Tarmac is an incomplete dynamically recompiling emulator of an ARM microprocessor. It aims
to emulate ARM architecture version 2a (an ARM3 processor) by decomposing sequences of ARM instructions to many
sub-instructions known as armlets. These armlets are then more easily recompiled to x86 code with specialised
optimisations being applied throughout the conversion. The x86 code is then executed to emulate the ARM instructions.
By caching the generated x86 code and reusing it when the same sequence of ARM instructions is executed again, a
significant speed up of the emulation can be achieved. The project was always intended to be used in the emulation
of a full computer system meaning that real world problems such as synchronisation with subsystems, interrupts and
exceptions have all been taken into account.
Part of the project was to develop an interpreting ARM emulator which is accurate enough to emulate the RISC OS
Operating System. This was done in conjunction with Graeme Barnes' Red Squirrel, an
Acorn computer emulator for Windows which provides the Memory map, IO and video emulation needed to emulate complex
Screenshots of Tarmac's interpreting ARM emulator and Red Squirrel running RISC OS
Jon Welch borrowed the interpreting emulator and disassembler from Tarmac for use in
BeebEm emulating the Acorn ARM Second
processor board of a BBC Micro. Jon's made the original ARM Evaluation
disk images and manual available to download which is well worth getting if you intend playing with this.
Strictly speaking, Tarmac doesn't emulate the ARM1 processor
used in that board but the ARM3 emulation implemented is almost identical (only adding the MUL and SWP instructions). The second screenshot
that follows shows the ARM BASIC assembler failing to recognise the MUL instruction.
Screenshots of Tarmac's interpreting ARM emulator and BeebEm running ARM BASIC on a BBC Micro
To use the ARM emulation on BeebEm:
Select menu item: Hardware / BBC Model / BBC Master 128
Select menu item: Hardware / ARM Second Processor
Now you're ready to use the machine:
Type 'CON. TUBE' to use the ARM co-processor by default
Type 'CON. FILE 13' to make ADFS the default filing system (so you can read the disc image).
Type 'CON. MODE 0' to switch to a screen mode with smaller font.
Press Ctrl + F12 to reset the machine so configuration changes take effect (presses Ctrl+Break in the emulator).
To make sure all your settings are saved for next time so you don't have to repeat the process:
Select menu item: Options / Save Preferences
Select menu item: File / Load Disc 0
Then browse and select armdisc3.adl supplied.
Now you're ready to run some software:
Type 'MOUNT 4' to mount disc.
Type '*.' to list files on disc.
Type 'AB' to start ARM BASIC
Type 'HELP .' to list all BASIC commands
The interpreting emulator and disassembler are both usable in their current state though may contain a couple of bugs.
The actual dynamic recompiler was not completed in the time available for the project, though nearly all the
framework is in place. Code is generated for many ARM instructions and several optimisations are performed. A
comprehensive report of the background to the project with an explanation of the techniques used and examples
of recompilation, as well as full C++ source code is available. This software was developed for my final year
project as part of my degree in Computer Science at the University of Warwick.
As of early 2019, this promising open source dynamic recompiler for ARMv8 to x86-64 on github is in use in Nintendo 3DS and Nintendo Switch emulators. The developers cite Tarmac's intermediate representation as inspiring their approach.
In 2007 Tom Walker added a dynamically recompiling StrongARM emulator to his open source RPCemu which emulates
an Acorn RISC PC. In the same way as Red Squirrel, Tom took a more direct approach, generating x86 or x64 machine code directly
and getting very good performance as a result. Full source code is available from the subversion repository.
The year before Tarmac was developed, Julian Brown wrote a dynamically recompiling ARM emulator called
ARMphetamine. Although more complete than Tarmac since
it recompiles all ARM instructions to x86 code which does actually execute, at the time of writing it too
is buggy and incomplete in terms of support for the nastier side of emulating a real machine.
Following the collaboration on Tarmac, Graeme Barnes went on to develop his own JIT recompiler from ARM
to x86 code for Red Squirrel. It takes a simpler approach, not
performing many optimisations, but in doing so is far more successful and can run real world code for various
versions of ARM processor.