\chapter{Instruction Set Architecture}

\newcommand{\OpcodeTable}[5] {
	\noindent
	\begin{minipage}{\textwidth}
		\begin{tabularx}{\textwidth}{ | X | X | X | X | }
			\hline
			Opcode Hex & Instruction & Operand 1 & Operand 2 \\
			\hline
			#1 & #2 & #3 & #4 \\
			\hline
		\end{tabularx}
		\paragraph{Description} #5
	\end{minipage}
}

\def\minY{-1.5em} \def\maxY{1.5em}
\def\minX{-7} \def\maxX{7}

\section{Encoding Scheme}
Instructions are variable width with their total length being determined by the mode setting byte that immediately follows the opcode byte. A break down of what a nibble means in the mode setting byte can be found in Figure \ref{fig:ModeSettingEncodingFigure}. Figure \ref{fig:opencoding} shows the layout of an instruction with the mode setting byte split down the middle to indicate the top nibble defines what \textit{Operand One} is meant to be interpreted as, followed by the lower nibble for \textit{Operand Two}. Depending on what mode is set for each operand there may be no bytes to follow the instruction (like in the case of nop) or there may be 8 bytes to follow (like in the case of mov [address], [address], assuming absolute addresses).
% Placement info here: https://en.wikibooks.org/wiki/LaTeX/Floats,_Figures_and_Captions#Figures
% Paths: https://tikz.dev/tikz-paths
\begin{figure}[h]
	\begin{tikzpicture}
		%Opcode Byte
		\draw (-8, 0) -- (-4, 0) node[midway, below=1em] {Opcode};
		\draw (-8, -0.25) -- (-8, 0.25) node[above] {7};
		\draw (-4, -0.25) -- (-4, 0.25) node[above] {0};
		%Mode Setting Byte
		\draw (-3.75, 0) -- (0, 0) node[midway, below=1em] {Mode Setting};
		\draw (-1.9, -0.25) -- (-1.9, 0.25) node[above] {3};
		\draw (-3.75, -0.25) node[below] {7} -- (-3.75, 0.25) node[right] {Op. One};
		\draw (0, -0.25) node[below] {0} -- (0, 0.25) node[left] {Op. Two};
	\end{tikzpicture}
	\caption{Encoding}
	\label{fig:opencoding} % https://www.overleaf.com/learn/latex/Referencing_Figures
	\paragraph{Note} The \textit{Mode Setting} byte is split into an upper nibble and a lower nibble which will contain any of the values shown in Figure \ref{fig:ModeSettingEncodingFigure}.
\end{figure}

\begin{figure}
	\begin{tabularx}{\textwidth}{ | X | X | X | }
		\hline
		Operand & Immediate Value & Value As Address \\
		\hline
		Register & 0000 & 1000 \\
		8-Bit & 0001 & 1001 \\
		16-Bit & 0010 & 1010 \\
		32-Bit & 0100 & 1100 \\
		None & 1111 & 1111 \\
		\hline
	\end{tabularx}
	\caption{Operand Modes Bit Pattern}
	\label{fig:ModeSettingEncodingFigure} % https://www.overleaf.com/learn/latex/Referencing_Figures
\end{figure}

\begin{figure}
	\begin{tikzpicture}
		\draw (\minX, 0) -- (\maxX, 0);
		\draw (\minX, \minY / 2) -- (\minX, \maxY / 2); %{$y$}; Note the $'s, those make something math stylized.
		\draw (0, \minY / 2) -- (0, \maxY / 2); % Halfway mark
		\draw (\maxX, \minY / 2) -- (\maxX, \maxY / 2);
		\draw (\minX, -\maxY) node[below=1em] {31} [decorate, decoration={brace, amplitude=1em, mirror}] -- (0, -\maxY) node[below=1em] {16} node[midway, below=1em] {Op Code (16-bits)};
		\draw (0, \maxY) node[above=1em] {15} [decorate, decoration={brace, amplitude=1em}] -- (\maxX, \maxY) node[above=1em] {0} node[midway, above=1em] {Near Address (16-bits)};
	\end{tikzpicture}
	\caption{Short Valued Opcodes}
	\label{fig:OpShortEncodingFigure} % https://www.overleaf.com/learn/latex/Referencing_Figures
\end{figure}
\begin{figure}
	\begin{tikzpicture}
		\draw (\minX, 0) -- (\maxX, 0);
		\draw (\minX, \minY / 2) -- (\minX, \maxY / 2); %{$y$}; Note the $'s, those make something math stylized.
		\draw (0, \minY / 2) -- (0, \maxY / 2); % Halfway mark
		\draw (\maxX, \minY / 2) -- (\maxX, \maxY / 2);
		\draw (\maxX / 2, \minY / 2) -- (\maxX / 2, \maxY / 2); % Halfway mark for the lower 16 bits
		\draw (\minX, -\maxY) node[below=1em] {31} [decorate, decoration={brace, amplitude=1em, mirror}] -- (0, -\maxY) node[below=1em] {16} node[midway, below=1em] {Op Code (16-bits)};
		\draw (0, \maxY) node[above=1em] {15} [decorate, decoration={brace, amplitude=1em}] -- (\maxX / 2, \maxY) node[above=1em] {8} node[midway, above=1em] {Destination};
		\draw (\maxX / 2, -\maxY) node[below=1em] {7} [decorate, decoration={brace, amplitude=1em, mirror}] -- (\maxX, -\maxY) node[below=1em] {0} node[midway, below=1em] {Source};
	\end{tikzpicture}
	\caption{Register Based Opcodes}
	\label{fig:OpArgEncodingFigure} % https://www.overleaf.com/learn/latex/Referencing_Figures
	\paragraph{Note} How the \textit{Destination} and \textit{Source} are interpreted is dependent on the opcode and operand mode bits as shown in Figure \ref{fig:AddressingModeBitPattern}.
\end{figure}
\clearpage
\section{Instructions}
The /r represents a byte that specifies a register. /iX is for the immediate data type, where \textit{X} defines the width, which are 8, 16, or 32 bits.
\subsection{Add}
\OpcodeTable{01 /r /r}{add /r, /r}{Register}{Register}{Sums \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{Sub}
\OpcodeTable{01 /r /r}{sub /r, /r}{Register}{Register}{Subtracts \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{Mul}
\OpcodeTable{01 /r /r}{mul /r, /r}{Register}{Register}{Multiplies \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{Div}
\OpcodeTable{01 /r /r}{div /r, /r}{Register}{Register}{Divides \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{Mov}
\OpcodeTable{04 /mod /r /r}{mov /mod /r, /r}{Register}{Register}{Copies the 32-bit value from \textit{Operand 2} to \textit{Operand 1}.}
\subsection{AND}
\OpcodeTable{01 /r /r}{and /r, /r}{Register}{Register}{Bit-wise ANDs \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{OR}
\OpcodeTable{01 /r /r}{or /r, /r}{Register}{Register}{Bit-wise ORs \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{XOR}
\OpcodeTable{01 /r /r}{xor /r, /r}{Register}{Register}{Bit-wise exclusive ORs \textit{Operand 1} and \textit{Operand 2} placing the result into \textit{Operand 1}.}
\subsection{NOT}
\OpcodeTable{01 /r}{not /r}{Register}{None}{Bit-wise negates \textit{Operand 1} placing the result into \textit{Operand 1}.}
\subsection{SHL}
\OpcodeTable{01 /r /i8}{shl /r, /i8}{Register}{Immediate}{Shifts the bits of \textit{Operand 1} left \textit{Operand 2} times.}
\subsection{SHR}
\OpcodeTable{01 /r /i8}{shr /r, /i8}{Register}{Immediate}{Shifts the bits of \textit{Operand 1} right \textit{Operand 2} times.}

\subsection{NOP}
\OpcodeTable{01}{nop}{None}{None}{No operation, increments the Program Counter by one.}
%Oh this control flow will be interesting, juggling flags is not something I'm used to.
\subsection{CMP}
\OpcodeTable{01 /mod /r /r}{cmp /r, /r}{Register}{Register}{Subtracts \textit{Operand 2} from \textit{Operand 1} setting the Zero Flag if the result is zero. If \textit{Operand 2} is larger then the Sign Flag is set, otherwise the Sign Flag will be cleared.}
\subsection{JMP}
\OpcodeTable{01 /mod /i32}{jmp /i32}{Immediate}{None}{Unconditionally jumps to address \textit{Operand 1}.}
\subsection{JZ}
\OpcodeTable{01 /i32}{jz /i32}{Immediate}{None}{Jumps to address \textit{Operand 1} if the Zero Flag is set.}
\subsection{JG}
\OpcodeTable{01 /mod /i32}{jg /i32}{Immediate}{None}{Jumps to address \textit{Operand 1} if the Sign Flag is set.}
\subsection{JL}
\OpcodeTable{01 /mod /i32}{jl /i32}{Immediate}{None}{Jumps to address \textit{Operand 1} if the Sign Flag is not set.}

%Maybe these in/out instructions could have a mode setting for choosing the width of the data written to the port.
\subsection{Outb}
\OpcodeTable{01 /r /r}{add /r, /r}{8-Bit}{Register}{Writes an 8-bit value from \textit{Operand 2} into port number \textit{Operand 1}.}
\subsection{Inb}
\OpcodeTable{01 /r /r}{inb /r, /r}{Register}{8-Bit}{Reads an 8-bit byte from the port number \textit{Operand 2}, writing it into \textit{Operand 1}.}

\subsection{HLT}
\OpcodeTable{01}{hlt}{None}{None}{Halts the processor until a new interrupt is received.}
\subsection{CLI}
\OpcodeTable{01}{cli}{None}{None}{Clears the interrupt flag preventing the processor from receiving interrupts.}
\subsection{ENI}
\OpcodeTable{01}{eni}{None}{None}{Sets the interrupt flag allowing the processor to receive interrupts.}
\subsection{INT}
\OpcodeTable{01 /i8}{int /i8}{Immediate}{None}{Triggers software interrupt number \textit{Operand 1}.}