\documentclass[a4paper,12pt]{book}
\usepackage{tikz}

\title{Unnamed Machine}
\author{A Very Terrible 16-bit Machine}

\newcommand{\OpcodeTable}[4] {
	\begin{tabular}{ c c c c }
		\hline
		Opcode & Mnemonic & Operand 1 & Operand 2 \\
		\hline\hline
		#1 & #2 & #3 & #4 \\
		\hline
	\end{tabular}
}

\begin{document}
	\maketitle
	\tableofcontents
	\chapter{Overview}
	\section{Introduction}
	This is a very poorly thought out 16-bit machine, but you've got to start somewhere. Currently debating between a CPU status register, 80x86 style, or just placing arithmetic results into a predetermined register. This is more of a load–store architecture to try and keep the instruction set simple. The machine will be big-endian.
	\section{Registers}
	The following are the general purpose registers that can be used.
	\begin{itemize}
		\item[] R1
		\item[] ...
		\item[] R8
	\end{itemize}
	\section{Memory Model}
	Memory will be implicitly mapped I/O. The bottom 3201 bytes of memory will be reserved for the keyboard input and graphics.
	A single byte is reserved for the keyboard's input. The current key will be stored in byte 0xF37E, with the most significant bit being a flag indicating that the keyboard 
	is ready to be read from. This means that the character encoding is actually 7 bits.
	Video memory starts at 0xF37F (62335 decimal), and every byte represents an ASCII character in monochrome.
	\begin{figure}
		\centering
		\begin{tikzpicture}
			\fill[gray!5] (0,0)rectangle(5,10);
			%\draw (0,10) .. controls (-2,6) and (-2,4) .. (0,1);
			%\draw (0,10) arc (0:180:3cm);
			\draw (0,10) -- (5,10);
			\draw (0,1) --  node[above] {Video 0xF37F} (5,1);
			\draw (0,0) -- (5,0);
			\node[label=right:Top 0x0000] at (5,10) {};
			\node[label=right:Bottom 0xFFFF] at (5,0) {};
		\end{tikzpicture}
		\caption{Memory Layout}
	\end{figure}
	\chapter{Instruction Set Architecture}
	\section{Instruction Encoding}
	Instructions are fixed to exactly one byte (8 bits).
	Instructions that work with two operands the register for operand one will be encoded in the three least significant bits. So an instruction with format XXXX X000 will use Register 1 and so forth all the way to XXXX X111, which will be Register 8.
	\begin{figure}
		\centering
		\begin{tabular}{ c c }
			\hline
			Instruction & Register \\
			\hline\hline
			0000 0 & 000 \\
			\hline
		\end{tabular}
		\caption{Encoding Layout}
	\end{figure}
	\section{STOB (Store Byte)}
		\OpcodeTable{0x20}{stob}{Register}{Address}\\[6pt]
		Stores a single byte (the lower nibble) from a register to a memory address.
	\section{STOW (Store Machine Word)}
		\OpcodeTable{0x20}{stow}{Register}{Address}\\[6pt]
		Stores a machine word from a register to a memory address.
	\section{LODB (Load Byte)}
		\OpcodeTable{0x20}{lodb}{Register}{Address}\\[6pt]
		Loads a single byte into a register from a memory address, zeroing out the high nibble.
	\section{LODW (Load Machine Word)}
		\OpcodeTable{0x20}{lodw}{Register}{Address}\\[6pt]
		Loads a word into a register from a memory address.
	\section{LODBI (Load Immediate Byte)}
		\OpcodeTable{0x00}{lodbi}{Register}{Constant}\\[6pt]
		Loads an immediate byte into the register, zeroing the high nibble.
	\section{LODWI (Load Immediate Word)}
		\OpcodeTable{0x00}{lodwi}{Register}{Constant}\\[6pt]
		Loads an immediate machine word into the register.
	\section{CMP (Compare)}
		\OpcodeTable{0x20}{cmp}{Register}{Register}\\[6pt]
		Compares two registers and somewhere sets a result, maybe in register 1?
	\section{ADD (Add)}
		\OpcodeTable{0x20}{add}{Register}{Register}\\[6pt]
		Performs addition on a register with a value from another (or the same) register.
	\section{SUB (Subtract)}
		\OpcodeTable{0x20}{sub}{Register}{Register}\\[6pt]
		Performs subtraction on a register with a value from another (or the same) register.
	\section{JMP (Jump)}
		\OpcodeTable{0x20}{jmp}{Address}{None}\\[6pt]
		Jumps unconditionally to a memory address.
	\section{JZ (Jump if Zero)}
		\OpcodeTable{0x20}{jz}{Register}{None}\\[6pt]
		Jumps to a memory address if some register is zero.
	\section{JG (Jump if Greater Than)}
		\OpcodeTable{0x70}{jg}{Address}{None}\\[6pt]
		Jump to the Address if some register is greater than zero.
	\section{JL (Jump if Less Than)}
	\section{AND (Logical AND)}
	\section{XOR (Logical Exclusive OR)}
	\section{OR (Logical OR)}
	\section{NOT (Logical Negation)}
	\section{SHR (Shift Right)}
	\section{SHL (Shift Left)}
	\section{INC (Increment)}
	\section{DEC (Decrement)}
	
\end{document}
