The problem with the conditional jump instructions in 16-bit assembler is that they’re limited to +127 or -128 bytes for the offset. 386 introduced the jcc rel16 encoding that is available in 16-bit mode, but only on 386 and later. Different assemblers have different options for enabling 386 instructions in 16-bit code Some also have … Read more
That assembly language is MASM, not NASM. For starters, NASM segments are defined differently. Instead of Code segment word public ‘CODE’ we write .section text And that “ASSUME” declaration… You must have an ancient book. That is old, old MASM code. Brings back memories from the early 1980s for me! There are many differences between … Read more
You’re right, a modern x86 will decode add dword [mem], 1 to 3 uops: a load, an ALU add, and a store. (This is actually a simplification of various things, including Intel’s micro-fusion and how AMD always keeps a load+ALU together in some parts of the pipeline…) Those 3 dependent operations can’t happen at the … Read more
The number of instructions executed in program 1) is high because of linking the program with system library’s at runtime? Yep, dynamic linking plus CRT (C runtime) startup files. used -static and which reduces the count by a factor of 1/10. So that just left the CRT start files, which do stuff before calling main, … Read more
X86-64 system calls use syscall instruction. This instruction saves return address to rcx, and after that it loads rip from IA32_LSTAR MSR. I.e. rcx is immediately destroyed by syscall. This is the reason why rcx had to be replaced for system call ABI. This same syscall instruction also saves rflags into r11, and then masks … Read more
BTS (and the other BT* insns) with a memory destination are slow. (>10 uops on Intel). You’ll probably get faster code from doing the address math to find the right byte, and loading it into a register. Then you can do the BT / BTS with a register destination and store the result. Or maybe … Read more
If your main function is being called from the C runtime library, then exiting will result in a call to ExitProcess() and the process will exit. If your main function is being called directly by Windows, as may well be the case with assembly code, then exiting will only cause the thread to exit. The … Read more
An x86 instruction stream is not self-synchronizing, and can only be unambiguously decoded forward. You need to know a valid start-point to decode. The last byte of an immediate can be a 0x90 which decodes as a nop, or in general a 4-byte immediate or displacement can have byte-sequences that are valid instructions, or whatever … Read more
If you are on Linux you can do it with DD utility. There is a version of DD for Microsoft Windows as well. General DD usage If you wish to make a zero filled virtual disk image the size of a 720K floppy you can use dd like this: dd if=/dev/zero of=disk.img bs=1024 count=720 This … Read more
You can’t vpinsrq into a YMM register. Only an xmm destination is available, so it unavoidably zeros the upper lane of the full YMM register. It was introduced with AVX1 as the VEX version of the 128-bit instruction. AVX2 and AVX512 did not upgrade it to YMM/ZMM destinations. I’m guessing they didn’t want to provide … Read more