Recent questions tagged Assembly

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assembly - How to produce a minimal BIOS hello world boot sector with GCC that works from a USB stick on real hardware?

I have managed to produce a minimal boot sector that works with QEMU 2.0.0 Ubuntu 14.04 ... /cirosantilli/d47d35bacc9be588009f#file-lenovo-t400 See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Enable the boot loader to load the second sector of a USB

I am learning the assembly language. I wrote a simple bootloader. After testing it out, it didn't work. ... something wrong? Any suggestions? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Where is VPERMB in AVX2?

AVX2 has lots of good stuff. For example, it has plenty of instructions which are pretty much strictly ... functionally they are superseded. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Relative performance of x86 inc vs. add instruction

Quick question, assuming beforehand mov eax, 0 which is more efficient? inc eax inc eax or add eax, 2 ... this is merely academic interest. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Is there a specification of x86 I/O port assignment?

I found some information on the web as follows, but incomplete. http://en.wikipedia.org/wiki/Input/ ... to download the specification? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Why are x86 registers named the way they are?

For example, the accumulator is named EAX and, while the instruction pointer is called IP. I also know that there ... names, but what is it? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What is a microcoded instruction?

I have seen a lot of literature referencing microcoded instructions. What are these and why they are used? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What is the purpose of segment registers in x86 protected mode?

I need to modify some dll, but i don't know, what excatly does segment registers (DS, SS, ...) in protected ... mov eax, dword ptr ds:[20037DA0] See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How does the GCC implementation of modulo (%) work, and why does it not use the div instruction?

I was trying to work out how to calculate modulo 10 in assembly so i compiled the following c code in gcc ... better than using the div operand. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Why did GCC generate mov %eax,%eax and what does it mean?

GCC 4.4.3 generated the following x86_64 assembly. The part that confuses me is the mov %eax,%eax. Move the ... a pointer of the correct type. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What does ordered / unordered comparison mean?

Looking at the SSE operators CMPORDPS - ordered compare packed singles CMPUNORDPS - unordered compare packed singles ... unordered (FUCOM). See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What does `dword ptr` mean?

Could someone explain what this means? (Intel Syntax, x86, Windows) and dword ptr [ebp-4], 0 See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How to tell the length of an x86 instruction?

I was looking at the different instructions in assembly and I am confused on how the lengths of different operands ... its opcode will require? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - NASM Error Parsing, Instruction Expected

I asked a similar question here but I changed assemblers and the question almost entirely changed, so in an effort to avoid ... mov ah,9 int 21h jmp short EndZTimerReport ; ; C...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How to write an absolute target for a near direct relative call/jmp in MASM

To make a normal (near direct relative) call to an absolute address, in NASM or AT&T syntax you write call ... code I want to actually use. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How to code a far absolute JMP/CALL instruction in MASM?

How can I write a far absolute JMP or CALL instruction using MASM? Specifically how do I get it to emit ... generate the right opcode (EA)? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How to read image file and display on screen in windows tasm dosbox

Im currently learning graphical programming in tasm using dosbox. Ive been using a bunch of loops to draw square blocks ... is a start. Pls help See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Why can't I move directly a byte to a 64 bit register?

Why can't I directly move a byte from memory to a 64-bit register in Intel x86-64 assembly? For instance, ... simpler way of doing this? Thanks. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Counting 1 bits (population count) on large data using AVX-512 or AVX-2

I have a long chunk of memory, say, 256 KiB or longer. I want to count the number of 1 bits in this ... single (thread on a single) core. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - How do vararg functions find out the number of arguments in machine code?

How can variadic functions like printf find out the number of arguments they got? The amount of arguments obviously isn' ... . What's the trick? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - tasm: operand type does not match

.model small .stack .data buff label byte maxchar dw 50 readchar dw 0 name1 db 48 dup(0) m1 db 10,13,"enter ... int 21h mov ah, 4ch int 21h end See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Unexpectedly poor and weirdly bimodal performance for store loop on Intel Skylake

I'm seeing unexpectedly poor performance for a simple store loop which has two stores: one with a forward ... so trivial. Pull requests accepted. See Question&Answers more detail:...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What is the latency and throughput of the RDRAND instruction on Ivy Bridge?

I cannot find any info on agner.org on the latency or throughput of the RDRAND instruction. However, this ... and throughput is variable. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Which is a better write barrier on x86: lock+addl or xchgl?

The Linux kernel uses lock; addl $0,0(%%esp) as write barrier, while the RE2 library uses xchgl (%0),%0 ... available. When is lfence required? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What do the brackets mean in x86 asm?

Given the following code: L1 db "word", 0 mov al, [L1] mov eax, L1 What do the brackets ([L1]) represent? See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - What are IN & OUT instructions in x86 used for?

I've encoutered these to instructions IN & OUT while reading "Understanding Linux Kernel" book. I've looked ... ? Give some practical examples. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Does it make any sense to use the LFENCE instruction on x86/x86_64 processors?

Often in internet I find that LFENCE makes no sense in processors x86, ie it does nothing , so instead ... command SFENCE flushes Store-Buffer. See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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assembly - Turn float into string

I have proceeded to state when I need to turn IEEE-754 single and double precision numbers into ... (betweenResult, decimalExponent); See Question&Answers more detail:os...

asked Oct 17, 2021 in Technique[技术] by 深蓝 (71.8m points)

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