- 1 year ago
Subtract or add a number from a variable in the bss segment in windows x64 assembly
In Windows x64 assembly, the BSS (Block Started by Symbol) segment is typically used for uninitialized data. Variables in the BSS segment are initialized to zero by default. To subtract or add a number from a variable in the BSS segment, you can follow these steps:
Define and declare the variable in the BSS segment:
assemblysection .bss
myVariable resq 1 ; Define a quadword (8 bytes) variable in the BSS segment
Access and manipulate the variable in the code section:
assemblysection .text
global _start
_start:
; Subtract a number from the variable
mov rax, qword [myVariable] ; Load the value of myVariable into RAX
sub rax, 42 ; Subtract 42 from the value
mov qword [myVariable], rax ; Store the result back to myVariable
; Add a number to the variable
mov rax, qword [myVariable] ; Load the value of myVariable into RAX
add rax, 10 ; Add 10 to the value
mov qword [myVariable], rax ; Store the result back to myVariable
; Exit the program
mov eax, 60 ; System call number for exit
xor edi, edi ; Exit status code 0
syscall ; Invoke the system call
In the code above, we assume you're using NASM syntax for x64 assembly. The _start
label is the program entry point, where the execution begins. The variable myVariable
is defined in the BSS segment as a quadword (8 bytes) using the resq
directive.
To subtract a number from the variable, you load the value of myVariable
into the RAX
register, subtract the desired value (42
in this case) from RAX
, and store the result back to myVariable
. Similarly, to add a number to the variable, you load the value of myVariable
into RAX
, add the desired value (10
in this case) to RAX
, and store the result back to myVariable
.
Finally, the program calls the exit
system call to terminate the program with an exit status code of 0
.
Remember to assemble and link the program properly using the appropriate tools for x64 assembly on Windows, such as NASM and the Microsoft linker (link.exe
).