Added EE modes and registers + enhanced layout

src/central-execution-engine-spec.adoc

@@ -7,7 +7,7 @@ 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/registers.adoc[]
 include::execution-engine-spec/instructions.adoc[]

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

@@ -1,15 +1,16 @@
 == Data Manipulation
-=== Data width
+=== Data Width
 All the registers of the {central-arch-name} are 32bit wide.
 
-=== Address width
+=== Address Width
 Memory addresses are 24bit wide thus an _Execution Engine_ can address up to 16MB of memory.
 
-=== Memory alignment
+=== 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.
+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

@@ -0,0 +1,11 @@
+== 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

@@ -3,4 +3,5 @@
 
 [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.
+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.
+

src/execution-engine-spec/registers.adoc

@@ -0,0 +1,68 @@
+== 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_*.
+
+=== Special Purpose Registers
+
+[cols="1,1,1,1,3"]
+|===
+|Name |User-mode |Supervisor-mode |Fault-mode |Description
+
+|lr
+|X
+|X
+|X
+|Link register. Store the return address from a Branch-with-Link.
+
+|pc
+|X
+|X
+|X
+|Program counter alias register. It is used as an alias to the current processor mode program counter register.
+
+|sp
+|X
+|X
+|X
+|Stack pointer alias register. It is used as an alias to the current processor mode stack pointer register.
+
+|pc_user
+|X
+|X
+|X
+|User-mode program counter.
+
+|sp_user
+|X
+|X
+|X
+|User-mode stack pointer.
+
+|pc_sys
+|
+|X
+|X
+|Supervisor-mode program counter.
+
+|sp_sys
+|
+|X
+|X
+|Supervisor-mode stack pointer.
+
+|pc_fault
+|
+|X
+|X
+|Fault-mode program counter.
+
+|sp_fault
+|
+|X
+|X
+|Fault-mode stack pointer.
+|===
+
+#Add a Program Status Register (ALU flags C, Z, N, V and processor modes U, S, F), an Exception Status Register and a Channel Status Register. --> Or do we put it in subsystems?#
+

src/global-config.adoc

@@ -3,4 +3,6 @@
 :icons: font
 :lang: en
 :toc: left
-:toclevels: 5
+:toclevels: 5
+:sectnums:
+:sectnumlevels: 5