You included stuff at the top of your bootloader, where it will executes first. Instead include extra functions where they aren’t in the main path of execution and are only reached by call. This should work, placing the %include directives where it’s safe to put extra function or data, just like if you were writing … Read more
LBA Disk Access Availability Not all BIOSes support the extended disk read and write functions (although on modern hardware they almost all likely will). Not all BIOSes support extended disk reads of floppies via Int 13h/AH=42h. This is true of BOCHS as well. You can test whether extended disk functions are available on a drive … 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
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
The shadow space must be provided directly previous to the call. Imagine the shadow space as a relic from the old stdcall/cdecl convention: For WriteFile you needed five pushes. The shadow space stands for the last four pushes (the first four arguments). Now you need four registers, the shadow space (just the space, contents don’t … Read more
Looking at the nasm source, it looks like: ‘oword”https://stackoverflow.com/”DO’ is 8 times as big as “word” (O for “octoword”), synonymous with dqword (“double-quad”); that would be 128 bits, corresponding to the size of an SSE vector register. ‘tword”https://stackoverflow.com/”DT’ is 80 bits (T for “ten bytes”), the full size of an Intel x87 floating point register. … Read more
If you’re using nasm 2.11.08, there is a issue documented here to do with relative addressing combined with multiple entries in the data section. You can do one (or both) of two things to be certain. First, you can have a look at the generated assembler code to investigate what it’s actually churning out. That’s … Read more
In x86 Intel syntax [expression] means content of memory at address expression. (Except in MASM when expression is a numeric literal or equ constant with no registers, then it’s still an immediate) expression without brackets depends on Assembler you are using. NASM-style (NASM, YASM): mov eax,variable ; moves address of variable into eax lea eax,[variable] … Read more
It can be done from assembly, but it isn’t easy. You can’t use int 21h, that’s a DOS system call and it isn’t available under Linux. To get characters from the terminal under UNIX-like operating systems (such as Linux), you read from STDIN (file number 0). Normally, the read system call will block until the … Read more
NOSPLIT: Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to be absent and space to be saved; in fact, it will also split [eax*2+offset] into [eax+eax+offset]. You can combat this behaviour by the use of the NOSPLIT keyword: [nosplit eax*2] will force [eax*2+0] to be generated literally. [nosplit eax*1] also … Read more