Created a proper intro and refactored the doc

src/central-execution-engine-spec.adoc

@@ -7,7 +7,6 @@ This document presents the public interface of an _Execution Engine_ of the {cen
 This means that the amount of information pertaining to the internals of an _Execution Engine_ hardware implementation are kept at a minimum except when a choice in the public interface is specifically made to simplify said implementation.
 
 include::execution-engine-spec/glossary.adoc[]
-include::execution-engine-spec/data-manipulation.adoc[]
-include::execution-engine-spec/ee-modes.adoc[]
+include::execution-engine-spec/intro.adoc[]
 include::execution-engine-spec/registers.adoc[]
 include::execution-engine-spec/instructions.adoc[]

src/execution-engine-spec/data-manipulation.adoc

@@ -1,16 +0,0 @@
-== Data Manipulation
-=== Data Width
-All the registers of the {central-arch-name} are 32bit wide.
-
-=== Address Width
-Memory addresses are 24bit wide thus an _Execution Engine_ can address up to 16MB of memory.
-
-=== Memory Alignment
-The {central-arch-name} uses byte-addressable memory. Under the hood, memory accesses are done on a memory-word boundary.
-A memory-word is 32bit wide. To maximize performance, memory accesses should be done on a 32bit alignment.
-
-NOTE: The term "`memory accesses`" encompasses both read and write operations.
-
-=== Endianness
-Data is encoded in memory with the little endian scheme. For a given value, the least significant byte (LSB) is stored in the lowest address and the most significant byte (MSB) in the highest.
-

src/execution-engine-spec/ee-modes.adoc

@@ -1,11 +0,0 @@
-== Execution Engine Modes
-
-User-mode::
-    Lowest privilege level. User programs execute in this mode. These programs can access services provided in a higher privilege level by making supervisor calls (SVCs).
-
-System-mode::
-    Highest privilege level (on par with fault-mode). The kernel executes its code in this mode. Control is passed to code executing in this mode when exceptions occur or when a user program makes a supervisor call (SVC).
-
-Fault-mode::
-    Code only executes in this mode when a double fault occurs, i.e. when an exception is generated in system-mode code. The code executing under this mode should do the bare minimum to log or report the error and then reset/halt the system.
-

src/execution-engine-spec/glossary.adoc

@@ -2,6 +2,21 @@
 == Glossary
 
 [glossary]
-Central Processing Unit:: An hardware unit containing one or more _Execution Engines_, a memory controller, an interrupt controller, an operator facility controller and various other minor components.
-Execution Engine:: The hardware responsible for the execution of user-written code, be it kernel code or user program code. An _Execution Engine_ contains a fetch unit, a decode unit, a micro-instruction sequencer, an Arithmetic & Logic Unit, various registers and subsystems. An _Execution Engine_ is analogous to an _hart_ in the RISC-V nomenclature or to a _core_ in other CPU specifications.
+[horizontal]
+Processing Complex::
+    The hardware enclosure and the hardware inside said enclosure.
+    Composed of _Processing Units_, memory modules & controllers, an I/O subsystem, an _HMC_ communications controller and various other components.
+
+Processing Unit::
+    An hardware unit containing one or more _Execution Engines_, cache controllers, an interrupt controller, ...
+
+Execution Engine::
+    The hardware responsible for the execution of user-written code, be it kernel code or user program code.
+    An _Execution Engine_ contains a fetch unit, a decode unit, a micro-instruction sequencer, an Arithmetic & Logic Unit, various registers and subsystems.
+    An _Execution Engine_ is analogous to an _hart_ in a RISC-V processor or to a _core_ in other CPU specifications.
+
+HMC::
+    Stands for Hardware Management Console.
+    This is a console directly attached to the _Processor Complex_ that is used to manage the different aspects of the hardware.
+    For example, the HMC is reponsible for loading the _Boot Code_ into main memory and instructing the _Processing Units_ to begin executing code at a given address.
 

src/execution-engine-spec/instructions.adoc

@@ -1,5 +1,7 @@
 == Instructions
-
 === Instruction Encoding Formats
+Multiple instruction encoding formats are used to encode multiple kinds of instructions.
+The source (rs1 and rs2) and destination (rd) registers are kept at the same position in all formats to simplify decoding.
+
+include::images/instruction-formats.adoc[]
 
-include::images/instruction-formats.adoc[]

src/execution-engine-spec/intro.adoc

@@ -0,0 +1,92 @@
+== Introduction
+=== Overview
+The {central-arch-name} is a 32-bits architecture meaning that most of the registers present in an _execution engine_ are 32-bits wide.
+
+=== Memory
+The {central-arch-name} uses byte-addressable memory.
+While an _execution engine_ handles data 32-bits wide, memory addresses are only 24-bits wide.
+An _Execution Engine_ can thus address up to 16MB of main memory.
+
+NOTE: We use the terms "`memory`" and "`main memory`" interchangably. Main memory refers to the RAM while we use the term "`secondary memory`" to refer to HDD or SSD storage.
+
+At the hardware level, memory accesses are done on a memory-word boundary.
+A memory-word is 32-bits wide and memory accesses should be done at a 32-bits alignment to avoid wasting cycles doing double the amount of memory operations.
+
+NOTE: The term "`memory accesses`" encompasses both read and write operations.
+
+Data is encoded in memory with the little endian scheme.
+For a given value, the least significant byte (LSB) is stored in the lowest address and the most significant byte (MSB) in the highest.
+
+=== System modes and privilege levels
+There are three system modes divided into two privilege levels.
+
+The privilege levels are `privileged` and `unprivileged`.
+Some instructions can only be executed and some registers can only be accessed while running under a system mode belonging to the `privileged` privilege level.
+
+.System modes
+[cols="1,1"]
+|===
+|Name |Privilege level
+
+|User-mode
+|Unprivileged
+
+|Supervisor-mode
+|Privileged
+
+|Fault-mode
+|Privileged
+|===
+
+User-mode::
+    User programs execute in this mode.
+    These programs can access services provided in a higher privilege level by making supervisor calls (SVCs).
+
+Supervisor-mode::
+    The kernel executes its code in this mode.
+    Control is passed to code executing in this mode when exceptions occur.
+    Software executing in this mode provides context switching, I/O, process management and inter-process communications.
+
+Fault-mode::
+    Code executes in this mode when a double fault occurs, i.e. when an exception is generated in system-mode code.
+    Code executing under this mode can be used to log/report double faults and then reset/halt the system.
+    Debug exceptions generated in supervisor-mode code are also handled in this mode, in which case control is passed back to supervisor-mode after handling.
+
+=== Exceptions
+Exceptions are events that need to be acknowledged and handled by the system.
+Exceptions change the state of the _execution engine_ so that privileged software can be executed to handle them.
+Exceptions thus always suspend user code for the duration of their handling.
+
+Exceptions can be of two types: *synchronous* and *asynchronous*.
+
+==== Synchronous exceptions
+Synchronous exceptions are generated from inside an _execution engine_.
+They are a conditional or unconditional response to the execution of an instruction.
+
+SVC::
+    Supervisor call exceptions are generated when a user program calls the `svc` instruction.
+    Supervisor calls are used to perform privileged actions in a secure manner.
+
+DataFault::
+    A data fault is generated when an instruction references a memory address that is not mapped in virtual memory or that does not exist.
+    This exception is generated when read or writing memory as well as fetching an instruction.
+
+SysTick::
+    This exception is generated each time a system programed timer ticks at regular intervals.
+    This exception is used to implement context switching.
+
+Debug::
+    TODO
+
+==== Asynchronous exceptions
+Asynchronous exceptions are generated from outside an _execution engine_.
+These exceptions enable the system to react to its environment.
+
+HMC::
+    This exception is generated when the _hardware management console_ communicates with a _Processing Unit_ and that the _Processing Unit_ relays the event to an _Execution Engine_.
+    Data can be passed alongside the exception and would be stored in main memory by the _Processing Unit_ communications controller.
+
+IO::
+    This exception is generated when an I/O device communicates with the _Execution Engine_.
+    Data can be passed alongside the exception and would be stored in main memory by the I/O system.
+

src/execution-engine-spec/registers.adoc

@@ -1,10 +1,11 @@
 == Registers
 === General Purpose Registers
-
-General purpose registers (GPRs) are used to perform calculations and store intermediate values. There are 8 GPRs in an Execution Engine. These registers are named *_r0_* through *_r7_*.
+General purpose registers (GPRs) are used to perform calculations and store intermediate values.
+There are 8 GPRs in an Execution Engine. These registers are named *_r0_* through *_r7_*.
 
 === Special Purpose Registers
 
+.Special purpose registers
 [cols="1,1,1,1,3"]
 |===
 |Name |User-mode |Supervisor-mode |Fault-mode |Description