Description. movsx reads the contents of the register or effective address as a word or byte. movsx then sign-extends the 16- or 32-bit value to the operand-size attribute of the instruction. The result is stored in the destination register by movsx.
What is the AL register?
The least significant byte of AX can be used as a single 8-bit register called AL, while the most significant byte of AX can be used as a single 8-bit register called AH. These names refer to the same physical register. … For example, the names EAX and eax refer to the same register.
What is the purpose of Movsx extension while we have Movzx instruction available?
The 80386/486/Pentium processors provide instructions that move and extend a value to a larger data size in a single step. MOVSX moves a signed value into a register and sign-extends it with 1. MOVZX moves an unsigned value into a register and zero-extends it with zero.
What is CDQE?
The CDQE instruction sign-extends a DWORD (32-bit value) in the EAX register to a QWORD (64-bit value) in the RAX register. The MOVZX instruction zero-extends the source to the destination. In this case, it sign-extends the BYTE loaded from memory at [rbp-528+rax] to the DWORD destination register, EAX .What is RDI register?
Architectures define a calling convention which dictates where parameters to a function and its return value are stored. In Objective-C, the RDI register is the reference of the calling NSObject , RSI is the Selector, RDX is the first parameter and so on.
What is EDX register?
edx. edx is a volatile general-purpose register that is occasionally used as a function parameter. Like ecx, edx is used for “__fastcall” functions. Besides fastcall, edx is generally used for storing short-term variables within a function.
What is SS register?
The stack segment register (SS) is usually used to store information about the memory segment that stores the call stack of currently executed program. SP points to current stack top. By default, the stack grows downward in memory, so newer values are placed at lower memory addresses.
What is Lea Assembly?
lea — Load effective address. The lea instruction places the address specified by its first operand into the register specified by its second operand. Note, the contents of the memory location are not loaded, only the effective address is computed and placed into the register.What is Movzx?
MOVSX (Move with sign extension) and MOVZX (Move with zero extension) are special versions of the mov instruction that perform sign extension or zero extension from the source to the destination. This is the only instruction that allows the source and destination to be different sizes.
How does CDQ work?The CDQ instruction copies the sign (bit 31) of the value in the EAX register into every bit position in the EDX register. … The CWD instruction is intended for use when the operand-size attribute is 16 and the CDQ instruction for when the operand-size attribute is 32.
Article first time published onWhat is Movsx instruction?
Description. movsx reads the contents of the register or effective address as a word or byte. movsx then sign-extends the 16- or 32-bit value to the operand-size attribute of the instruction. The result is stored in the destination register by movsx.
What does JNE do in assembly?
The jnz (or jne) instruction is a conditional jump that follows a test. It jumps to the specified location if the Zero Flag (ZF) is cleared (0). jnz is commonly used to explicitly test for something not being equal to zero whereas jne is commonly found after a cmp instruction.
How does XOR work in assembly?
The XOR instruction performs a bit wise Exclusive OR operation between corresponding bits in the two operands and places the result in the first operand. reg, mem, and immed can be 8, 16, or 32 bits. The XOR instruction can be used to reverse selected bits in an operand while preserving the remaining bits.
What is x86 RDX?
General-Purpose Registers. The 64-bit versions of the ‘original’ x86 registers are named: … rdx – register d extended. rbp – register base pointer (start of stack) rsp – register stack pointer (current location in stack, growing downwards)
What is BX ax Al and CX register?
For example, in multiplication operation, one operand is stored in EAX or AX or AL register according to the size of the operand. BX is known as the base register, as it could be used in indexed addressing. CX is known as the count register, as the ECX, CX registers store the loop count in iterative operations.
What is bit size %r8?
64-bit registerLower 32 bitsLower 8 bitsr8r8dr8br9r9dr9br10r10dr10br11r11dr11b
What is DS in x86?
The ds: means the instruction is referencing memory in the Data Segment – and can pretty much be ignored on modern OSes, since they run with a flat address space model (code, data and stack segments all refer to the same memory range, and memory protection is handled with paging).
What is the size of SP register?
The register used to access the stack is called the SP (stack pointer) register. The stack pointer in the 8051 is only 8 bits wide, which means that it can take values of 00 to FFH. When the 8051 is powered up, the SP register contains value 07.
What does SS mean in assembly?
In the 16-bit Intel x86 architecture: SS is the stack-segment register. It identifies the block of memory that will be used for the stack. SP is the stack pointer register. It points to the precise location within the stack segment which is at any given moment the ‘top’ of the stack.
What is size of register EAX?
eax is the 32-bit, “int” size register. It was added in 1985 during the transition to 32-bit processors with the 80386 CPU.
Why is MOV Turing complete?
Of course, on an actual x86 processor the mov instruction can be used to write arbitrary code into memory after the instruction pointer which the processor will then execute, making it in some sense trivially “Turing-complete”. … Executing a finite sequence of mov instructions will complete in a finite amount of time.
What does test EAX EAX do?
eax contains the return value of strcmp. test is like bitwise and except it only sets the flags. Anding a value with itself gives the same value, so test eax, eax sets the flags based on whatever eax contains. ZF is set when the result of an operation is zero.
What is byte ptr?
byte ptr -> it simply means that you want to fetch a byte from the address. if it said word ptr or dword ptr , you would get a word or dword from the address in source index.
What is rip in assembly?
The instruction pointer register (%rip) points to the next instruction to execute; it cannot be directly accessed by the programmer, but is heavily used as the base for position-independent code addressing.
What is CLD in assembly language?
CLD: clear direction flag so that string pointers auto increment after each string operation. STD: std is used to set the direction flag to a 1 so that SI and/or DI will automatically be decremented to point to the next string element when one of the string instruction executes.
What is SHL in assembly language?
The SHL (shift left) instruction performs a logical left shift on the destination operand, filling the lowest bit with 0. CF.
What does CDQ mean in assembly?
The CDQ (Convert Doubleword to Quadword) instruction extends the sign bit of EAX into the EDX register.
How do you use IDIV?
The IDIV (signed divide) instruction performs signed integer division, using the same operands as the DIV instruction. For both DIV and IDIV, all of the arithmetic status flags are undefined after the operation. When doing 8-bit division, you must sign-extend the dividend into AH before using IDIV.
How does a Div assembly work?
div executes unsigned division. div divides a 16-, 32-, or 64-bit register value (dividend) by a register or memory byte, word, or long (divisor). The quotient is stored in the AL, AX, or EAX register respectively. The remainder is stored in AH, Dx, or EDX.
What is neg in assembly?
Description. Replace the value of the byte, word, or long with its two’s complement; that is, neg subtracts the byte, word, or long value from 0, and puts the result in the byte, word, or long respectively. neg sets the carry flag to 1, unless initial value of the byte, word, or long is 0.
How does CMP work Assembly?
The CMP instruction compares two operands. … This instruction basically subtracts one operand from the other for comparing whether the operands are equal or not. It does not disturb the destination or source operands. It is used along with the conditional jump instruction for decision making.
