Changes

When you program in [[Assembler|assembly language]], you're directly programming the "bare metal" hardware. This means that many of the compile-time and run-time checks, error messages, and diagnostics that are present in other languages are not available. The computer will follow your instructions exactly, even if they are completely wrong (like executing data), and when something goes wrong, your program won't terminate until it tries to do something that's not permitted, such as execute an invalid opcode or attempt to access a protected or unmapped region of memory. When that happens, the CPU will signal an exception, and in most cases the operating system will shut down the offending process.

The traditional extension for assembly-language source files is <code>.s</code> (e.g., <code>example.s</code>), or <code>.S</code> for files that need to go through the C preprocessor (<code>cpp</code>).

Assembler '''directives''' are used to control the assembly of the code, by specifying output file sections (such as .text (machine code), .data (read/write data), or .rodata (read-only data /constants) in an ELF file) and data formats (such as word size for numeric values), and by defining macros.

<font color="green">mov</font> $<font color="orange">1</font>,%<font color="blue">rax</font> /* [[Syscalls|syscall ]] sys_write */

<font color="red">.datarodata</font>

<font color="red">.set</font> <font color="blue">len</font> , <font color="orange">. - msg</font>

* .start text is a directive (equivalent to the longer directive ".section .starttext") which specifies that the following instructions/data should be placed in the ".starttext" section of the output ELF file.* .data rodata is a similar directive which specifies that the following instructions/data should be placed in the .data rodata section of the output ELF file. In the case of this program, they could alternately be placed in the .rodata data section, which is for read-only write data (data which is write-protected in memory), but .rodata was used because the string is not modified by the program(it's a constant).

* .set is a directive which sets a symbol (len) equal to the value of an expression (in this example, ". - msg" meaning the current memory location minus the value of the label "msg"). Note that the GNU assembler accepts <code>a=1</code> as equivalent to <code>.set a , 1</code> -- both are counted as directives regardless of the presence of the <code>.set</code> keyword.

# Code must be placed in the <code>.text</code> section of the ELF file.# Data must be placed in either the <code>.rodata</code> (read-only data) or <code>.data</code> (read/write data) sections of the ELF file.# There must be a globally-defined symbol which the linker (<code>ld</code>) will use to find the entry point to your program. If the code is being directly compiled by the assembler, this symbol must be <code>_start</code> -- but if the code is being compiled by gcc, this symbol must be called <code>main</code>(a preamble will be located at <code>_start</code> which will then transfer control to <code>main</code>).

# Run the linker: <code>ls ld -o ''test'' ''test''.o</code>