16 THEN use DI for dest-index; ELSE (* AddressSize
16 Jun 2009 The x86 architecture offers different types of instructions to perform various string operations . Scan string instruction is one of them. There are different flavors of the scan string instruction: scasb (byte form), scasw(word), scasd(double word) and scasq(quad word). scasb: Will compare the byte at AL with the
At the assembly-code level, two forms of this instruction are allowed. The explicit-operand form and the no-operands form. The explicit-operand form (specified using the SCAS mnemonic) allows a memory operand to be specified explicitly. The memory operand must be a symbol that indicates the size and location of the
The SCAS instruction subtracts the destination string element from the contents of the EAX, AX, or AL register (depending on operand length) and updates the status flags according to the results. The string element and register contents are not modified. The following “short forms" of the SCAS instruction

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March 2018