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67 lines
4.9 KiB
TeX
67 lines
4.9 KiB
TeX
\subsection{Operations}
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Assembly operations include performing arithmetic or logic functions on registers or memory data,
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transferring data between memory and registers and transferring control (conditional or unconditional jumps)
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Most of the memory-related instructions in \texttt{x86} assembly have the format
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\mint{asm}|movq dest, src|
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whereas arithmetic / logic operations have the format inversed (below \texttt{X} is to be replced with size postfix).
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\begin{tables}{lll}{Mnemonic & Format & Computation}
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\texttt{addl} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest + Src} \\
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\texttt{subX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest - Src} \\
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\texttt{imulX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest * Src} \\
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\texttt{salX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest << Src} \\
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\texttt{sarX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest >> Src} (arithmetic) \\
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\texttt{shrX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest >> Src} (logical) \\
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\texttt{xorX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest \string^ Src} \\
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\texttt{andX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest \& Src} \\
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\texttt{orX} & \texttt{Src, Dest} & \texttt{Dest} $\gets$ \texttt{Dest | Src} \\
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\texttt{incX} & \texttt{Dest} & \texttt{Dest} $\gets$ \texttt{Dest + 1} \\
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\texttt{decX} & \texttt{Dest} & \texttt{Dest} $\gets$ \texttt{Dest - 1} \\
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\texttt{negX} & \texttt{Dest} & \texttt{Dest} $\gets$ \texttt{-Dest} \\
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\texttt{notX} & \texttt{Dest} & \texttt{Dest} $\gets$ \texttt{\string~Dest} \\
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\end{tables}
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\newpage
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\subsubsection{Condition Codes and jumping}
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Any arithmetic operation (that is truly part of the arithmetic operations group, so not including \texttt{lea} for example) implicitly sets the \bi{condition codes}.
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The following condition codes were covered in the lecture (operation: \texttt{t = a + b}):
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\begin{itemize}
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\item \texttt{CF} (Carry Flag): Set if carry out from MSB (unsigned overflow)
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\item \texttt{ZF} (Zero Flag): Set if \texttt{t == 0}
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\item \texttt{SF} (Sign Flag): Set if \texttt{(a - b) < 0} (for signed)
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\item \texttt{OF} (Overflow Flag): Set if two's complement overflow (i.e. \verb+(a>0 && b>0 && t<0) || (a<0 && b<0 && t>=0)+)
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\end{itemize}
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\content{Explicit computation}
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In the below explanations, we always assume \texttt{src2 = b} and \texttt{src1 = a}
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To explicitly compute them, we can use the \texttt{cmpX src2, src1} instruction, that essentially computes $(a - b)$ without setting a destination register.
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When we execute that instruction, \texttt{CF} is set if \texttt{a < b} (unsigned), \texttt{ZF} is set if \texttt{a == b}, \texttt{SF} is set if \texttt{a < b} (signed)
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and \texttt{OF} is set as above, where \texttt{t = a - b}.
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Another instruction that is used is \texttt{testX src2, src1}, and acts like computing \texttt{a \& b} and where \texttt{ZF} is set if \texttt{a \& b == 0}
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and \texttt{SF} is set if \texttt{a \& b < 0}.
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\content{Zeroing register} We can use a move instruction, but that is less efficient than using \texttt{xorl reg, reg}, where \texttt{reg} is the 32-bit version of the reg we want to zero.
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\content{Reading condition codes} To read condition codes, we can use the \texttt{setX} instructions, where the \texttt{X} is to be substituted by an element of table \ref{tab:condition-codes}
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\content{Jumping} To jump, we have the \texttt{jmp <label>} instruction (unconditional jump) or the \texttt{jX} instructions, with \texttt{X} from table \ref{tab:condition-codes}
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\begin{table}[h!]
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\begin{tables}{lll}{\texttt{setX} & Condition & Description}
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\texttt{e} & \verb|ZF| & Equal / Zero \\
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\texttt{ne} & \verb+~ZF+ & Not Equal / Not Zero \\
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\texttt{s} & \verb|SF| & Negative \\
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\texttt{ns} & \verb+~SF+ & Nonnegative \\
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\texttt{g} & \verb+~(SF^OF)&~ZF+ & Greater (signed) \\
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\texttt{ge} & \verb+~(SF^OF)+ & Greater or equal (signed) \\
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\texttt{l} & \verb+SF^OF+ & Less (signed) \\
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\texttt{le} & \verb+(SF^OF)|ZF+ & Less or equal (signed) \\
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\texttt{a} & \verb+~CF&~ZF+ & Above (unsigned) \\
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\texttt{b} & \verb|CF| & Below (unsigned) \\
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\end{tables}
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\caption{Condition code postfixes for jump and set instructions}
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\label{tab:condition-codes}
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\end{table}
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