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79 lines
4.5 KiB
TeX
79 lines
4.5 KiB
TeX
\newpage
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\subsection{The Stack}
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The Stack is the main way to dynamically allocate memory in Assembly, i.e. for temporary values. This process is completely manual: Allocating/Deallocating memory on the stack is entirely explicit. The stack grows \textit{downwards}: Addresses decrease as the stack grows.
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The Stack pointer \verb|%rsp| points to the current top of the stack.
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\content{Using the Stack} \verb|pushX|, \verb|popX| exist for $x = 1, 2, 4, 8$, each corresponding to a size prefix that is set with \texttt{X}
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\bi{Stack push} \texttt{pushX src}: Fetch operand at \texttt{src}, decrement \texttt{\%rsp} by $x$, then writes the operand at address of \texttt{\%rsp}
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\bi{Stack pop} \texttt{popX dest}: Fetch operand at address of \texttt{\%rsp}, increment \texttt{\%rsp} by $x$, then writes the operand into \texttt{dest}
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So intuitively, \verb|pushX| and \verb|popX| do exactly what is expected.
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\content{Procedure call / return} Use \texttt{call LABEL}. This pushes the return label to the stack and jumps to \verb|LABEL|.
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After this instruction, we also may use the \texttt{pushX} instruction to store further registers.
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Just remember to pop in the correct order with the correct size again!
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The \texttt{ret} instruction is the return instruction and it will jump back to the caller and execution will continue there.
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\subsubsection{Calling Conventions}
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Even though Assembly will never stop you from using registers in any way, \hlhref{cs61.seas.harvard.edu/site/pdf/x86-64-abi-20210928.pdf}{System V ABI}\ specifies a few conventions, standardizing especially how functions are allowed to use registers.
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\content{Caller/Callee} The callee is the function that is called and the caller is the code / function that calls the function.
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\begin{itemize}
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\item \texttt{\%rax} and \texttt{\%eax} are caller saved (usually used as return)
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\item \texttt{\%rdi, \%rsi, \%rdx, \%rcx} are caller saved (usually used for arguments)
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\item \texttt{\%rsp} should not be modified manually
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\item \texttt{\%rbp} is callee saved and used as frame pointer: usually set to \texttt{\%rsp} at start of procedure and used to access frame elements
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(should always point to the start of the frame during function)
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\end{itemize}
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\content{Register Conventions}
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\begin{multicols}{2}
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\begin{tables}{ll}{Name & Description}
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\texttt{\%rax} & Return value, \#variable args \\
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\texttt{\%rbx} & Base pointer, Callee saved \\
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\texttt{\%rcx} & Argument 4 \\
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\texttt{\%rdx} & Argument 3 (and return 2) \\
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\texttt{\%rsi} & Argument 2 \\
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\texttt{\%rdi} & Argument 1 \\
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\texttt{\%rsp} & Stack pointer \\
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\texttt{\%rbp} & Frame pointer, Callee saved \\
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\end{tables}
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\begin{tables}{ll}{Name & Description}
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\texttt{\%r8} & Argument 5 \\
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\texttt{\%r9} & Argument 6 \\
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\texttt{\%r10} & Static chain pointer \\
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\texttt{\%r11} & Temporary \\
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\texttt{\%r12} & Callee saved \\
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\texttt{\%r13} & Callee saved \\
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\texttt{\%r14} & Callee saved \\
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\texttt{\%r15} & GOT pointer, callee saved \\
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\end{tables}
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\end{multicols}
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If we have more than 6 arguments to be passed, we can use the stack for this.
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If we can do all accesses to the stack relative to the stack pointer, we do not need to
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update \texttt{\%rbp} and not even \texttt{\%rbx}, or we can use it for other purposes.
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\content{Manual stack management} We can also allocate the entire stack frame directly by incrementing \texttt{\%rsp} to the final position and then store data relative to it.
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To deallocate a stack frame, simply increment the stack pointer.
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\newpage
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\subsubsection{Examples}
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The stack is commonly used for recursive functions. A recursive factorial function might compile like this:
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Note how \texttt{\%rbx} is saved on the stack, since \texttt{rbx} is callee-saved by convention. \\
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\texttt{\%eax} is used directly, since it is caller-saved by convention.
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\inputcodewithfilename{gas}{code-examples/01_asm/}{06_factorial.s}
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A more complex example, passing addresses as arguments: \\
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This function swaps 2 array elements (using a \texttt{swap} function) and adds the first value to an accumulator.
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\inputcodewithfilename{gas}{code-examples/01_asm/}{07_swap_and_sum.s}
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