[PATCH] docs/system: add a little more detail to ARM machines

[Alex Bennée]

Taking from the wiki page mention:

  - both 32 and 64 bit architectures
  - the roll of A/M profile chips
  - warn about kernel portability
  - gently suggest -M virt
  - add headlines for groups of boards

This is still incomplete but hopefully is a less of a straight dive
into a dry list of peripherals.

Signed-off-by: Alex Bennée <address@hidden>
---
 docs/system/target-arm.rst | 47 +++++++++++++++++++++++++++++++++++---
 1 file changed, 44 insertions(+), 3 deletions(-)

diff --git a/docs/system/target-arm.rst b/docs/system/target-arm.rst
index d2a3b44ce88..28aaef011d6 100644
--- a/docs/system/target-arm.rst
+++ b/docs/system/target-arm.rst
@@ -3,8 +3,35 @@
 ARM System emulator
 -------------------
 
-Use the executable ``qemu-system-arm`` to simulate a ARM machine. The
-ARM Integrator/CP board is emulated with the following devices:
+Use the executable ``qemu-system-arm`` to simulate a 32 bit ARM
+machine. The ``qemu-system-aarch64`` executable is used to simulate
+the 64 bit ARM AArch64 architecture. AArch64 CPUs can often include an
+AArch32 execution unit and execute a mix of 64 and 32 bit code.
+
+The emulator supports both "A-profile" and "M-profile" CPUs. The
+A-profile CPUs have a full MMU and can run things like the Linux while
+the M-profile CPUs are typically used in embedded micro-controller
+boards.
+
+Because ARM systems differ so much and in fundamental ways, typically
+operating system or firmware images intended to run on one machine
+will not run at all on any other.
+
+If you don't care about running on a particular piece of hardware the
+``-M virt`` board provides a PCI based virtio board which can be
+configured with a range of RAM sizes, CPU types and virtio based
+peripherals. It is generally the target you want to use if general
+purpose operating systems.
+
+Otherwise a range of other machine types are available. Passing ``-M
+help`` to the command line will list them all. They include the
+following:
+
+ARM Integrator/CP
+~~~~~~~~~~~~~~~~~
+
+This is a development board intended for prototyping and developing
+ARM-based devices.
 
 -  ARM926E, ARM1026E, ARM946E, ARM1136 or Cortex-A8 CPU
 
@@ -18,7 +45,11 @@ ARM Integrator/CP board is emulated with the following 
devices:
 
 -  PL181 MultiMedia Card Interface with SD card.
 
-The ARM Versatile baseboard is emulated with the following devices:
+ARM Versatile/Versatile Express
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Another family of development boards. The more recent Versatile
+Express boards are designed for modern Cortex processors.
 
 -  ARM926E, ARM1136 or Cortex-A8 CPU
 
@@ -45,6 +76,9 @@ The ARM Versatile baseboard is emulated with the following 
devices:
 
 -  PL181 MultiMedia Card Interface with SD card.
 
+ARM RealView
+~~~~~~~~~~~~
+
 Several variants of the ARM RealView baseboard are emulated, including
 the EB, PB-A8 and PBX-A9. Due to interactions with the bootloader, only
 certain Linux kernel configurations work out of the box on these boards.
@@ -77,6 +111,9 @@ The following devices are emulated:
 
 -  PL181 MultiMedia Card Interface with SD card.
 
+Various PDA machines
+~~~~~~~~~~~~~~~~~~~~
+
 The XScale-based clamshell PDA models (\"Spitz\", \"Akita\", \"Borzoi\"
 and \"Terrier\") emulation includes the following peripherals:
 
@@ -156,6 +193,10 @@ Nokia N800 and N810 internet tablets (known also as RX-34 
and RX-44 /
 -  Nokia RETU and TAHVO multi-purpose chips with an RTC, connected
    through CBUS
 
+
+M-profile boards
+~~~~~~~~~~~~~~~~
+
 The Luminary Micro Stellaris LM3S811EVB emulation includes the following
 devices:
 
-- 
2.20.1