You’re running into one surprising difference between i386 and x86_64: they don’t use the same system call mechanism. The correct code is: movq $60, %rax movq $2, %rdi ; not %rbx! syscall Interrupt 0x80 always invokes 32-bit system calls. It’s used to allow 32-bit applications to run on 64-bit systems. For the purposes of learning, … Read more
See Why Does GCC LEA EIZ?: Apparently %eiz is a pseudo-register that just evaluates to zero at all times (like r0 on MIPS). … I eventually found a mailing list post by binutils guru Ian Lance Taylor that reveals the answer. Sometimes GCC inserts NOP instructions into the code stream to ensure proper alignment and … Read more
Labels “xb” and “xf”, where “x” is a number are a smart extension to the GNU assembly. It branches to the first found label “x” searching “forward” for “f” or “backward” for “b”. That means that in your first listing using “1b” as a target will search for “1” BEFORE the instruction that uses it. … Read more
GNU gas behavior for ARMv7 depends on .syntax The docs say https://sourceware.org/binutils/docs-2.26/as/ARM_002dInstruction_002dSet.html#ARM_002dInstruction_002dSet : Two slightly different syntaxes are support for ARM and THUMB instructions. The default, divided, uses the old style where ARM and THUMB instructions had their own, separate syntaxes. The new, unified syntax, which can be selected via the .syntax directive, and has … Read more
The information about a function size is stored in the ELF Attributes for the corresponding symbol (name). C example code how to parse this programmatically is at the bottom of the Solaris manpage forgelf_getsym(3ELF) (libelf does exist in Linux, *BSD and MacOS as well, you need to look for the st_size field of the GElf_Sym … Read more
There are a few possibilities… here are a couple I can think of off the top of my head: You could access it relative to a label that comes after the .byte directive. Example: .byte 0x0a label: mov (label – 1), %eax Based on the final linked layout of the program, maybe the .byte directives … Read more
As of Binutils 2.29 the instruction suffixes are now deprecated in favor of pseudo-prefixes. You can find the older suffixes documented in the GNU Assembler (pre-2.29) info pages. Earlier info as pages say this: 9.15.4.1 Instruction Naming [snip] Different encoding options can be specified via optional mnemonic suffix. .s suffix swaps 2 register operands in … Read more
As mentioned by @Jester, I had to zero DS with: @@ -4,2 +4,4 @@ _start: cli + xor %ax, %ax + mov %ax, %ds mov $msg, %si Note that it is not possible to mov immediates to ds: we must pass through ax: 8086- why can’t we move an immediate data into segment register? So … Read more
The problem with using GCC’s inline asm to learn assembly is that you spend half your time learning about how gcc’s inline assembly works instead of actually learning assembly. For example here’s how I might write this same code: #include <stdio.h> int getStringLength(const char *pStr){ int len; __asm__ ( “repne scasb\n\t” “not %%ecx\n\t” “dec %%ecx” … Read more
I believe that you want to load the address of your string into %rsi; your code attempts to load a quadword from that address rather than the address itself. You want: lea msg(%rip), %rsi if I’m not mistaken. I don’t have a linux box to test on, however.