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1. Top results related to why was x86 called x86?

   • x86 IMUL result is correct, why?

     Top Answer

     Answered Jan 08, 2012 · 6 votes

     MOV EAX,0xB504F333MOV ECX,0xB504F333IMUL ECX-

     Must indeed produce 0x15F61998 in EDX and 0x9EA1DC29 in EAX. That's because IMUL treats its operands as signed.

     In other words, since 0xB504F333 represents a negative value in 2's complement code (because its most significant bit is set), the instruction actually multiplies 0xB504F333-0x100000000=-1257966797 instead of 0xB504F333=3037000499 by itself.

     And so the correct result is 0x15F619989EA1DC29=1582480462354439209 instead of 0x7FFFFFFF9EA1DC29=9223372030926249001.

     Since the 64-bit signed product in EDX:EAX is not the sign extended value of EAX, which means that the signed product doesn't fit into 32 bits, IMUL sets the carry and overflow flags to 1. This all is explained in the Intel and AMD CPU manuals.

     If you use MUL ECX instead of IMUL ECX, you will get 0x7FFFFFFF9EA1DC29=9223372030926249001 in EDX:EAX and the overflow and carry flags will be set again because the now unsigned product still doesn't fit into 32 bits.

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   • x86-64 assembly: why offset 25 bytes?

     Top Answer

     Answered Mar 10, 2018 · 10 votes

     Looking at this fragment:

     leaq -25(%rbp), %raxmovq %rax, %rdicall _ZNSaIiEC1Ev-

     _ZNSaIiEC1Ev is demangled to std::allocator<int>::allocator(), so -25(%rbp) is the address of an allocator<int> object which is passed to the constructor. If we print the sizeof of this object in GCC we will get 1. Since the size of the object is 1, there is no need to align it to 8 bytes and it can be placed in any memory address.

     The -24(%rbp) which you see later is the address of a different object, and the compiler is not reading across an 8-byte boundary.

     Note that the lea instruction does not actually access memory - it only calculates an address. Therefore, the fact that it has a q suffix does not mean that it accesses 8 bytes.

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     2/5

   • x86 Assembly: Why Do I Need Stack Frames?

     Top Answer

     Answered Jun 21, 2013 · 7 votes

     Well, in fact, you don't need stack frames.

     Stack frames are convenient when you save registers and store local variables in stack - to make writing and debugging easier: you just set ebp to a fixed point in stack and address all stack data using ebp. And it's easier to restores esp at the end.

     Also, debuggers often expect stack frames to be present, otherwise you can get inaccurate stack call for example.

     So, the answer to 1 is yes, the answer to 2 and 3 is above.

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     3/5

   • x86 Assembly - Why is [e]bx preserved in calling conventions?

     Top Answer

     Answered Oct 03, 2016 · 8 votes

     Not all registers make good candidates for preserving:

     no (e)ax -- Implicitly used in some instructions; Return valueno (e)dx -- edx:eax is implicity used in cdq, div, mul and in return values- (e)bx -- generic register, usable in 16-bit addressing modes (base) (e)cx -- shift-counts, used in loop, rep (e)si -- movs operations, usable in 16-bit addressing modes (index) (e)di -- movs operations, usable in 16-bit addressing modes (index)Must (e)bp -- frame pointer, usable in 16-bit addressing modes (base)Must (e)sp -- stack pointer, not addressable in 8086 (other than push/pop)

     Looking at the table, two registers have good reason to be preserved and two have a reason not to be preserved. accumulator = (e)ax e.g. is the most often used register due to short encoding. SI,DI make a logical register pair -- on REP MOVS and other string operations, both are trashed.

     In a half and half callee/caller saving paradigm the discussion would basically go only if bx/cx is preferred over si/di. In other calling conventions, it's just EDX,EAX and ECX that can be trashed.

     EBX does have a few obscure implicit uses that are still relevant in modern code (e.g. CMPXGH8B / CMPXGH16B), but it's the least special register in 32/64-bit code.

     EBX makes a good choice for a call-preserved register because it's rare that a function will need to save/restore EBX because they need EBX specifically, and not just any non-volatile register. As Brett Hale's answer points out, it makes EBX a great choice for the global offset table (GOT) pointer in ABIs that need one.

     In 16-bit mode, addressing modes were limited to (any subset of) [BP|BX + DI|SI + disp8/disp16]), so BX is definitely special there.

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     4/5

   • Why was NOP assigned to 0x90 in x86 assembly?

     Top Answer

     Answered May 01, 2019 · 16 votes

     0x00000000 is not actually NOP on ARM A32 though it behaves like one; it's actually andeq r0, r0, r0 whereas a real NOP is 0xe1a00000 (mov r0, r0). It's a rather bad idea to have the NUL byte be a NOP because that makes every empty memory region a free NOP slide, greatly increasing the risk for attacks. Modern architectures generally try to make all zeroes be a break instruction or similar to avoid this attack vector. For example, ARM A64 makes 0x00000000 a permanently undefined instruction udf 0.

     0x90 is actually just one instruction of the 0x90 + r opcode family which stands for xchg r32, eax. eax is register 0, so 0x90 stands for xchg eax, eax which is a NOP. In long mode, 0x90 is special cased not to zero out the high 32 bit of rax to keep its function as a single byte NOP.

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     5/5

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2. People also ask

   What is not called x86?

   • 32-bit is NOT called x86. There are tens of 32-bit architectures such as MIPS, ARM, PowerPC, SPARC which are not called x86. x86 is a term meaning any instruction set which derived from the instruction set of Intel 8086 processor.

   Why is 32-bit called x86 and not x32? - DEV Community

   dev.to/alphaolomi/why-is-32-bit-called-x86-and-not-x32-46ae

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   Is x86 a 32-bit processor?

   • x86 began as a 16-bit instruction set for 16-bit processors (the 8086 and 8088), and was later expanded to a 32-bit instruction set for 32-bit processors (80386 and 80486). But the term x86 had already been consistent with all processors that used the instruction set family.

   Explain Why Windows 32-bit called Windows x86 and not Windows x32

   www.geeksforgeeks.org/explain-why-windows-32-bit-called-windows-x86-and-not-windows-x32/

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   Which 32-bit architecture is not called x86?

   • There are tens of 32-bit architectures such as MIPS, ARM, PowerPC, SPARC which are not called x86. x86 is a term meaning any instruction set which derived from the instruction set of Intel 8086 processor. It’s successors were named 80186, 80286, 80386, 80486, and were all compatible with the original 8086, capable of executing code made for it.

   Why is 32-bit called x86 and not x32? - DEV Community

   dev.to/alphaolomi/why-is-32-bit-called-x86-and-not-x32-46ae

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   What is x86 architecture?

   • x86 is the name of the architecture that it's built to run on (the name comes from a series of old Intel processors, the names of which all ended in 86, The first of which was the 8086). Although x86 was originally a 16-bit architecture, the version in use today is the 32-bit extension.

   Why is Windows 32-bit called Windows x86 and not Windows x32?

   stackoverflow.com/questions/29974425/why-is-windows-32-bit-called-windows-x86-and-not-windows-x32

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3. stackoverflow.com › questions › 29974425Why is Windows 32-bit called Windows x86 and not Windows x32?

   stackoverflow.com › questions › 29974425

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   x86 is the name of the architecture that it's built to run on (the name comes from a series of old Intel processors, the names of which all ended in 86, The first of which was the 8086). Although x86 was originally a 16-bit architecture, the version in use today is the 32-bit extension.

   • Intel CPUs at the heart of the PC architecture used to have model numbers ending with number 86. There was the 8086, 80286, 80386, 80486, 80586 (th...
   • When the processor was invented known as 8086. That was firstly introduced for 16-bit machines later it was converted to 32-bit machines. Due to it...

4. en.wikipedia.org › wiki › X86x86 - Wikipedia

   en.wikipedia.org › wiki › X86

   • Cached

   x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the 8086 microprocessor and its 8088 variant.

5. www.geeksforgeeks.org › explain-why-windows-32-bitExplain Why Windows 32-bit called Windows x86 and not Windows ...

   www.geeksforgeeks.org › explain-why-windows-32-bit

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   Jun 14, 2021 · x86 began as a 16-bit instruction set for 16-bit processors (the 8086 and 8088), and was later expanded to a 32-bit instruction set for 32-bit processors (80386 and 80486). But the term x86 had already been consistent with all processors that used the instruction set family.

6. superuser.com › questions › 179919x86 vs x64 - Why is 32-bit called x86? - Super User

   superuser.com › questions › 179919

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   The term x86 refers to a family of instruction set architectures [2] based on the Intel 8086. The 8086 was launched in 1978 as a fully 16-bit extension of Intel's early 8-bit based microprocessors and also introduced segmentation to overcome the 16-bit addressing barrier of earlier chips.

   • Technically x86 simply refers to a family of processors and the instruction set they all use. It doesn't actually say anything specific about data...
   • Yes that's correct: The term x86 refers to a family of instruction set architectures[2] based on the Intel 8086. The 8086 was launched in 1978 as...

7. dev.to › alphaolomi › why-is-32-bit-called-x86-andWhy is 32-bit called x86 and not x32? - DEV Community

   dev.to › alphaolomi › why-is-32-bit-called-x86-and

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   Jun 14, 2020 · Why is 32-bit called x86 and not x32? 32-bit is NOT called x86. There are tens of 32-bit architectures such as MIPS, ARM, PowerPC, SPARC which are not called x86. x86 is a term meaning any instruction set which derived from the instruction set of Intel 8086 processor.

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9. superuser.com › questions › 61798032 bit - Why does x86 stands for 32bit OS - Super User

   superuser.com › questions › 617980

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   As the term became common after the introduction of the 80386, it usually implies binary compatibility with the 32-bit instruction set of the 80386. This may sometimes be emphasized as x86-32 or x32 to distinguish it either from the original 16-bit "x86-16" or from the 64-bit x86-64.

10. phoenixnap.com › kb › xx64 vs. x86: Key Differences {Features, Limitations, and Use ...

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    Jul 20, 2022 · Introduction. The x86 and x64 architectures refer to the two most widely-used types of instruction set architectures ( ISA) created by Intel and AMD. An ISA specifies the behavior of machine code and defines how the software controls the CPU. ISA is the hardware and software interface, defining what the CPU can do and how.