![]() Commands that can accept only a single file name are said to accept a filename parameter. In the list below, commands that can accept more than one file name, or a filename including wildcards (* and ?), are said to accept a filespec (file specification) parameter. Copies of these utility command programs had to be on an accessible disk, either on the current drive or on the command path set in the command interpreter. Such utility programs would be stored on disk and loaded just like regular application programs but were distributed with the operating system. If no match is found, an error message is printed, and the command prompt is refreshed.Įxternal commands were too large to keep in the command processor, or were less frequently used. When the user types a line of text at the operating system command prompt, COMMAND.COM will parse the line and attempt to match a command name to a built-in command or to the name of an executable program file or batch file on disk. Some commands are internal-built into COMMAND.COM others are external commands stored on disk. When an application exits, if the transient portion of the command interpreter in memory was overwritten, DOS will reload it from disk. So main and printf will become _main and _printf respectively in the Windows development environment.The command interpreter for DOS runs when no application programs are running. Microsoft will prefix functions using the cdecl calling convention with a underscore.gcc expects the entry point for execution to be main.This also means we do not need to explicitly specify its length anymore. The "hello, world" string now becomes the format string for printf(3) and therefore needs to be null terminated.In the Windows world this has the additional benefit that the linking step will be the same in the Windows command prompt and cygwin. This has the advantage that going from Linux to Windows requires minimal source code changes and a slightly different assemble and link steps. In this example we will rewrite Hello World to use printf(3) from the C library and link using gcc. Hello World (Using C libraries and Linking with gcc) One last note, WriteConsole() does not behave well within a cygwin console, so in order to see output the final exe should be run within a Windows command prompt. This example was run under cygwin, in a Windows command prompt the link step would be different. Otherwise we would have to use as the entry point rather than _start. In this example we use the -e command line option when invoking ld to specify the entry point for program execution. $ ld -e _start helloWorldwin32.obj -lkernel32 -o helloWorldWin32.exe text _start: HANDLE WINAPI GetStdHandle( _In_ DWORD nStdHandle ) push dword -11 Arg1: request handle for standard output call Result: in eax BOOL WINAPI WriteConsole( _In_ HANDLE hConsoleOutput, _In_ const VOID *lpBuffer, _In_ DWORD nNumberOfCharsToWrite, _Out_ LPDWORD lpNumberOfCharsWritten, _Reserved_ LPVOID lpReserved ) push dword 0 Arg5: Unused so just use zero push numCharsWritten Arg4: push pointer to numCharsWritten push dword strLen Arg3: push length of output string push str Arg2: push pointer to output string push eax Arg1: push handle returned from _GetStdHandle call VOID WINAPI ExitProcess( _In_ UINT uExitCode ) push dword 0 Arg1: push exit code call order to assemble, link and run the program we need to do the following: data str: db ' hello, world ', 0x0D, 0x0A strLen: equ $-str section. Global _start extern extern extern section. NASM has powerful macro functions, similar to C's preprocessor. For example,įor those using gdb with nasm, you can set gdb to use Intel-style disassembly by issuing the command:Ī single semi-colon is used for comments, and functions the same as double slash in C++: the compiler ignores from the semicolon to the next newline. ![]() ![]() This means that the operand order is dest then src, as opposed to the AT&T style used by the GNU Assembler. The Netwide Assembler (NASM) uses a syntax "designed to be simple and easy to understand, similar to Intel's but less complex". Wikipedia has related information at Netwide Assembler 5 Hello World (Using C libraries and Linking with gcc).4 Hello World (Using only Win32 system calls). ![]()
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