MASM Over Explained | Assembly x86

ThiccDaddyLOAF

June 28th, 2021 07:07:30 PM

					
; Sample project from the video "Setup Visual Studio for Assembly MASM" https://www.youtube.com/watch?v=LqyVybUodXE ; Basically reverses a string using the stack. It stores the result where the string was defined, modifying the string itself in memory. .386 ;x86 instruction set. This is only nessesary in x32 MASM. .model flat, stdcall ;Defines the memory model, and the calling convention http://masm32.com/board/index.php?topic=6942.msg74381#msg74381 .stack 4096 ;Sets the size of the stack. 4096 is 1024*4 which is 4 kilobytes. ;option casemap:none ;Make labels case sensitive, so things like ExitProcess and exitprocess and myName and myname are different. ;include \masm32\include\windows.inc ;include \masm32\include\kernel32.inc ;includelib \masm32\lib\kernel32.lib ;Some system calls or something are defined here. Not needed if you just use assembly along side c++ or c. Also can't be used if on anything other than windows. ExitProcess PROTO, dwEXITCODE:DWORD ;Define the ExitProcess function prototype. ExitProcess is defined somewhere in the Windows API on the OS level. .data ;Starts the data section of the program, where a lot of read-write memory is needed myName BYTE "Leif Messinger", 0 ;Allocates a string named myName initialized with LeifMessinger followed by a terminating null character myNameLength = ($ - myName) - 1 ;Allocates a 32 bit number which is initialized to be the current pointer ($), minus the pointer to the start of the string, minus one (because of the null terminating character) .code ;Starts the section of the program where your code goes. Is the same as .text, and in theory, the name doesn't even matter. The importance is that it ends the data section, which was read-write. Code should be read only. main PROC ;States the start of the main procedure. This is where the program starts executing. mov ecx, myNameLength ;Sets the loop counter to myNameLength. The loop counts backwards until it reaches zero, meaning that it loops myNameLength times. mov esi, 0 ;Sets the general purpose register esi to 0. esi was named because it's a "source index" when moving stuff. It's basically like an i variable in c. StackIt: ;Flag to the start of the loop. Ahh, tss push it movzx eax, myName[esi] ;movzx stands for move with zero extend. It pads the source data (myName[esi]) with enough zeros to fill 32 bits, or enough for the push and pop instructions. push eax ;Pushes the character we just got out of that string (and padded with zeros) to the stack inc esi ;Increments our index loop StackIt ;Decrements ecx and checks if it is equal to 0. If it isn't, jump to StackIt mov ecx, myNameLength ;Sets the loop counter to myNameLength. The loop counts backwards until it reaches zero, meaning that it loops myNameLength times. mov esi, 0 ;Sets the general purpose register esi to 0. esi was named because it's a "source index" when moving stuff. It's basically like an i variable in c. PopIt: ;Flag to the start of the loop. pop eax ;Pops a 32 bit number off the stack and places it in eax mov myName[esi], al ;Move the lower byte of eax (al) to the string (myName) at the index esi. This overwrites one character of the string. inc esi ;Increments our index loop PopIt ;Decrements ecx and checks if it is equal to 0. If it isn't, jump to StackIt INVOKE ExitProcess, 0 ;Exits the program with the error code of 0. The return type was declared in that prototype at the top, and defined by the operating system main ENDP ;States the end of the main procedure END main ;Declares the end of the file. Also defines where the start of the program is, which is main. ;Normally the linker looks for main as a place to start, but if you are writing pure assembly, that is need. ;Saying END without a pointer to the start is also alowed for people who like to live dangerously.

Featured Posts