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Added some information about the memory layout and a figure for the bit ordering in the registers.

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glm94
2025-02-03 23:12:00 -06:00
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chapter 1.tex
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\chapter{Introduction}
This is an attempt to distill what I know about how computers work into some kind of emulator / simulator. I'm curious to see if what I know at the moment is enough to allow me to emulate a general purpose computer. It turns out that it take quite a different skillset compared to writing web applications or desktop applications. But I realize that, while I understand the concepts that underpin what I do day to day, I don't have an intuitive understanding of the hardware. This has become painfully evident pretty much immediately after I started this project. An ISA should be simple, add, subtract, copy, etc., but the you have to consider the state the processor will be in after each instruction.
This is an attempt to distill what I know about how computers work into some kind of emulator / simulator. I'm curious to see if what I know at the moment is enough to allow me to emulate a general purpose computer. It turns out that it take quite a different skillset compared to writing web applications or desktop applications. But I realize that, while I understand the concepts that underpin what I do day to day, I don't have an intuitive understanding of the hardware. This has become painfully evident pretty much immediately after I started this project. An ISA should be simple, add, subtract, copy, etc., but the you have to consider the state the processor will be in after each instruction.
\section{Registers}
Thirty-two general purpose registers are available for program code to use however it wishes. The base name for the general purpose registers is \quotes{r} follows by an unpadded number, such as \quotes{r28}. These numbers do not follow a zero-based index scheme, \quotes{r29} is the twenty-ninth register. Using the base name implies full width, a \quotes{word}, when reading or writing from or to the register respectively. In addition to the general purpose registers there is also a base pointer (bp), stack pointer (sp) and an instruction pointer (ip). The base pointer and the stack pointer may be set directly using the \hyperref[sec:mov]{mov} instruction, which allows the programmer to set up a stack frame. The instruction pointer can only be set with a branching operation like \hyperref[sec:jmp]{jmp} or a call to a subroutine with the \hyperref[sec:call]{call} instruction.
\begin{figure}[h]
\begin{tikzpicture}
%\draw[help lines] (-8,-3) grid (8,3);
\node [draw, fit={(-6, 0) (-1, -0.5)}, label=center:r1] (r1) {};
\node [draw=none, fit={(-6, -1) (-1, -1.5)}, label=center:...] (ellipsis) {};
\node [draw, fit={(-6, -2) (-1, -2.5)}, label=center:r32] (r32) {};
\node [draw, fit={(-3.0, 1.5) (2.0, 1.0)}, label=center:Flags Register] (flags) {};
\node [draw, fit={(0, 0) (5, -0.5)}, label=center:Instruction Pointer (ip)] (ip) {};
\node [draw, fit={(0, -1.0) (5, -1.5)}, label=center:Base Pointer (ip)] (bp) {};
\node [draw, fit={(0, -2.0) (5, -2.5)}, label=center:Stack Pointer (ip)] (sp) {};
\draw (r1);
\node at (r1.north) [above] {General Purpose};
\node at (r1.north west) [above] {31};
\node at (r1.north east) [above] {0};
\draw (ellipsis);
\draw (r32);
\draw (flags);
\node at (flags.north) [above] {Status Register};
\node at (flags.north west) [above] {31};
\node at (flags.north east) [above] {0};
\draw (ip);
\node at (ip.north) [above] {Program Status};
\node at (ip.north west) [above] {31};
\node at (ip.north east) [above] {0};
\draw (bp);
\draw (sp);
\end{tikzpicture}
\caption{Bit Ordering}
\label{fig:registerbitlayout} % https://www.overleaf.com/learn/latex/Referencing_Figures
\end{figure}
\begin{description}
\item In summary, the available registers are as follows:
\item[General Purpose] Thirty-two general purpose registers that are the width of a \textit{word} and numbered \quotes{r1} through \quotes{r32}
@@ -11,4 +48,5 @@ Thirty-two general purpose registers are available for program code to use howev
\item[Instruction Pointer (ip)] Points to the next instruction to execute and advanced by the width of an instruction. Set by jump or call instructions to the location specified by the instruction.
\end{description}
\section{Memory}
\section{Memory}
The memory layout is quite simple, much like many modern machines it is a linear memory map. The address bus is thrity-two bits wide, the same width of the data bus, meaning at most there is 2\textsuperscript{32} of addressable memory. Like contemprary machines the top of memory starts at address 0x00000000 and grows downward toward 0xFFFFFFFF.